What is Corona Effect


Introduction:

One of the phenomena associated with all energized electrical devices, including high-voltage transmission lines, is corona. The localized electric field near a conductor can be sufficiently concentrated to ionize air close to the conductors. This can result in a partial discharge of electrical energy called a corona discharge, or corona.

  What is Corona?

  •  Electric transmission lines can generate a small amount of sound energy as a result of corona.
  • Corona is a phenomenon associated with all transmission lines. Under certain conditions, the localized electric field near energized components and conductors can produce a tiny electric discharge or corona that causes the surrounding air molecules to ionize, or undergo a slight localized change of electric charge.
  • Utility companies try to reduce the amount of corona because in addition to the low levels of noise that result, corona is a power loss, and in extreme cases, it can damage system components over time.
  • Corona occurs on all types of transmission lines, but it becomes more noticeable at higher voltages (345 kV and higher). Under fair weather conditions, the audible noise from corona is minor and rarely noticed.
  • During wet and humid conditions, water drops collect on the conductors and increase corona activity. Under these conditions, a crackling or humming sound may be heard in the immediate vicinity of the line.
  • Corona results in a power loss. Power losses like corona result in operating inefficiencies and increase the cost of service for all ratepayers; a major concern in transmission line design is the reduction of losses.

 Source of Corona:

  • The amount of corona produced by a transmission line is a function of the voltage of the line, the diameter of the conductors, the locations of the conductors in relation to each other, the elevation of the line above sea level, the condition of the conductors and hardware, and the local weather conditions. Power flow does not affect the amount of corona produced by a transmission line.
  • The electric field gradient is greatest at the surface of the conductor. Large-diameter conductors have lower electric field gradients at the conductor surface and, hence, lower corona than smaller conductors, everything else being equal. The conductors chosen for the Calumet to the line were selected to have large diameters and to utilize a two conductor bundle. This reduces the potential to create audible noise.
  • Irregularities (such as nicks and scrapes on the conductor surface or sharp edges on suspension hardware) concentrate the electric field at these locations and thus increase the electric field gradient and the resulting corona at these spots. Similarly, foreign objects on the conductor surface, such as dust or insects, can cause irregularities on the surface that are a source for corona.
  • Corona also increases at higher elevations where the density of the atmosphere is less than at sea level. Audible noise will vary with elevation. An increase in 1000 feet of elevation will result in an increase in audible noise of approximately 1 dB (A). Audible noise at 5000 feet in elevation will 5 dB (A) higher than the same audible noise at sea level, all other things being equal. The new Calumet to Comanche 345 kV double circuit line was modeled with an elevation of 6000 feet.
  • Raindrops, snow, fog, hoarfrost, and condensation accumulated on the conductor surface are also sources of surface irregularities that can increase corona. During fair weather, the number of these condensed water droplets or ice crystals is usually small and the corona effect is also small.
  • However, during wet weather, the number of these sources increases (for instance due to rain drops standing on the conductor) and corona effects are therefore greater.
  • During wet or foul weather conditions, the conductor will produce the greatest amount of corona noise. However, during heavy rain the noise generated by the falling rain drops hitting the ground will typically be greater than the noise generated by corona and thus will mask the audible noise from the transmission line.
  • Corona produced on a transmission line can be reduced by the design of the transmission line and the selection of hardware and conductors used for the construction of the line. For instance the use of conductor hangers that have rounded rather than sharp edges and no protruding bolts with sharp edges will reduce corona. The conductors themselves can be made with larger diameters and handled so that they have smooth surfaces without nicks or burrs or scrapes in the conductor strands. The transmission lines proposed here are designed to reduce corona generation.

 TYPES OF CORONA:

There are three types of corona.

  • A glow discharge occurs at a gradient of approximately 20 kV rms/cm. Glow discharge is a light glow off sharp points that does not generate objectionable RIV/TVI or cause any audible noise.
  • At about 25 kV rms/cm, negative polarity “brush” discharges occur. So named because the appearance is similar to the round ends of a bottle brush. The audible noise associated with brush corona is generally a continuous background type of hissing or frying noise.
  • At a gradient of around 30 kVrms/cm positive polarity plume corona is generated; so named because of its general resemblance to a plume. When viewed in the dark it has a concentrated stem that branches and merges into a violet-colored, tree-like halo. The audible noise associated with plume corona is a rather intense snapping and hissing sound. Plume corona generates significant RIV/TVI.
  • These observations are based on fair weather conditions. Under wet conditions virtually all energized electrodes will be in corona of one form or another.
  • Many are under the impression that the dielectric strength of air is greater under dry conditions. That is not true. In fact, the dielectric strength of air increases with increased moisture up to the dew point when moisture begins to condense on the surface of insulators and other components of the line.

 Physical Parameters of Corona:

  • Corona is caused by the ionization of the media (air) surrounding the electrode (conductor)
  • Corona onset is a function of voltage
  • Corona onset is a function of relative air density
  • Corona onset is a function of relative humidity

1. Corona and the Electric Field

  • Corona is NOT solely a function of the Electric Field
  • Corona is a function of the electric field on the surface of the electrode (conductor)
  • Corona is also a function of the radius of curvature of the electrode (conductor)
  • Corona is also a function of the rate of decay of the electric field away from the electrode (conductor)
  • For the preceding reasons, selecting the conductor with the smallest electric field at its surface is not correct.

2. Corona and the Relative Air Density

  • Corona has an inverse relationship with air density
  • Standard line designs that perform well at sea level, may have significant corona issues if used on lines that are installed over mountainous areas

3. Corona and the Humidity

  • Corona has an inverse relationship with humidity at power frequencies
  • Fair weather corona is more prevalent in low humidity environments

 4. Corona is Dependent Surface Condition of the Conductors

  • Corona is enhanced by irregularities on the conductor surface
  • Irregularities include: dust, insects, burrs and scratches and water drops present on new conductors
  • Corona will generally be greater on new conductors and will decrease to a steady-state value over a period of approximately one year in-service
  • Corona is significantly increased in foul weather.

What’s The Fuss?

  • Corona from conductors and hardware may cause audible noise and radio noise
  • Audible noise from conductors may violate noise standards
  • Radio noise from conductors may interfere with communications or navigation
  • Corona loss may be significant when compared with resistive loss of conductors
  • Corona can cause possible damage to polymeric insulators

Methods to reduce Corona Discharge Effect:

  •  Corona can be avoided
    1.  By minimizing the voltage stress and electric field gradient.: This is accomplished by using utilizing good high voltage design practices, i.e., maximizing the distance between conductors that have large voltage differentials, using conductors with large radii, and avoiding parts that have sharp points or sharp edges.
    2. Surface Treatments: Corona inception voltage can sometimes be increased by using a surface treatment, such as a semiconductor layer, high voltage putty or corona dope.
    3. Homogenous Insulators: Use a good, homogeneous insulator. Void free solids, such as properly prepared silicone and epoxy potting materials work well.
    4. If you are limited to using air as your insulator, then you are left with geometry as the critical parameter. Finally, ensure that steps are taken to reduce or eliminate unwanted voltage transients, which can cause corona to start.
    5. Using Bundled Conductors: on our 345 kV lines, we have installed multiple conductors per phase. This is a common way of increasing the effective diameter of the conductor, which in turn results in less resistance, which in turn reduces losses.
    6. Elimination of sharp points: electric charges tend to form on sharp points; therefore when practicable we strive to eliminate sharp points on transmission line components.
    7. Using Corona rings: On certain new 345 kV structures, we are now installing corona rings. These rings have smooth round surfaces which are designed to distribute charge across a wider area, thereby reducing the electric field and the resulting corona discharges.
    8. Whether: Corona phenomena much worse in foul weather, high altitude
    9. New Conductor: New conductors can lead to poor corona performance for a while.
    10. By increasing the spacing between the conductors: Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phase’s results in heavier metal supports. Cost and Space requirement increases.
    11. By increasing the diameter of the conductor: Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved.

Sources of Corona and Arcing in Polymer Insulators:

  •  Loose hardware
  • Contamination and surface tracking
  • Missing corona rings
  • Damaged or incorrectly installed corona ring
  • Damaged end fittings or end fitting seal
  • Exposed internal rod due to: Carbonized internal rod by internal discharges Split sheath due to weathering

 Electro Magnetic Inductions:

  •  EM1 field or radio noise field from high-voltage transmission lines are caused by corona, which is essentially due to the electrical breakdown of the air surrounding the conductors at higher voltage.
  •  When the conductor surface electric field exceeds the corona onset electric field, a partial electrical breakdown occurs in the surrounding air medium near the conductor surface and is called the corona discharge. The increase of conductor surface gradient takes place with increase of supply voltage. In addition, organic contamination or attachment -of water droplets also may contribute to localized field enhancement.
  • When organic particles or water droplets are attached to the conductor surface, the charge accumulation at that point increases which enhances the local electric field. The intensification of surface gradient locally leads to the corona discharge.
  • The streamer generated during corona discharge, transports electric charge into the surrounding air during the discharge cycle. These moving charges contribute directly to the noise fields. ‘They also cause currents to be induced on the transmission line conductors. Since the charge is moved by a time varying electric field, it is equivalent to a current pulse and this When a communication line passes near the corridor of a HV or EHV transmission line, if the frequency of the radiated EM signal due to corona matches with that of the transmitted signal on the communication line, then the communication signal may get distorted. To mitigate this effect, the communication line should pass at a safe distance away from the transmission line.
  • Hence there is a need to estimate the radiated EM1 signal in dB at a given distance from the HV or EHV transmission line. In this paper, radiated EM1 in dB is computed for a single conductor high voltage over headline. This theoretical result is compared with the published experimental results available in the literature. In the computational work, earth is considered as an infinitely conducting ground.

 Physical description of corona and Electro Magnetic Induction:

  •  When alternating supply voltage energizes the conductor, the conductor surface electric field exceeds the corona on set electric field of the conductor. The corona discharge occurs in both positive and negative half cycle. So the corona is divided into positive and negative corona depending upon the polarity of the supply voltage.
  • When the conductor is positive with respect to ground, an electron avalanche moves rapidly into the conductor leaving the heavy positive-ion charge cloud close to conductor, which drifts away.
  • The rapid movements of electrons and motion of positive ions gives the steep front of the pulse, while the further drift of positive ions will give slow tail of the corona pulse.
  • When conductor is negative with respect to the ground, an electron avalanche moves away from the energized conductor and the positive heavy ions move towards the conductor. Since the heavy positive ions are, moving towards the higher electric field, their motion is very rapid which gives rise to a much sharper pulse than the positive pulse. Due to rapid moment of the electrons from the conductor surface, the electric field regains its original value at conductor surface very quickly than in the case of positive polarity. Thus the negative corona pulses are lower in amplitude and lower in rise and fall times as compared to positive corona pulses. They have also higher repetition rates than the positive pulse

 Corona Detection:

  •  Light Ultraviolet radiation: Corona can be visible in the form of light, typically a purple glow, as corona generally consists of micro arcs. Darkening the environment can help to visualize the corona.
  • Sound (hissing, or cracking as caused by explosive gas expansions):  You can often hear corona hissing or cracking Sound.
  • In addition, you can sometimes smell the presence of ozone that was produced by the corona.
  • Salts, sometimes seen as white powder deposits on Conductor.
  • Mechanical erosion of surfaces by ion bombardment
  •  Heat (although generally very little, and primarily in the insulator)
  • Carbon deposits, thereby creating a path for severe arcing
  • The corona discharges in insulation systems result in voltage transients. These pulses are superimposed on the applied voltage and may be detected, which is precisely what corona detection equipment looks for. In its most basic form, the following diagram is a corona (or partial discharge) measuring system:
  • It is important that the voltage source and the coupling capacitor exhibit low noise so as not to obscure the corona. In its simplest form the pulse detection network is a resistor monitored by an oscilloscope. Don’t dismiss this simple technique as crude, as we once used this method to observe the presence of corona in an improperly terminated high voltage connector, even after a dedicated corona tester failed to find any. Commercially available corona detectors include electronic types (as above) as well as ultrasonic types.

 Corona Calculations

  •  The following corona calculations are from Dielectric Phenomena in High Voltage Engineering

1.     For Concentric Cylinders in Air:

  • Corona will not form when RO / RI < 2.718. (Arcing will occur instead when the voltage is too high.)

2.     For Parallel Wires in Air:

  • Corona will not form when X / r < 5.85. (Arcing will occur instead when the voltage is too high.)

3.     For Equal Spheres in Air:

  • Corona will not form when X / R < 2.04. (Arcing will occur instead when the voltage is too high.)
  • Arcing difficult to avoid when X / R < 8

Where

  • RO = Radius of outer concentric sphere
  • RI = Radius of inner concentric sphere
  • R = Sphere radius
  • r = wire radius
  • X = Distance between wires or between spheres

 Effects of Corona:

 (1) Audible Noise

  •  During corona activity, transmission lines (primarily those rated at 345 kV and above) can generate a small amount of sound energy. This audible noise can increase during foul weather conditions. Water drops may collect on the surface of the conductors and increase corona activity so that a crackling or humming sound may be heard near a transmission line. Transmission line audible noise is measured in decibels using a special weighting scale, the “A” scale that responds to different sound characteristics similar to the response of the human ear. Audible noise levels on typical 230 kV lines are very low and are usually not noticeable. For example, the calculated rainy weather audible noise for a 230 kV transmission line at the right-of-way edge is about 25 dBA, which is less than ambient levels in a library and much less than background noise for wind and rain.

 (2)Radios and Television Interference:

  •  Overhead transmission lines do not, as a general rule, interfere with radio or TV reception.
  • There are two potential sources for interference: corona and gap discharges. As described above, corona discharges can sometimes generate unwanted electrical signals.
  • Corona-generated electrical noise decreases with distance from a transmission line and also decreases with higher frequencies (when it is a problem, it is usually for AM radio and not the higher frequencies associated with TV signals).
  • Corona interference to radio and television reception is usually not a design problem for transmission lines rated at 230 kV and lower. Calculated radio and TV interference levels in fair weather and in rain are extremely low at the edge of the right-of-way for a 230 kV transmission line.
  • Gap discharges are different from corona. Gap discharges can develop on power lines at any voltage. They can take place at tiny electrical separations (gaps) that can develop between mechanically connected metal parts. A small electric spark discharges across the gap and can create unwanted electrical noise. The severity of gap discharge interference depends on the strength and quality of the transmitted radio or TV signal, the quality of the radio or TV set and antenna system, and the distance between the receiver and power line. (The large majority of interference complaints are found to be attributable to sources other than power lines: poor signal quality, poor antenna, door bells, and appliances such as heating pads, sewing machines, freezers, ignition systems, aquarium thermostats, fluorescent lights, etc.).
  • Gap discharges can occur on broken or poorly fitting line hardware, such as insulators, clamps, or brackets. In addition, tiny electrical arcs can develop on the surface of dirty or contaminated insulators, but this interference source is less significant than gap discharge.
  • Hardware is designed to be problem-free, but corrosion, wind motion, gunshot damage, and insufficient maintenance contribute to gap formation. Generally, interference due to gap discharges is less frequent for high-voltage transmission lines than lower-voltage lines. The reasons that transmission lines have fewer problems include: predominate use of steel structures, fewer structures, greater mechanical load on hardware, and different design and maintenance standards.
  • Gap discharge interference can be avoided or minimized by proper design of the transmission line hardware parts, use of electrical bonding where necessary, and by careful tightening of fastenings during construction. Individual sources of gap discharge noise can be readily located and corrected. Arcing on contaminated insulators can be prevented by increasing the insulation in high contamination areas and with periodic washing of insulator strings.

 (3) Gaseous Effluents

  •  Corona activity in the air can produce very tiny amounts of gaseous effluents: ozone and NOX. Ozone is a naturally occurring part of the air, with typical rural ambient levels ranging from about 10 to 30 parts per billion (ppb) at night and peaks at approximately 100 ppb. In urban areas, concentrations exceeding 100 ppb are common. After a thunderstorm, the air may contain 50 to 150 ppb of ozone, and levels of several hundred ppb have been recorded in large cities and in commercial airliners.
  • Ozone is also given off by welding equipment, copy machines, air fresheners, and many household appliances. The National Ambient Air Quality Standard for Oxidants (ozone is usually 90 to 95 percent of the oxidants in the air) is 120 ppb, not to be exceeded as a peak concentration on more than one day a year.
  •  In general, the most sensitive ozone measurement instrumentation can measure about 1 ppb. Typical calculated maximum concentrations of ozone at ground level for 230 kV transmission lines during heavy rain are far below levels that the most sensitive instruments can measure and thousands of times less than ambient levels. Therefore, the proposed transmission lines would not create any significant adverse effects in the ambient air quality of the project area.

 (4) Induced Currents

  •  Small electric currents can be induced by electric fields in metallic objects close to transmission lines. Metallic roofs, vehicles, vineyard trellises, and fences are examples of objects that can develop a small electric charge in proximity to high voltage transmission lines. Object characteristics, degree of grounding, and electric field strength affect the amount of induced charge.
  •  An electric current can flow when an object has an induced charge and a path to ground is presented. The amount of current flow is determined by the impedance of the object to ground and the voltage induced between the object and ground.
  • The amount of induced current that can flow is important to evaluate because of the potential for nuisance shocks to people and the possibility of other effects such as fuel ignition.
  • The amount of induced current can be used to evaluate the potential for harmful or other effects. As an example, when an average woman or man grips an energized conductor, the threshold for perception of an electric current is 0.73 milli ampere (mA) and 1.1 mA, respectively. If the current is gradually increased beyond a person’s perception threshold, it becomes bothersome and possibly startling.
  • However, before the current flows in a shock situation, contact must be made, and in the process of establishing contact a small arc occurs. This causes a withdrawal reaction that, in some cases, may be a hazard if the involuntary nature of the reaction causes a fall or other accident.
  • The proposed 230 kV transmission lines will have the highest electric field within the right-of-way, approximately 0.2 to 1.5 kV per meter (kV/m), and approximately 0.1 to 0.9 kV/m at the edge of the right-of-way. These fields are less than many other 230 kV transmission lines due to the use of cross-phasing on the double-circuit lines and higher clearance above ground. Induced currents have been calculated for common objects for a set of worst-case theoretical assumptions: the object is perfectly insulated from ground, located in the highest field, and touched by a perfectly grounded person. Even though the maximum electric field only occurs on a small portion of the right-of-way, and perfect insulation and grounding states are not always common, the calculated induced current values are very low therefore, in most situations, even in the highest field location, induced currents are below the threshold of perception and are far below hazardous levels.
  • Agricultural operations can occur on or near a transmission line right-of-way. Irrigation systems often incorporate long runs of metallic pipes that can be subject to magnetic field induction when located parallel and close to transmission lines. Because the irrigation pipes contact moist soil, electric field induction is generally negligible, but annoying currents could still be experienced from magnetic field coupling to the pipe. Pipe runs laid at right angles to the transmission line will minimize magnetically induced currents, although such a layout may not always be feasible. If there are induction problems, they can be mitigated by grounding and/or insulating the pipe runs. Operation of irrigation systems beneath transmission lines presents another safety concern. If the system uses a high-pressure nozzle to project a stream of water, the water may make contact with the energized transmission line conductor. Generally, the water stream consists of solid and broken portions. If the solid stream contacts an energized conductor, an electric current could flow down the water stream to someone contacting the high-pressure nozzle. Transmission line contact by the broken-up part of the water stream is unlikely to present any hazard.

 (5)  Fuel Ignition

  •  If a vehicle were to be refueled under a high-voltage transmission line, a possible safety concern could be the potential for accidental fuel ignition. The source of fuel ignition could be a spark discharge into fuel vapors collected in the filling tube near the top of the gas tank.
  • The spark discharge would be due to current induced in a vehicle (insulated from ground) by the electric field of the transmission line and discharged to ground through a metallic refueling container held by a well-grounded person. Theoretical calculations show that if a number of unlikely conditions exist simultaneously, a spark could release enough energy to ignite gasoline vapors. This could not occur if a vehicle were simply driven or parked under a transmission line. Rather, several specific conditions would need to be satisfied: A large gasoline-powered vehicle would have to be parked in an electric field of about 5 kV/m or greater. A person would have to be refueling the vehicle while standing on damp earth and while the vehicle is on dry asphalt or gravel. The fuel vapors and air would have to mix in an optimum proportion. Finally, the pouring spout must be metallic. The chances of having all the conditions necessary for fuel ignition present at the same time are extremely small.
  • Very large vehicles (necessary to collect larger amounts of electric charge) are often diesel-powered, and diesel fuel is less volatile and more difficult to ignite. The proposed 230 kV transmission line electric field levels are too low (about 0.2-1.5 kV/m on the right-of-way) for the minimum energy necessary for fuel ignition under any practical circumstances.

 (6) Cardiac Pacemakers

  •  One area of concern related to the electric and magnetic fields of transmission lines has been the possibility of interference with cardiac pacemakers. There are two general types of pacemakers: asynchronous and synchronous. The asynchronous pacemaker pulses at a predetermined rate. It is practically immune to interference because it has no sensing circuitry and is not exceptionally complex. The synchronous pacemaker, on the other hand, pulses only when its sensing circuitry determines that pacing is necessary.
  • Interference resulting from the transmission line electric or magnetic field can cause a spurious signal in the pacemaker’s sensing circuitry. However, when these pacemakers detect a spurious signal, such as a 60 hertz (Hz) signal, they are programmed to revert to an asynchronous or fixed pacing mode of operation and return to synchronous operation within a specified time after the signal is no longer detected. The potential for pacer interference depends on the manufacturer, model, and implantation method, among other factors.
  • Studies have determined thresholds for interference of the most sensitive units to be about 2,000 to 12,000 milli gauss (mG) for magnetic fields and about 1.5 to 2.0 kV/m for electric fields. The electric and magnetic fields at the right-of-way edge are below these values, and on the right-of-way, only the lower bound electric field value of 1.5 kV/m is reached. Therefore, the potential impact would not be significant.

 (7) Computer Interference

  •  Personal computer monitors can be susceptible to 60 Hz magnetic field interference. Magnetic field interference results in disturbances to the image displayed on the monitor, often described as screen distortion, “jitter,” or other visual defects. In most cases it is annoying, and at its worst, it can prevent use of the monitor. Magnetic fields occur in the normal operation of the electric power system.
  • This type of interference is a recognized problem by the video monitor industry. As a result, there are manufacturers who specialize in monitor interference solutions and shielding equipment. Possible solutions to this problem include: relocation of the monitor, use of magnetic shield enclosures, software programs, and replacement of cathode ray tube monitors with liquid crystal displays that are not susceptible to 60 Hz magnetic field interference. Because these solutions are widely available to computer users, potential impacts would be less than  significant

CORONA RING:                                                     

  • The ring, which surrounds the energized end of the transformer bushing, serves two functions.
  • It is a corona ring that is intended to electrically shield the bushing terminal and connections. It does so by reducing the voltage gradient to a level well below the ionizing gradient of the surrounding air at the maximum transformer output voltage.
  • It’s also a grading ring, which helps electrically grade the external voltage on the bushing from line to ground (at the bushing flange). The bushing is likely a condenser bushing, which contains a capacitance-graded core to grade the voltage radically from 100% at the central conductor to ground at the flange and, axially from ground to the top and bottom ends of the core.
  • Grounding the test transformer following a circuit breaker test is necessary for safety but you are grounding the entire test circuit; not just the corona ring. I suspect the corona ring just happens to be a convenient attachment point for the hook on your ground stick.
  • Die cast are usually 380, sand and permanent mold 356 or A356, and fabricated rings are usually made from 6061 thin wall tubing or pipe that is formed and welded; with appropriate brackets and other mounting provisions.
  • Corona grading ring should be designed to reduce the critical dielectric voltage gradient (typ. 20 to 30 kVrms/cm) to prevent corona effect, internal discharge and reduce E-field in live parts and fitting that cause radio/ TV interference (RIV), audio noise and losses. Corona ring could also help to smooth the voltage profile distributing the voltage more uniform along the insulator preventing concentration of over stresses.
  • For porcelain post insulators, some manufacturer recommends one corona ring and for 500 kV and above two rings. However, for composite insulator the corona ring is recommended for 220/230 kV. Most equipment manufacturer provide corona ring base on testing such surge arrester, switches, CT’s/PTs, etc.

Difference between Arcing Horn Gap and Corona Ring:

  • At transmission line voltage the arcing horns, when the breaker is closed normally have nothing except corona from the tips and arc marks, the instant the breaker begins to open an arc is established across the gap between the arc horns, when the gap is long enough the arc breaks. The plan is to keep the sliding contacts from getting arc metal removal so the contacts maintain low resistance, arcing horns are sacrificial.
  • At switchgear voltage, there are arc chutes and usually puffers to extinguish the arc during breaker opening, the arc chutes may be of a sand-crystal cast material (like space shuttle heat tiles), asbestos layers, and electrical insulating board to protect the works during an explosive event when temperatures get hotter than the sun. There is specific NFPA training for arc flash exposure.
  • Arcing horns are also commonly used to protect insulation from impulse and other overvoltages. The horn gap (distance between arcing horns) is set to ensure that flashover occurs across the gap rather than along the insulation surface thereby protecting the insulation surface and preventing arc termination and associated damage to the end terminals or line and ground end hardware. They may also be used to connect a surge arrester to protect transformers and other equipment from overvoltage surges (gapped arrester). A gapped connection is one method of preventing line lockout in the event of arrester failure
  • Corona rings are meant to distribute the electrical field and neither the hardware protected or the corona ring should have corona, the typical line voltage that corona rings are applied is 150KV and higher, altitude or high temperatures can reduce the voltage to 138KV lines. Properly designed corona rings do not have corona.
  • Corona can appear to start and stop at essentially the same voltage, there are other variables. Corona produces light (from UV thru visible and into the infrared), sound (thru all wavelengths), ozone, and nitric acid (in the presence of moisture).
  • Arcing arrestors were used long ago, some of the old-old transmission lines. They were opposing arcing fingers mounted in parallel with the insulators; the gap determined the flash-over voltage. The intent was to protect insulators from lightening surges. I don’t know if those old lines are energized anymore. You don’t see arcing fingers on modern (post WWII war) transmission lines.
  • To break an arc the voltage must be decreased below about 60% of the voltage an arc starts at, thus if a transmission line insulator arcing arrestor flashes over and maintains an arc the line is going to be shutdown. Thus arcing arrestors (without an arc extinguishing capability) decrease the reliability of a transmission line.
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Electrical Thumb Rules-(Part-7)


Overhead Conductor /Cable Size:

Voltage

Overhead Conductor

Cable Size

33 KV ACSR-Panther/Wolf/Dog , AAAC 150,185,300,400,240 mm2 Cable
11 KV ACSR-Dog/Recon/Rabbit , AAAC 120, 150,185,300 mm2 Cable
LT ACSR-Dog/Recon/Rabbit , AAC,AAAC 95,120, 150,185,300 mm2 Cable

 Transmission / Distribution Line:

Span

Height of Tower

400KV=400 Meter

400KV=30Meter (Base 8.8 Meter)

220KV=350 Meter

220KV=23Meter (Base 5.2 Meter)

132KV=335 Meter

220KV Double Circuit=28 Meter

66KV=210 Meter

66KV=13Meter

Conductor Ampere

Voltage wise Conductor

Dog=300Amp

400KV=Moose ACSR=500MVA Load

Panther=514Amp

220KV=Zebra ACSR=200MVA Load

Zebra=720Amp

132KV=Panther ACSR=75MVA Load

Rabbit=208Amp

66KV=Dog ACSR=50MVA Load

Moose=218Amp

 

 Type of Tower:

Type

Used

Angle/Deviation

A

Suspension Tower Up to 2°

B

Small Angle Tower 2° to 15°

C

Medium Angle Tower 15° to 30°

D

Large Angle / Dead End Tower 30° to 60° & Dead End

 Tower Swing Angle Clearance (Metal Part to Live Part):

Swing Angle

Live Part to Metal Part Clearance (mm)

66KV

132KV

220KV

400KV

915mm

1530mm

2130mm

3050mm

15°

915mm

1530mm

2130mm

22°

3050mm

30°

760mm

1370mm

1830mm

44°

1860mm

44°

610mm

1220mm

1675mm

 Cable Coding (IS 1554) 😦 A2XFY / FRLS / FRPVC / FRLA / PILC)

A

Aluminium

2X

XLPE

F

Flat Armoured

W

Wire Armoured

Y

Outer PVC Insulation Sheath

W

Steel Round Wire

WW

Steel double round wire Armoured

YY

Steel double Strip Armoured

FR

Fire Retardation

LS

Low Smoke

LA

Low Acid Gas Emission

WA

Non Magnetic round wire Armoured

FA

Non Magnetic Flat wire Armoured

FF

Double Steel Round Wire Armoured

 Corona Ring Size:

Voltage

Size

<170 KV 160mm Ring put at HV end
>170 KV 350mm Ring put at HV end
>275 KV 450mm Ring put at HV end & 350 mm Ring put at Earth end

 Load as per Sq.Ft:

Type of Load

Load/Sq.Ft

Diversity Factor

Industrial

1000 Watt/Sq.Ft

0.5

Commercial

30 Watt/Sq.Ft

0.8

Domestic

15 Watt/Sq.Ft

0.4

Lighting

15 Watt/Sq.Ft

0.8

  Size of Ventilation Shaft:

Height of Building in meter Size of ventilation shaft in sq meter Minimum size of shaft in meter

9.0

1.5

1.0

12.5

3.0

1.2

15 and above

4.0

1.5

 

Total Losses in Power Distribution & Transmission Lines-Part 1


Introduction:

  • Power generated in power stations pass through large & complex networks like transformers, overhead lines, cables & other equipments and reaches at the end users. It is fact that the Unit of electric energy generated by Power Station does not match with the units distributed to the consumers. Some percentage of the units is lost in the Distribution network. This difference in the generated & distributed units is known as Transmission and Distribution loss.
  • Transmission and Distribution loss are the amounts that are not paid for by users.
  • T&D Losses= (Energy Input to feeder(Kwh)-Billed Energy to Consumer(Kwh)) / Energy Input kwh x100
  • Distribution Sector considered as the weakest link in the entire power sector. Transmission Losses is approximate 17% while Distribution Losses is approximate 50%.
  • There are two types of Transmission and Distribution Losses
  1. Technical Losses
  2. Non Technical Losses (Commercial Losses)

(1) Technical Losses:

  • The technical losses are due to energy dissipated in the conductors, equipment used for transmission Line, Transformer, sub- transmission Line and distribution Line and magnetic losses in transformers.
  • Technical losses are normally 22.5%, and directly depend on the network characteristics and the mode of operation.
  • The major amount of losses in a power system is in primary and secondary distribution lines. While transmission and sub-transmission lines account for only about 30% of the total losses. Therefore the primary and secondary distribution systems must be properly planned to ensure within limits.
  • The unexpected load increase was reflected in the increase of technical losses above the normal level
  • Losses are inherent to the distribution of electricity and cannot be eliminated.
  • There are two Type of Technical Losses.

 (a)   Permanent / Fixed Technical losses:

  • Fixed losses do not vary according to current. These losses take the form of heat and noise and occur as long as a transformer is energized.
  • Between 1/4 and 1/3 of technical losses on distribution networks are fixed losses. Fixed losses on a network can be influenced in the ways set out below.
  • Corona Losses.
  • Leakage Current Losses.
  • Dielectric Losses.
  • Open-circuit Losses.
  • Losses caused by continuous load of measuring elements
  • Losses caused by continuous load of control elements.

(b) Variable Technical losses

  • Variable losses vary with the amount of electricity distributed and are, more precisely, proportional to the square of the current. Consequently, a 1% increase in current leads to an increase in losses of more than 1%.
  • Between 2/3 and 3/4 of technical (or physical) losses on distribution networks are variable Losses.
  • By increasing the cross sectional area of lines and cables for a given load, losses will fall. This leads to a direct trade-off between cost of losses and cost of capital expenditure. It has been suggested that optimal average utilization rate on a distribution network that considers the cost of losses in its design could be as low as 30 per cent.
  • joule losses in lines in each voltage level
  • impedance losses
  • Losses caused by contact resistance.

 Main Reasons for Technical Losses:

(1) Lengthy Distribution lines:

  • In practically 11 KV and 415 volts lines, in rural areas are extended over long distances to feed loads scattered over large areas. Thus the primary and secondary distributions lines in rural areas are largely radial laid usually extend over long distances. This results in high line resistance and therefore high I2R losses in the line.
  • Haphazard growths of sub-transmission and distribution system in to new areas.
  • Large scale rural electrification through long 11kV and LT lines.

(2) Inadequate Size of Conductors of Distribution lines:

  • The size of the conductors should be selected on the basis of KVA x KM capacity of standard conductor for a required voltage regulation but rural loads are usually scattered and generally fed by radial feeders. The conductor size of these feeders should be adequate.

(3) Installation of Distribution transformers away from load centers:

  • Distribution Transformers are not located at Load center on the Secondary Distribution System.
  • In most of case Distribution Transformers are not located centrally with respect to consumers. Consequently, the farthest consumers obtain an extremity low voltage even though a good voltage levels maintained at the transformers secondary. This again leads to higher line losses. (The reason for the line losses increasing as a result of decreased voltage at the consumers end Therefore in order to reduce the voltage drop in the line to the farthest consumers, the distribution transformer should be located at the load center to keep voltage drop within permissible limits.

(4) Low Power Factor of Primary and secondary distribution system:

  • In most LT distribution circuits normally the Power Factor ranges from 0.65 to 0.75. A low Power Factor contributes towards high distribution losses.
  • For a given load, if the Power Factor is low, the current drawn in high  And  the losses proportional to square of the current will be more. Thus, line losses owing to the poor PF can be reduced by improving the Power Factor. This can be done by application of shunt capacitors.
  • Shunt capacitors can be connected either in secondary side (11 KV side) of the 33/11 KV power transformers or at various point of Distribution Line.
  • The optimum rating of capacitor banks for a distribution system is 2/3rd of the average KVAR requirement of that distribution system.
  • The vantage point is at 2/3rd the length of the main distributor from the transformer.
  • A more appropriate manner of improving this PF of the distribution system and thereby reduce the line losses is to connect capacitors across the terminals of the consumers having inductive loads.
  • By connecting the capacitors across individual loads, the line loss is reduced from 4 to 9% depending upon the extent of PF improvement.

(5) Bad Workmanship:

  • Bad Workmanship contributes significantly role towards increasing distribution losses.
  •  Joints are a source of power loss. Therefore the number of joints should be kept to a minimum. Proper jointing techniques should be used to ensure firm connections.
  • Connections to the transformer bushing-stem, drop out fuse, isolator, and LT switch etc. should be periodically inspected and proper pressure maintained to avoid sparking and heating of contacts.
  •  Replacement of deteriorated wires and services should also be made timely to avoid any cause of leaking and loss of power.

(6) Feeder Phase Current and Load Balancing:

  • One of the easiest loss savings of the distribution system is balancing current along three-phase circuits.
  • Feeder phase balancing also tends to balance voltage drop among phases giving three-phase customers less voltage unbalance. Amperage magnitude at the substation doesn’t guarantee load is balanced throughout the feeder length. Feeder phase unbalance may vary during the day and with different seasons. Feeders are usually considered “balanced” when phase current magnitudes are within 10.Similarly, balancing load among distribution feeders will also lower losses assuming similar conductor resistance. This may require installing additional switches between feeders to allow for appropriate load transfer.
  • Bifurcation of feeders according to Voltage regulation and Load.

(7)  Load Factor Effect on Losses:

  • Power consumption of Customer varies throughout the day and over seasons. Residential customers generally draw their highest power demand in the evening hours. Same commercial customer load generally peak in the early afternoon. Because current level (hence, load) is the primary driver in distribution power losses, keeping power consumption more level throughout the day will lower peak power loss and overall energy losses. Load variation is Called load factor and It varies from 0 to 1.
  • Load Factor=Average load in a specified time period / peak load during that time period.
  • For example, for 30 days month (720 hours) peak Load of the feeder is 10 MW. If the feeder supplied a total energy of 5,000 MWH, the load factor for that month is (5,000 MWh)/ (10MW x 720) =0.69.
  • Lower power and energy losses are reduced by raising the load factor, which, evens out feeder demand variation throughout the feeder.
  • The load factor has been increase by offering customers “time-of-use” rates. Companies use pricing power to influence consumers to shift electric-intensive activities during off-peak times (such as, electric water and space heating, air conditioning, irrigating, and pool filter pumping).
  • With financial incentives, some electric customers are also allowing utilities to interrupt large electric loads remotely through radio frequency or power line carrier during periods of peak use. Utilities can try to design in higher load factors by running the same feeders through residential and commercial areas

(8)  Transformer Sizing and Selection:

  • Distribution transformers use copper conductor windings to induce a magnetic field into a grain-oriented silicon steel core. Therefore, transformers have both load losses and no-load core losses.
  • Transformer copper losses vary with load based on the resistive power loss equation (P loss = I2R).
  • For some utilities, economic transformer loading means loading distribution transformers to capacity-or slightly above capacity for a short time-in an effort to minimize capital costs and still maintain long transformer life.
  • However, since peak generation is usually the most expensive, total cost of ownership (TCO) studies should take into account the cost of peak transformer losses. Increasing distribution transformer capacity during peak by one size will often result in lower total peak power dissipation-more so if it is over Loaded.
  • Transformer no-load excitation loss(iron loss) occurs from a changing magnetic field in the transformer core whenever it is energized. Core loss varies slightly with voltage but is essentially considered constant. Fixed iron loss depends on transformer core design and steel lamination molecular structure. Improved manufacturing of steel cores and introducing amorphous metals (such as metallic glass) have reduced core losses.

(9)  Balancing 3 phase loads

  • Balancing 3-phase loads periodically throughout a network can reduce losses significantly. It can be done relatively easily on overhead networks and consequently offers considerable scope for Cost effective loss reduction, given suitable incentives.

(10)   Switching off transformers

  • One method of reducing fixed losses is to switch off transformers in periods of low demand. If two transformers of a certain size are required at a substation during peak periods, only one might be required during times of low demand so that the other transformer might be switched off in order to reduce fixed losses.
  • This will produce some offsetting increase in variable losses and might affect security and quality of supply as well as the operational condition of the transformer itself. However, these trade-offs will not be explored and optimized unless the cost of losses are taken into account.

(11)    Other Reasons for Technical Losses:

  • Unequal load distribution among three phases in L.T system causing high neutral currents.
  • leaking and loss of power
  • Over loading of lines.
  • Abnormal operating conditions at which  power and distribution transformers are operated
  • Low voltages at consumer terminals causing higher drawl of currents by inductive loads.
  • Poor quality of equipment used in agricultural pumping in rural areas, cooler air-conditioners and industrial loads in urban areas.

Electrical Q&A Part-4


1)    What value AC meters show, is it the RMS or peak voltage?

  • AC voltmeters and ammeters show the RMS value of the voltage or current. DC meters also show the RMS value when connected to varying DC providing the DC is varying quickly, if the frequency is less than about 10Hz you will see the meter reading fluctuating instead.

2)    Why in the transmission tower construction Middle arm is longer than the upper and lower Arm.

  • Conductor of Upper Arm and Lower Arm will stay apart.
  • To prevent big birds (Ostriches etc) from bumping their heads against the conductor above when they sit on the wire below.
  • Designed to maintain the mechanical requirement to prevent arching between conductors while maintaining a tower height that is manageable, and of course preventing head injuries to birds
  • The arms are of different links to prevent a broken upper line from falling on one or more of the phase lines below.
  • The clearance from other phase.
  • Mutual inductance minimization.
  • Preventing droplet of water/ice to fall on bottom conductor.

3)    What is the difference between Surge Arrester & Lightning Arrestor

  • LA is installed outside and the effect of lightning is grounded, where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.
  • Transmission Line Lightning Protection:
  • The transmission line towers would normally be higher than a substation structure, unless you have a multi-storey structure at your substation.
  • Earth Mats are essential in all substation areas, along with driven earth electrodes (unless in a dry sandy desert site).
  • It is likewise normal to run catenaries’ (aerial earth conductors) for at least 1kM out from all substation structures. Those earth wires to be properly electrically to each supporting transmission tower, and bonded back to the substation earth system.
  • It is important to have the catenaries’ earth conductors above the power conductor lines, at a sufficient distance and position that a lightning strike will not hit the power conductors.
  • In some cases it is thus an advantage to have two catenary earth conductors, one each side of the transmission tower as they protect the power lines below in a better manner.
  • In lightning-prone areas it is often necessary to have catenary earthing along the full distance of the transmission line.
  • Without specifics, (and you could not presently give tower pictures in a Post because of a CR4 Server graphics upload problem), specifics would include:
  • Structure Lightning Protection:
  • At the Substation, it is normal to have vertical electrodes bonded to the structure, and projecting up from the highest points of the structure, with the location and number of those electrodes to be sufficient that if a lightning strike arrived, it would always be a vertical earthed electrode which would be struck, rather than any electrical equipment.
  • In some older outdoor substation structures, air-break isolator switches are often at a very high point in the structure, and in those cases small structure extension towers are installed, with electrodes at the tapered peak of those extension towers.
  • The extension towers are normally 600mm square approximately until the extension tower changes shape at the tapered peak, and in some cases project upwards from the general structure 2 to 6 metres, with the electrode some 2 to 3 metres projecting upwards from the top of the extension tower.
  • The substation normally has a Lightning Counter – which registers a strike on the structure or connected  to earth conductors, and the gathering of that information (Lightning Days, number per Day/Month/Year, Amperage of each strike)

4)    How Corona Discharge Effect Occur in Transmission Line?

  • In a power system transmission lines are used to carry the power. These transmission lines are separated by certain spacing which is large in comparison to their diameters.
  • In Extra High Voltage system (EHV system ) when potential difference is applied across the power conductors in transmission lines then air medium present between the phases of the power conductors acts as insulator medium however the air surrounding the conductor subjects to electro static stresses. When the potential increases still further then the atoms present around the conductor starts ionize. Then the ions produced in this process repel with each other and attracts towards the conductor at high velocity which intern produces other ions by collision.
  • The ionized air surrounding the conductor acts as a virtual conductor and increases the effective diameter of the power conductor. Further increase in the potential difference in the transmission lines then a faint luminous glow of violet color appears together along with hissing noise. This phenomenon is called virtual corona and followed by production of ozone gas which can be detected by the odor. Still further increase in the potential between the power conductors makes the insulating medium present between the power conductors to start conducting and reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium results in breakdown of the insulating medium and flash over is observed. All this above said phenomenon constitutes CORONA DISCHARGE EFFECT in electrical Transmission lines.

5)    Methods to reduce Corona Discharge Effect:

  • Critical Breakdown voltage can be increased by following factors
  • By increasing the spacing between the conductors:
  • Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phases results in heavier metal supports. Cost and Space requirement increases.
  • By increasing the diameter of the conductor:
  • Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved.
  • By using Bundled Conductors:
  • By using Bundled Conductors also corona effect can be reduced this is because bundled conductors will have much higher effective diameter compared to the normal conductors.
  • By Using Corona Rings or Grading Rings:
  • This is of having no greater significance but i presented here to understand the Corona Ring in the Power system. Corona Rings or Grading Rings are present on the surge arresters to equally distribute the potential along the Surge Arresters or Lightning Arresters which are present near the Substation and in the Transmission lines.

6)    How to test insulators?

  • Always remember to practice safety procedures for the flash-over voltage distance and use a sturdy enclosure to contain an insulator that may shatter, due to steam build-up from moisture in a cavity, arcing produces intense heat, an AM radio is a good RFI/arcing detection device, a bucket truck AC dielectric test set (130KV) is a good test set for most pin and cap type insulators. A recent article said the DC voltage required to “search out defects can be 1.9 times the AC voltage.
  • Insulators have a normal operating voltage and a flash-over voltage. Insulators can have internal flash-over that are/are not present at normal operating voltage. If the RFI is present, de-energize the insulator (line) and if the RFI goes away, suspect the insulator (line). Then there can be insulators that have arcing start when capacitor or other transients happen, stop when the line is de-energized or dropped below 50% of arc ignition voltage. Using a meg-ohm-meter can eliminate defective insulators that will immediately arc-over tripping the test set current overload.

7)    How to identify the starting and ending leads of winding in a motor which is having 6 leads in the      terminal box

  • If it is a single speed motor then we have to identify 6 leads.
  • Use IR tester to identify 3 windings and their 6 leads. Then connect any two leads of two winding and apply small voltage across it and measure the current.
  • Then again connect alternate windings of same two windings and apply small amount of voltage (same as before) and measure current.
  • Check in which mode you get the max current and then mark it as a1-a2 & b1-b2. You get max current when a2-b1 will be connected and voltage applied between a1-b2.
  • Follow the same process to identify a1-a2, b1-b2, c1-c2.now we will be able to connect it in delta or star.

8)    How to measure Transformer Impedance?

  • Follow the steps below:
  • (1) Short the secondary side of the transformer with current measuring devices (Ammeter)
  • (2) Apply low voltage in primary side and increase the voltage so that the secondary current is the rated secondary current of the transformer. Measure the primary voltage (V1).
  • (3) Divide the V1 by the rated primary voltage of the transformer and multiply by 100. This value is the percentage impedance of the transformer.
  • When we divide the primary voltage V1 with the full load voltage we will get the short circuit impedance of the transformer with refereed to primary or Z01. For getting the percentage impedance we need to use the formula = Z01*Transformer MVA /(Square of Primary line voltage).

9)    Why Bus Couplers are normally 4-Pole. Or When Neutral Isolation is required?

  • Neutral Isolation is mandatory when you have a Mains Supply Source and a Stand-by Power Supply Source. This is necessary because if you do not have neutral isolation and the neutrals of both the sources are linked, then when only one source is feeding and the other source is OFF, during an earth fault, the potential of the OFF Source’s Neutral with respect to earth will increase, which might harm any maintenance personnel working on the OFF source. It is for this reason that PCC Incomers & Bus Couplers are normally 4-Pole. (Note that only either the incomer or the bus coupler needs to be 4-pole and not both).
  • 3pole or 4pole switches are used in changing over two independant sources ,where the neutral of one source and the neutral of another source should not mix the examples are electricity board power supply and standalone generator supply etc. the neutral return current from one source should not mix with or return to another source. as a mandatory point the neutral of any transformer etc are to be earthed, similarly the neutral of a generator also has to be earthed. While paralling (under uncontrolled condition) the neutral current between the 2 sources will crises cross and create tripping of anyone source breakers.
  • also as per IEC standard the neutral of a distribution system shall not be earthed more than once. means earthing the neutral further downstream is not correct,

10) Why Three No’s of Current transformer in 3 phase Star point is grounded.

  • For CT’s either you use for 3 phase or 2 phase or even if you use only 1 CT’s for the Over current Protection or for the Earth Faults Protection, their neutral point is always shorted to earth. This is NOT as what you explain as above but actually it is for the safety of the CT’s when the current is passing thru the CT’s.
  • In generally, tripping of Earth faults and Over current Protection has nothing to do with the earthing the neutral of the CT’s. Even these CT’s are not Grounded or Earthed, these Over current and the Earth Faults Protection Relay still can operated.
  • Operating of the Over current Protection and the Earth Faults Relays are by the Kirchhoff Law Principle where the total current flowing into the points is equal to the total of current flowing out from the point.
  • Therefore, for the earth faults protection relays operating, it is that, if the total current flowing in to the CT’s is NOT equal total current flowing back out of the CT’s then with the differences of the leakage current, the Earth Faults Relays will operated.

11) What is tertiary winding of Transformer?

  • Providing a tertiary winding for a transformer may be a costly affair. However, there are certain constraints in a system which calls for a tertiary transformer winding especially in the case of considerable harmonic levels in the distribution system. Following is an excerpt from the book “The J&P Transformer Book”.
  • Tertiary winding is may be used for any of the following purposes:
  • (A)To limit the fault level on the LV system by subdividing the infeed that is, double secondary transformers.
  • (B)The interconnection of several power systems operating at different supply voltages.
  • (C) The regulation of system voltage and of reactive power by means of a synchronous capacitor connected to the terminals of one winding.
  • It is desirable that a three-phase transformer should have one set of three-phase windings connected in delta thus providing a low-impedance path for third-harmonic currents. The presence of a delta connected winding also allows current to circulate around the delta in the event of unbalance in the loading between phases, so that this unbalance is reduced and not so greatly fed back through the system.
  • Since the third-order harmonic components in each phase of a three-phase system are in phase, there can be no third-order harmonic voltages between lines. The third-order harmonic component of the magnetising current must thus flow through the neutral of a star-connected winding, where the neutral of the supply and the star-connected winding are both earthed, or around any delta-connected winding. If there is no delta winding on a star/star transformer, or the neutral of the transformer and the supply are not both connected to earth, then line to earth capacitance currents in the supply system lines can supply the necessary harmonic component. If the harmonics cannot flow in any of these paths then the output voltage will contain the harmonic distortion.
  • Even if the neutral of the supply and the star-connected winding are both earthed, then although the transformer output waveform will be undistorted, the circulating third-order harmonic currents flowing in the neutral can cause interference with telecommunications circuits and other electronic equipment as well as unacceptable heating in any liquid neutral earthing resistors, so this provides an added reason for the use of a delta connected tertiary winding.
  • If the neutral of the star-connected winding is unearthed then, without the use of a delta tertiary, this neutral point can oscillate above and below earth at a voltage equal in magnitude to the third-order harmonic component. Because the use of a delta tertiary prevents this it is sometimes referred to as a stabilizing winding.
  • When specifying a transformer which is to have a tertiary the intending purchaser should ideally provide sufficient information to enable the transformer designer to determine the worst possible external fault currents that may flow in service. This information (which should include the system characteristics and details of the earthing arrangements) together with a knowledge of the impedance values between the various windings, will permit an accurate assessment to be made of the fault currents and of the magnitude of currents that will flow in the tertiary winding. This is far preferable to the purchaser arbitrarily specifying a rating of, say, 33.3%, of that of the main windings.

12) Why do transformers hum?

  • Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon is scientifically referred to as magnetostriction.
  • A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization. The magnetization of any given point on the sheet varies, so the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to reduce losses and moderate the ensuing heating effect.
  • The extensions and contractions are taking place erratically all over a sheet and each sheet is behaving erratically with respect to its neighbour, so you can see what a moving, writhing construction it is when excited. These extensions are miniscule proportionally and therefore not normally visible to the naked eye. However, they are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of magnetostriction and hence, the noise level Why not reduce the noise in the core by reducing the amount of flux? Transformer voltages are fixed by system requirements. The ratio of these voltages to the number of turns in the winding determines the amount of magnetization. This ratio of voltage to turns is determined mainly for economical soundness. Therefore the amount of flux at the normal voltage is fixed. This also fixes the level of noise and vibration. Also, increasing (or decreasing) magnetization does not affect the magnetostriction equivalently. In technical terms the relationship is not linear.

13) How can we reduce airborne noise?

  • Put the transformer in a room in which the walls and floor are massive enough to reduce the noise to a person listening on the other side. Noise is usually reduced (attenuated) as it tries to pass through a massive wall. Walls can be of brick, steel, concrete, lead, or most other dense building materials.
  • Put the object inside an enclosure which uses a limp wall technique. This is a method which uses two thin plates separated by viscous (rubbery) material. As the noise hits the inner sheet some of its energy is used up inside the viscous material. The outer sheet should not vibrate.
  • Build a screen wall around the unit. This is cheaper than a full room. It will reduce the noise to those near the wall, but the noise will get over the screen and fall elsewhere (at a lower level). Screens have been made from wood, concrete, brick and with dense bushes (although the latter becomes psychological)
  • Do not make any reflecting surface coincident with half the wave length of the frequency. What does this mean? Well, every frequency has a wave length. To find the wave length in air, for instance, you divide the speed of sound, in air (generally understood as 1130 feet per second) by the frequency. If a noise hits a reflecting surface at these dimensions it will produce what is called a standing wave. Standing waves will cause reverberations (echoes) and an increase in the sound level. If you hit these dimensions and get echoes you should apply absorbent materials to the offending walls (fibreglass, wool, etc.)

14) What is polarity, when associated with a transformer?

  • Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity. This leaves one high voltage and one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings. This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.

15) What is exciting current?

  • Exciting current is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and less to approximately 2% on larger sizes of 750 KVA.

16) Can a three phase transformer be loaded as a single phase transformer?

  • Yes, but the load cannot exceed the rating per phase and the load must be balanced. (KVA/3 per phase)
  • For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each secondary. If you need a 30 kVA load, 10 kVA of load should be supplied from each secondary.

17) What are taps and when are they used?

  • Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals.
  • Standard tap arrangements are at two-and-one-half and five percent of the rated primary voltage for both high and low voltage conditions.
  • For example, if the transformer has a 480 volt primary and the available line voltage is running at 504 volts, the primary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at the proper rating.

18) What is the difference between “Insulating,” “Isolating,”and“Shielded Winding” transformers?

  • Insulating and isolating transformers are identical. These terms are used to describe the isolation of the primary and secondary windings, or insulation between the two.
  •  A shielded transformer is designed with a metallic shield between the primary and secondary windings to attenuate transient noise.
  • This is especially important in critical applications such as computers, process controllers and many other microprocessor controlled devices.
  •  All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers, whose primary and secondary are connected to each other electrically, are not of the insulating or isolating variety.

19) Can transformers be operated at voltages other than nameplate voltages?

  • In some cases, transformers can be operated at voltages below the nameplate rated voltage.
  •  In NO case should a transformer be operated at a voltage in excess of its nameplate rating, unless taps are provided for this purpose. When operating below the rated voltage, the KVA capacity is reduced correspondingly.
  • For example, if a 480 volt primary transformer with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts. If the transformer was originally rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage.

20) Can a Single Phase Transformer be used on a Three Phase source?

  • Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire. The transformer output will be single phase.

21) Can Transformers develop Three Phase power from a Single Phase source?

  • No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phases.

22) Can Single Phase Transformers be used for Three Phase applications?

  • Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock.
  • Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer. For example: Three 10 KVA single phase transformers will accommodate a 30 KVA three phase load

23) Difference between Restricted Earth Fault & Unrestricted Earth Fault protections?

  • Restricted earth fault is normally given to on star connected end of power equipment like generators, transformers etc. mostly on low voltage side. For REF protection 4 no’s CTs are using one each on phase and one in neutral. It is working on the principle of balanced currents between phases and neutral. Unrestricted E/F protection working on the principle of comparing the unbalance on the phases only. For REF protection PX class CT are using but for UREF 5P20 Cts using.
  • For Differential Protection CTs using on both side HT & LV side each phase, and comparing the unbalance current for this protection also PX class CTs are using.

24) Can transformers be operated at voltages other than nameplate voltages?

  • In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated in excess of its nameplate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly.

25) How many types of cooling system it transformers?

  • ONAN (oil natural,air natural)
  • ONAF (oil natural,air forced)
  • OFAF (oil forced,air forced)
  • ODWF (oil direct,water forced)
  • OFAN (oil forced,air natural)

26) What is the function of anti-pumping in circuit breaker?

  • when breaker is close at one time by close push button, the anti pumping contactor prevent re close the breaker by close push button after if it already close.

27) There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum?

  • The motor has max load current compare to that of transformer because the motor consumes real power.. and the transformer is only producing the working flux and it’s not consuming. Hence the load current in the transformer is because of core loss so it is minimum.

28) Where the lighting arrestor should be placed in distribution lines?

  • Near distribution transformers and out going feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations.

29) Why Delta Star Transformers are used for Lighting Loads?

  • For lighting loads, neutral conductor is must and hence the secondary must be star winding. and this lighting load is always unbalanced in all three phases.
  • To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.

30) NGR grounded system vs. solidly grounded system

  • In India, at low voltage level (433V) we must do only Solid Earthing of the system neutral. This is by IE Rules 1956, Rule No. 61 (1) (a).Because, if we have opt for impedance earthing, during an earth fault, there will be appreciable voltage present between the faulted body & the neutral, the magnitude of this voltage being determined by the fault current magnitude and the impedance value.
  • This voltage might circulate enough current in a person accidentally coming in contact with the faulted equipment, as to harm his even causing death. Note that, LV systems can be handled by non-technical persons too.
  • In solid earthing, you do not have this problem, as at the instant of an earth fault, the faulted phase goes to neutral potential and the high fault current would invariably cause the Over current or short circuit protection device to operate in sufficiently quick time before any harm could be done.

31) Why Do not We Break Neutral in AC Circuits?

  • Neutral is connected to earth at some point, thus it has some value as a return path in the event of say and equipment earth being faulty. It’s a bit like asking ‘why don’t we break the Earth connection’
  • It was stupid and dangerous, as it was possible for the neutral fuse to blow; giving the appearance of ‘no power’ when in fact the equipment was still live.

32) What is Minimum Value of Insulation Resistance / Polarization Index?

  • Motor Insulation Resistance:
  • The acceptable meg-ohm value = motor KV rating value + 1 (For LV and MV Motor).
  • Example, for a 5 KV motor, the minimum phase to ground (motor body) insulation is 5 + 1 = 6 meg-ohm.
  • Panel Bus Insulation Resistance:
  • The acceptable meg-ohm value = 2 x KV rating of the panel.
  • Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 meg-ohm
  • IEEE 43 – INSULATION RESISTANCE AND POLARIZATION INDEX (min IR at 400C in MΩ)
Minimum Insulation Resistance TEST SPECIMEN
R1 min = kV+1 R1 min = 100 For most windings made before about 1970, all field windings, and others not described below For most dc armature and ac windings built after about 1970 (form wound coils)
R1 min = 5 For most machines with random -wound stator coils and form-wound coils rated below 1kV

33) What is service factor?

  • Service factor is the load that may be applied to a motor without exceeding allowed ratings. For example, if a 10-hp motor has a 1.25 service factor; it will successfully deliver 12.5 hp (10 x 1.25) without exceeding specified temperature rise. Note that when being driven above its rated load in this manner, the motor must be supplied with rated voltage and frequency.
  • Keep in mind, however, that a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the 10-hp motor is operated continuously at 12.5 hp, its insulation life could be decreased by as much as two-thirds of normal. If you need a 12.5-hp motor, buy one; service factor should only be used for short-term overload conditions.

34) Calculate the size the CT on the neutral point of the secondary side of 11/0.415 kV Transformer

  • For high impedance relays (differential or restricted earth fault relays), ‘Class X’ current transformers are recommended to be used.
  • Please note that both CTs (neutral & phase) shall have the same characteristics. The following is an example to size the CT:
  • Input data: 11/0.415 kV ,2500 KVA Power transformer ,Transformer impedance is 6% ,Length of cable from neutral CT to the relay is 200 m ,Cross section of CT cable to be used is 6 mm² -copper and resistance is 0.0032 Ω/m
  • Step  1: Calculation of CT Rated Primary Current
  • I = kVA/ (0.415×1.732) = 2500/ (0.415×1.732) = 3478.11 A, CT with primary current of 4000 A to be selected.
  • Select the secondary current of the CT 1 or 5 A. selecting 1 A secondary current, as the cross section and length of pilot wires can have a significant effect on the required knee voltage of the CT and therefore the size and cost of the CT. When the relay is located some distance from the CT, the burden is increased by the resistance of the pilot wires.
  • Step 2: Calculation of maximum Fault Current
  • Ift = kVA/ (0.415×1.732x Z)
  • Ift = 2500/ (0.415×1.732×0.06) = 57968.59 A (say 58000 A)
  • Step 3: Calculation of the Knee Voltage of the CT (Vkp)
  • Vkp = (2x Iftx (Rct+Rw)/CT transformation ratio)
  • Where: Rct  is the CT resistance (to be given by the manufacturer), Here Rct is1.02 Ω. 
  •  Rw: total CT cable resistance= 2x cable length (200 m) x wire resistance= 2x200x0.0032= 1.28 Ω
  • CT transformation ratio = CT Primary Current/CT Secondary Current
  • CT transformation ratio = 4000/5= 800 A, for CT with 5 A secondary current; or,
  • CT transformation ratio = 4000/1= 4000 A, for CT with 1 A secondary current. We will use 1 A in this example.
  • Vkp = (2x58000x (1.02+1.28)/4000)= 66.7 V.
  • The Vkp of the CT should be higher than the setting of relay stability voltage (Vs), to ensure stability of the protection during maximum Through fault current.
  • To calculate the stability voltage,we should follow the related formula given by the relay manufacturer, as each relay manufacturer has its own formula.
  • we may calculate the Vkp as above using a CT with secondary current of 5 A, and you will notice the difference in the Vkp.

35) When should we use Molded Case Circuit Breakers and Mini Circuit Breakers?

  • MCB is Miniature Circuit Breaker, since it is miniature it has limitation for Short Circuit Current and Amp Rating MCB:
  • MCB are available as Singe module and used for :-
  • Number of Pole :- 1,2,3,4 – 1+ N , & 3 + N
  • Usually Current range for A.C. 50-60 HZ, is from 0.5 Amp – 63 Amp. Also available 80A, 100A, and 125 Amp.
  • SC are limited 10 KA
  • Applications are as: – Industrial, Commercial and Residential application.
  • Tripping Curve:
  • (1) B Resistive and lighting load,
  • (2) C Motor Load,
  • (3) D Highly inductive load.
  • MCCB:
  • MCCB: – Moulded Case Circuit Breaker.
  • MCCB are available as Singe module and used for:
  • Number of Pole :- 3 pole , & 4 Pole
  • Current range for A.C:
  • For 3.2 /6.3/12.5/25/50/100/125/160 Amp and Short Circuit Capacity 25/35/65 KA.
  • For  200 250 Amp and Short Circuit Capacity 25/35/65 KA
  • For 400 630/800 Amp and Short Circuit Capacity 50 KA
  • Protection release :
  • Static Trip :- Continuous adjustable overload protection range 50 to 100 % of the rated current Earth fault protection can be add on with adjustable earth fault pick up setting 15 to 80 % of the current.
  • Micro processor Based release:
  • Over load rated current 0.4 to1.0 in steps of o.1 of in trip time at 600 % Ir (sec) 0.2.0.5,1, 1.5 , 2 ,3
  • Short Circuit :-2 to10 in steps of 1 lr , short time delay (sec) 0.02.0.05,0.1, 0.2 ,0.3
  • Instantaneous pick up :2 to10 in steps of 1 in Ground fault pick up Disable: 0.2 to 0.8 in steps of 0.1 of in Ground fault delay (sec): 0.1 to 0.4 in steps of 0.1
  • MCB (Miniature Circuit Breaker) Trip characteristics normally not adjustable, factory set but in case of MCCB (Moulded Case Circuit Breaker) Trip current field adjustable.

 

Sub Station Abstracts


Sub Station Equipments and Safety Clearance

Rating of Lighting Arrestor:

  • Size of L.A = 1.5 X Phase to Earth Voltage    OR   
  • Size of L.A =1.5 X system Voltage/1.732       OR
  • Size of L.A = 0.81 X highest System Voltage

Lighting Arrestor Protection Radius:

  • Protection Radius (Rp) = Sqrt (H X (2D-H)+L(2D+L))
  • H= Actual Height of L.A
  • D= 20 meter, 40 meter or 60 meter
  • L= V X T (T=Discharge Time & V= 1m/ms)

Creepage Distance:

  • 18 mm to 22 mm /KV for Moderate Polluted Air.
  • 25 mm to 33 mm /KV for Heavily Polluted Air.
  • In HVDC System The value is double from above value.

Lighting Arrestor Rating:

Rated Voltage

Highest Voltage

L.A Rating

132kv

145kv

120kv To 132kv

220kv

245kv

198kv To 216kv

400kv

420kv

336kv

Location of Lighting Arrestor:

Rated Voltage

Max Distance from Equipment

132kv

35 meter To 45 meter

220kv

Closed To Transformer

400kv

Closed To Transformer

 Size of Corona Ring:

Rated Voltage

Size of Corona Ring

Less than 170 KV

160mm Ring Put on HV end

170KV To 275KV

350mm Ring Put on HV end

More than 275KV

450mm Ring Put on HV end

More than 275KV

350mm Ring Put on HV end

 Capacity of Sub Station as per GERC:

Size of S/S

Electrical Load

66 KV

80 MVA

132 KV

150 MVA

220 KV

320 MVA

400 KV

1000 MVA

 Breaking / Short Circuit Capacity of Sub Station:

Size of S/S

Short Circuit Current

66 KV

25 KAmp  for 1 or 3 sec

132 KV

31.5 KAmp  for 1 or 3 sec

220 KV

40 KAmp  for 1 or 3 sec

400 KV

40 KAmp  for 1 or 3 sec

 Fault Clear Time:               

                Size of S/S

Fault Clear Time

66 KV

300 mili Sec

132 KV

160 mili Sec

220 KV

120 mili Sec

400 KV

100 mili Sec

 Normal Type of Conductors:

Voltage

Main Bus

Auxiliary Bus

11KV

Twin ACSR Zebra

ACSR Zebra

33KV

ACSR Zebra

ACSR Zebra

132KV

ACSR Zebra

ACSR Panthers

220KV

Twin ACSR Zebra

ACSR Zebra

400KV

1/14.2 mm Dia Alu Pipe

Twin ACSR Moose

 Number of Disc Insulator:

System

Number

Strength (KN)

11KV

4

120KN

33KV

4

120KN

132KV

10

120KN

220KV

14

70KN

220KV (Anti  Fog)

2 X 15

120KN

400KV (Anti  Fog)

2 X 25

120KN

 Minimum Clearance:

Voltage

Phase to Earth Wire

Phase to Phase

Section Clearance

2.2KV

28cm

33cm

27.45cm

33KV

380cm

43cm

27.7cm

132KV

107cm

12cm

25cm

220KV

178cm

20.6cm

42.8cm

400KV

350cm

40cm

65cm

 Ground Clearance:

                Voltage

Meter

33KV

3.7meter

66 KV

6.1meter

132 KV

6.1meter

220 KV

7.0meter

400 KV

8.8meter

 Conductor Spacing:

Voltage

Highest Voltage

Lighting Impulse Level (Kvp)

Min Clearance

Ground Clearance

Safety working Clearance

Phase to Earth

Phase to Phase

11KV

12kv

70

178mm

229mm

3700mm

2600mm

33KV

36kv

170

320mm

320mm

3700mm

2800mm

132 KV

145kv

550

1300mm

1300mm

4600mm

3700mm

220 KV

245kv

950

2100mm

2100mm

5500mm

4300mm

400 KV

420kv

1425

3400mm

4200mm

8000mm

6400mm

  Earthing Resistance Value:

Particular

Max Earthing Resistance

Power Station

0.5Ω

EHT Sub Station

33KV Sub Station

Double Pole Structure

Tower foot Resistance

10Ω

Distribution Transformer

220KV Sub Station

1Ω To 2Ω

400KV Sub Station

0.5Ω

 Losses in 11 kv Transformer at 75c (As per CBIP):

Transformer

No Load Loss(kw)

Load Loss (kw)

% Impedance

3.15MVA

2.9

20

6.25

4MVA

3.2

27

7.15

5.3MVA

3.9

31

7.15

6.3MVA

4.5

37

7.15

 Losses in 66 kv Transformer at 75c (As per CBIP):

Transformer

No Load Loss(kw)

Load Loss (kw)

% Impedance

6.3MVA

6

40

8.35

8MVA

7.1

48

8.35

10MVA

8.4

57

8.35

12.5MVA

9.7

70

8.35

20MVA

13

102

10.0

 Standard Rating of 66KV Transformer (As per CBIP):

Transformer

KV

Type of Cooling

6.3MVA

66KV/11KV

ONAN

8MVA

66KV/11KV

ONAN

10MVA

66KV/11KV

ONAN

12.5MVA

66KV/11KV

ONAN / ONAF

20MVA

66KV/11KV

ONAN / ONAF

 % Impedance for Transformer (As per IS 2026):

33KV Transformer

66KV Transformer

MVA

%Impedance

MVA

%Impedance

1MVA

5%

6.3MVA

8.35%

1.6MVA

6.25%

8MVA

8.35%

3.15MVA

6.25%

10MVA

8.35%

4MVA

7.15%

12.5MVA

8.35%

5MVA

7.15%

20MVA

10%

6.3MVA

7.15%

16MVA

10%

8MVA

8.35%

25MVA

10%

10MVA

8.35%

31.5MVA

12.5%

MVA

%impedance

Less Than 1MVA

5%

1MVA To 2.5MVA

6%

2.5MVA To 5MVA

7%

5MVA To 7MVA

8%

7MVA To 12MVA

9%

12MVA To 30MVA

10%

More Than 30MVA

12.5%

  Bus Bar Materials:

Description Bus Bar and Jumper Material
400 kV Main Bus 114.2 mm dia. Aluminium pipe
400 kV equipment interconnection 114.2 mm dia. Aluminium pipe
400 kV overhead bus & droppers in all bays. Twin ACSR Moose
220 kV Main Bus Quadruple / Twin ACSR Zebra / Twin AAC Tarantulla
220 kV Auxiliary Bus ACSR Zebra
220 kV equipment interconnection Twin ACSR Zebra / Single ACSR Zebra
220 kV overhead bus & droppers in all bays. Twin ACSR Zebra / Single ACSR Zebra
132 kV Main Bus ACSR Zebra
132 kV Auxiliary Bus ACSR Panther
132 kV equipment interconnection ACSR Zebra / ACSR Panther
132 kV overhead bus & droppers in all bays. ACSR Panther
33 kV Main Bus ACSR Zebra
33 kV Auxiliary Bus ACSR Zebra
33 kV equipment interconnection, overhead bus and droppers:  
(i) Bus coupler & transformer bay ACSR Zebra
(ii) Feeder bay. ACSR Panther
11 kV Main Bus Twin ACSR Zebra
11 kV Auxiliary Bus ACSR Zebra
11 kV equipment interconnection, overhead bus and droppers:  
(i) Transformer bay Twin ACSR Zebra / Single ACSR Zebra
(ii) Bus coupler ACSR Zebra
   

Electrical Notes


 

Notes on Electrical Engineering:

All Notes ,Calculations & Abstracts are Based on Some Electrical References. All References are mention at end of each Notes.

BUY All Electrical Notes & Calculation Sheets in PDF Format (US$)

BUY All Electrical Notes & Calculation Sheets in PDF Format (Indian Rs)

Electrical Calculation Sheets:

  1. Calculation of Cable Tray Size
  2. Calculation Short Circuit Current (Base KVA Method)
  3. Calculate Size of Bus bar & Panel Design
  4. Calculate Street Light Pole’s Distance / Fixture Watt / Lighting Area
  5. Calculate No of Street Light Poles
  6. Calculate No of Lighting Fixtures / Lumen for Indoor Lighting
  7. Calculate Size of Capacitor Bank / Annual Saving & Payback Period
  8. Calculate Technical Losses of Transmission / Distribution Line
  9. Calculate Cable Size and Voltage Drop
  10. Calculate IDMT over Current Relay Setting (50/51)
  11. Calculate TC Size & Voltage Drop due to starting of Large Motor
  12. Calculate Size of Contactor, Fuse, C.B, Over Load Relay of DOL Starter
  13. Calculate Voltage Regulation of Distribution Line
  14. Calculate Numbers of Plate/Pipe/Strip Earthings (Part-3)
  15. Calculate Numbers of Plate/Pipe/Strip Earthings (Part-2)
  16. Calculate Numbers of Plate/Pipe/Strip Earthings (Part-1)
  17. Calculate Lightening Protection for Building / Structure
  18. Calculate Voltage drop for Street Light Poles
  19. Calculate Size of Solar Panel
  20. Calculate Cable Trunking Size
  21. Calculate Conduit Size for Wire / Cables
  22. Calculate Size of Diesel Generator Set
  23. Calculate Size of Inverter / Battery Bank
  24. Calculate Size of Main ELCB/ Branch MCB of Distribution Box

Electrical Notes:

  1. Setting of overload, Short circuit & Ground Fault Protection of MCCB (PART-1)
  2. Type of Tripping Mechanism of MCB / MCCB-(Part-2)
  3. Type of Tripping Mechanism of MCB / MCCB-(Part-1)
  4. Type of Light Bulb base & Socket:Part-2
  5. Type of Light Bulb base & Socket:Part-1
  6. How to select MCB / MCCB (Part:3)
  7. How to select MCB / MCCB (Part:2)
  8. How to select MCB / MCCB (Part:1)
  9. What should you know before buying LED Bulbs (Part:3)
  10. What should you know before buying LED Bulbs (Part:2)
  11. What should You know before buying LED Bulb-Part:1
  12. Cable Construction & Cable Selection-Part:4
  13. Cable Construction & Cable Selection-Part:3
  14. Cable Construction & Cable Selection-Part:2
  15. Cable Construction & Cable Selection-Part:1
  16. Electrical Thumb Rule-13
  17. Selection of Surge Protective Device (SPD)- (Part 4)
  18. Selection of Surge Protective Device (SPD)- (Part 3)
  19. Selection of Surge Protective Device (SPD)- (Part 2)
  20. Selection of Surge Protective Device (SPD)- (Part 1) 
  21. Various Routine Test of Power Transformer-(Part-4)
  22. Various Routine Test of Power Transformer-(Part-3)
  23. Various Routine Test of Power Transformer-(Part-2)
  24. Various Routine Test of Power Transformer-(Part-1)
  25. Selection for Street Light Luminar-(PART-5)
  26. Selection for Street Light Luminar-(PART-4)
  27. Selection for Street Light Luminar-(PART-3)
  28. Selection for Street Light Luminar-(PART-2)
  29. Selection for Street Light Luminar-(PART-1)
  30. Cable Tray Size as per National Electrical Code-2002. Article 392
  31. Motor Name Plate Terminology
  32. Electrical Thumb Rule-(Part-11)
  33. Electrical Thumbs Rules (Part-10)
  34. Electrical Thumbs Rules (Part-9)
  35. Electrical Thumb Rules-(Part-8)
  36. Electrical Thumb Rules-(Part-7)
  37. Electrical Thumb Rules-(Part-6)
  38. Electrical Thumb Rules-(Part-5)
  39. Electrical Thumb Rules-(Part-4)
  40. Electrical Thumb Rules-(Part-3)
  41. Electrical Thumb Rules (Part-2)
  42. Electrical Thumb Rules-(Part 1).
  43.  IP Rating for Electrical Enclosure
  44. Pirating of Technical Works.
  45. Selection of 3P-TPN-4P MCB & Distribution Board
  46. Total Losses in Power Distribution and Transmission Lines-(Part 2)
  47. Total Losses in Power Distribution & Transmission Lines-(Part 1)
  48. Size and Location of Capacitor in Electrical System-(Part 2)
  49. Size and Location of Capacitor in Electrical System-(Part1)
  50. Difference between Unearthed Cable & Earthed Cables
  51. Over Current Relay(Type-Application-Connection)
  52. Types and Revolution of Electrical Relays
  53. Abstract of over current Protection of Transformer (NEC 450.3)
  54. Difference between Bonding, Grounding and Earthing
  55. Safety Clearance for Transformer
  56. Safety Clearance for Electrical Panel
  57. Electrical Safety Clearance (Part-6)
  58. Electrical Safety Clearance (Part-5)
  59. Electrical Safety Clearance (Part-4)
  60. Electrical Safety Clearance (Part-3)
  61. Electrical Safety Clearance (Part-2)
  62. Electrical Safety Clearance (Part-1)
  63. Impact of Floating Neutral in Power Distribution
  64. Parallel Operation of Transformer
  65. Vector Group of Transformer
  66. Auto Transformer Connection
  67. Scott-T Connection of Transformer
  68. Zig-zag Connection of Transformer
  69. Star-Delta Connection of Transformer
  70. Delta-Star Connection of Transformer
  71. Delta-Delta Connection of Transformer
  72. Star-Star Connection of Transformer
  73. Insulation Resistance (IR) Values of Electrical Equipments
  74. Star-Delta Starter
  75. Direct On Line Starter
  76. Effects of High Voltage Transmission Lines on Humans and Plants
  77. Analysis the Truth behind Household Power Savers
  78. Types of Neutral Earthing in Power Distribution Systems
  79. EHV/HV Cable Sheath Earthing
  80. Abstract of NEC:430 for Size of Cable for Single or Group of Motors
  81. HIPOT Testing
  82. What is Earthing
  83. Abstract of National Electrical Code for Transformer’s Protection
  84. Guideline to Design Electrical Network for Building / Small Area.
  85. Demand Factor-Diversity Factor-Utilization Factor-Load Factor
  86. Working Principle of ELCB and RCB
  87. Single Earthed Neutral and Multi Earthed Neutral.
  88. Type of Electrical Power Distribution systems
  89. Type of Cable Tray.
  90. 11KV/415V Overhead Line Specification as per REC
  91. Sub Station Abstracts
  92. Tampering Methods of Electrical Energy Meter.
  93. Specification for Re wirable Cut Out Fuse Unit
  94. Low Voltage and High Voltage Cable Testing
  95. Overload Relay Size & Contactor for Starter
  96. Type of Gland
  97. Gland Size Selection
  98. EHV XLPE – Current Rating
  99. XLPE Cable-Current Rating
  100. PVC Cable-Current Rating
  101. Type & Selection of Fuse
  102. Minimum Electrical Clearance.
  103. Difference between Power Transformer and Distribution Transformer
  104. Type & Selection of Current Transformer
  105. Electrical Clearance in Substation.
  106. Transformer
  107. Minimum Acceptable specification for Metering C.T
  108. Typical Limiting Values of Substation Equipments.
  109. Lighting Arr ester
  110. Standard Makes for Electrical Equipments
  111. Standard Electrical Motor Connections
  112. Power Quality
  113. Ferranti Effect in Transmission Line
  114. What is Corona Effect in Transmission Line 
  115. How Reactive Power helpful to maintain a System Healthy
  116. Automatic Power Factor Correction
  117. Harmonics and It’s Effects
  118. HID Lamps
  119. Types of Overhead Conductors
  120. Difference Between  MCB/MCCB/ELCB/RCCB
  121. Vibration Damper in Transmission Line
  122. Electrical Energy Saving Tips

Electrical Q&A Part-2


1)    Why We use of Stones/Gravel in electrical Switch Yard

  • Reducing Step and Touch potentials during Short Circuit Faults
  • Eliminates the growth of weeds and small plants in the yard
  • Improves yard working condition
  • Protects from fire which cause due to oil spillage from transformer and also protects from wild habitat.

2)    What is service factor?

  • Service factor is the load that may be applied to a motor without exceeding allowed ratings.
  • For example, if a 10-hp motor has a 1.25 service factor, it will successfully deliver 12.5 hp (10 x 1.25) without exceeding specified temperature rise. Note that when being driven above its rated load in this manner, the motor must be supplied with rated voltage and frequency.
  • However a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the 10-hp motor is operated continuously at 12.5 hp, its insulation life could be decreased by as much as two-thirds of normal. If you need a 12.5-hp motor, buy one; service factor should only be used for short-term overload conditions

3)     Why transmission line 11KV OR 33KV, 66KV not in 10KV 20KV?

  • The miss concept is Line voltage is in multiple of 11 due to Form Factor.  The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is 1.11.
  • The Main reason is something historical. In olden days when the electricity becomes popular, the people had a misconception that in the transmission line there would be a voltage loss of around 10%. So in order to get 100 at the load point they started sending 110 from supply side. This is the reason. It has nothing to do with form factor (1.11).
  • Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V.
  • Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of 11 does not arise.  Now a days when, we have voltage correction systems, power factor improving capacitors, which can boost/correct voltage to desired level, we are using the exact voltages like 400KV in spite of 444KV

4)    What is electrical corona?

  • Corona is the ionization of the nitrogen in the air, caused by an intense electrical field.
  • Electrical corona can be distinguished from arcing in that corona starts and stops at essentially the same voltage and is invisible during the day and requires darkness to see at night.
  • Arcing starts at a voltage and stops at a voltage about 50% lower and is visible to the naked eye day or night if the gap is large enough (about 5/8″ at 3500 volts).

5)    What are the indications of electrical corona?

  • A sizzling audible sound, ozone, nitric acid (in the presence of moisture in the air) that accumulates as a white or dirty powder, light (strongest emission in ultraviolet and weaker into visible and near infrared) that can be seen with the naked eye in darkness, ultraviolet cameras, and daylight corona cameras using the solar-blind wavelengths on earth created by the shielding ozone layer surrounding the earth.

6)    What damage does corona do?

  • The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating materials. Corona can also contribute to the chemical soup destruction of insulating cements on insulators resulting in internal flash-over.
  • The corona is the only indication. Defects in insulating materials that create an intense electrical field can over time result in corona that creates punctures, carbon tracks and obvious discoloration of NCI insulators.

7)    How long does corona require creating visible damage?

  • In a specific substation the corona ring was mistakenly installed backwards on a temporary 500kV NCI insulator, at the end of two years the NCI insulator was replaced because 1/3 of the insulator was white and the remaining 2/3 was grey.

8)    What voltage are corona rings typically installed at?

  • It varies depending upon the configuration of the insulators and the type of insulator, NCI normally start at 160kV, pin and cap can vary starting at 220kV or 345kV depending upon your engineering tolerances and insulators in the strings.

9)    How do we select transformers?

  • Determine primary voltage and frequency.
  • Determine secondary voltage required.
  • Determine the capacity required in volt-amperes. This is done by multiplying the load current (amperes) by the load voltage (volts) for single phase.
  • For example: if the load is 40 amperes, such as a motor, and the secondary voltage is 240 volts, then 240 x 40 equals 9600 VA. A 10 KVA (10,000 volt-amperes) transformer is required.
  • Always select Transformer Larger than Actual Load. This is done for safety purposes and allows for expansion, in case more loads is added at a later date. For 3 phase KVA, multiply rated volts x load amps x 1.73 (square root of 3) then divide by 1000.
  • Determine whether taps are required. Taps are usually specified on larger transformers.

10)   Why Small Distribution Transformers not used for Industrial Applications?

  • Industrial control equipment demands a momentary overload capacity of three to eight times’ normal capacity. This is most prevalent in solenoid or magnetic contactor applications where inrush currents can be three to eight times as high as normal sealed or holding currents but still maintain normal voltage at this momentary overloaded condition.
  • Distribution transformers are designed for good regulation up to 100 percent loading, but their output voltage will drop rapidly on momentary overloads of this type making them unsuitable for high inrush applications.
  • Industrial control transformers are designed especially for maintaining a high degree of regulation even at eight time’s normal load. This results in a larger and generally more expensive transformer.

11) Can 60 Hz transformers be used at higher frequencies?

  • Transformers can be used at frequencies above 60 Hz up through 400 Hz with no limitations provided nameplate voltages are not exceeded.
  •  However, 60 Hz transformers will have less voltage regulation at 400 Hz than 60 Hz.

12) What is meant by regulation in a transformer?

  • Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. This is usually expressed in terms of percentage.
  • For example: A transformer delivers 100 volts at no load and the voltage drops to 95 volts at full load, the regulation would be 5%. Distribution transformers generally have regulation from 2% to 4%, depending on the size and the application for which they are used.

13) Why is impedance important?

  • It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.
  • Example: Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source.
  • Calculate:
  • Normal Full Load Current = Nameplate Volt Amps / Line Volts = 10,000 VA / 480 V = 20.8 Amperes
  • Maximum Short Circuit Amps = Full Load Amps / 4% =20.8 Amps / 4%= 520 Amp
  • The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.
  • Example: Determine the interrupting capacity, in amperes, of a circuit breaker or fuse required for a 75 KVA, three phase transformer, with a primary of 480 volts delta and secondary of 208Y/120 volts. The transformer impedance (Z) = 5%. If the secondary is short circuited (faulted), the following capacities are required:
  • Normal Full Load Current =Volt Amps / √ 3 x Line Volts= 75,000 VA / √ 3 x Line Volts √ 3 x 480 V =90 Amps
  • Maximum Short Circuit Line Current = Full Load Amps / 5%=  90 Amps /  5% =1,800 Amps
  • The breaker or fuse would have a minimum interrupting rating of 1,800 amps at 480 volts.
  • Note: The secondary voltage is not used in the calculation. The reason is the primary circuit of the transformer is the only winding being interrupted.

14) What causes flash-over?

  • Flash-over causes are not always easily explained, can be cumulative or stepping stone like, and usually result in an outage and destruction. The first flash-over components are available voltage and the configuration of the energized parts, corona may be present in many areas where the flash-over occurs, and flash-over can be excited by stepping stone defects in the insulating path.

15) What are taps and when are they used?

  • Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Taps are generally set at two and a half and five percent above and below the rated primary voltage.

16) Can Transformers be reverse connected?

  • Dry type distribution transformers can be reverse connected without a loss of KVA rating, but there are certain limitations. Transformers rated 1 KVA and larger single phase, 3 KVA and larger three phases can be reverse connected without any adverse effects or loss in KVA capacity.
  • The reason for this limitation in KVA size is, the turns ratio is the same as the voltage ratio.
  • Example: A transformer with a 480 volt input, 240 volt output— can have the output connected to a 240 volt source and thereby become the primary or input to the transformer, then the original 480 volt primary winding will become the output or 480 volt secondary.
  • On transformers rated below 1 KVA single phase, there is a turn’s ratio compensation on the low voltage winding. This means the low voltage winding has a greater voltage than the nameplate voltage indicates at no load.
  • For example, a small single phase transformer having a nameplate voltage of 480 volts primary and 240 volts secondary, would actually have a no load voltage of approximately 250 volts, and a full load voltage of 240 volts. If the 240 volt winding were connected to a 240 volt source, then the output voltage would consequently be approximately 460 volts at no load and approximately 442 volts at full load. As the KVA becomes smaller, the compensation is greater—resulting in lower output voltages.
  • When one attempts to use these transformers in reverse, the transformer will not be harmed; however, the output voltage will be lower than is indicated by the nameplate.

17) What is the difference between “Insulating”, “Isolating”, and “Shielded Winding” transformers?

  • Insulating and isolating transformers are identical. These terms are used to describe the separation of the primary and secondary windings. A shielded transformer includes a metallic shield between the primary and secondary windings to attenuate (lessen) transient noise.

18) How many BTU’s of heat does a transformer generate?

  • The heat a transformer generates is dependent upon the transformer losses. To determine air conditioning requirements multiply the sum of the full load losses (obtained from factory or test report) of all transformers in the room by 3.41 to obtain the BTUs/hour.
    For example: A transformer with losses of 2000 watts will generate 6820 BTUs/hour.

19) What is a transformer and how does it work?

  • A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to “step up” or “step down” voltages and works on the magnetic induction principle.
  • A transformer has no moving parts and is a completely static solid state device, which insures, under normal operating conditions, a long and trouble-free life. It consists, in its simplest form, of two or more coils of insulated wire wound on a laminated steel core.
  • When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the turns ratio of the two coils.

20) Factors Affecting Corona Discharge Effect:

  • Corona Discharge Effect occurs because of ionization if the atmospheric air surrounding the voltage conductors, so Corona Discharge Effect is affected by the physical state of the atmosphere as well as by the condition of the lines.
  • (1) Conductor: Corona Discharge Effect is considerably affected by the shape, size and surface conditions of the conductor .Corona Discharge Effect decreases with increases in the size (diameter) of the conductor, this effect is less for the conductors having round conductors compared to flat conductors and Corona Discharge Effect is concentrated on that places more where the conductor surface is not smooth.
  • (2) Line Voltage: Corona Discharge effect is not present when the applied line voltages are less. When the Voltage of the system increases (In EHV system) corona Effect will be more.
  • (3) Atmosphere: Breakdown voltage directly proportional to the density of the atmosphere present in between the power conductors. In a stormy weather the ions present around the conductor is higher than normal weather condition So Corona Breakdown voltage occurs at low voltages in the stormy weather condition compared to normal conditions
  • (4)Spacing between the Conductors: Electro static stresses are reduced with increase in the spacing between the conductors. Corona Discharge Effect takes place at much higher voltage when the distance between the power conductors increases.

21) Will a transformer change Three Phases to Single Phase?

  • A transformer will not act as a phase changing device when attempting to change three phase to single phase.
  • There is no way that a transformer will take three phase in and deliver single phase out while at the same time presenting a balanced load to the three phase supply system.
  • There are, however, circuits available to change three phase to two phase or vice versa using standard dual wound transformers. Please contact the factory for two phase applications.

22) Can 60 Hz transformers be operated at 50 Hz?

  • Transformers rated below 1 KVA can be used on 50 Hz service.
  • Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service, due to the higher losses and resultant heat rise. Special designs are required for this service. However, any 50 Hz transformer will operate on a 60 Hz service.

23) Can transformers be used in parallel?

  • Single phase transformers can be used in parallel only when their impedances and voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers, which will cause excess heating and result in a shorter life of the transformer. In addition, impedance values of each transformer must be within 7.5% of each other.
  • For example: Transformer A has an impedance of 4%, transformer B which is to be parallel to A must have impedance between the limits of 3.7% and 4.3%. When paralleling three phase transformers, the same precautions must be observed as listed above, plus the angular displacement and phasing between the two transformers must be identical.

24) What are causes of insulator failure?

  • Electrical field intensity producing corona on contaminated areas, water droplets, icicles, corona rings, … This corona activity then contributes nitric acid to form a chemical soup to change the bonding cements and to create carbon tracks, along with ozone and ultraviolet light to change the properties of NCI insulator coverings. Other detrimental effects include water on the surface or sub-surface freezing and expanding when thawing, as a liquid penetrating into a material and then a sudden temperature change causes change of state to a gas and rapid expansion causing fracture or rupture of the material.

25)  Causes of Corona

  • Corona is causes by the following reasons:
  • The natural electric field caused by the flow of electrons in the conductor. Interaction with surrounding air.
    Poor or no insulation is not a major cause but increases corona.
  • The use of D.C (Direct Current) for transmission.(Reason why most transmission is done in form of AC)

26) Effects of Corona

1)     Line Loss – Loss of energy because some energy is used up to cause vibration of the air particles.

2)     Long term exposure to these radiations may not be good to health (yet to be proven).

3)     Audible Noise

4)     Electromagnetic Interference to telecommunication systems

5)     Ozone Gas production

6)     Damage to insulation of conductor.

27) What is polarity, when associated with a transformer?

  • Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding.
  • Transformers 600 volts and below are normally connected in additive polarity — that is, when tested the terminals of the high voltage and low voltage windings on the left hand side are connected together, refer to diagram below. This leaves one high voltage and one low voltage terminal unconnected.
  • When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings.
  • This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.

28) What is exciting current?

  • Exciting current, when used in connection with transformers, is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and smaller to approximately .5% to 4% on larger sizes of 750 KVA. The exciting current is made up of two components, one of which is a real component and is in the form of losses or referred to as no load watts; the other is in the form of reactive power and is referred to as KVAR.

29) What is Boucholz relay and the significance of it in to the transformer?

  • Boucholz relay is a device which is used for the protection of transformer from its internal faults,
  • it is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.

30) Why we do two types of earthing on transformer (Body earthing & neutral earthing)

  • The two types of earthing are Familiar as Equipment earthing and system earthing.
  • In Equipment earthing: body (non conducting part) of the equipment should be earthed to safeguard the human beings.
  • The System Earthing : In this neutral of the supply source ( Transformer or Generator) should be grounded. With this, in case of unbalanced loading neutral will not be shifted. So that unbalanced voltages will not arise. We can protect the equipment also. With size of the equipment ( transformer or alternator)and selection of relying system earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing.

31) Conductor corona is caused by?

  • Corona on a conductor can be due to conductor configuration (design) such as diameter too small for the applied voltage will have corona year-around and extreme losses during wet weather, the opposite occurs during dry weather as the corona produces nitric acid which accumulates and destroys the steel reinforcing cable (ACSR) resulting in the line dropping. Road salts and contaminants can also contribute to starting this deterioration.

32) What is flash-over and arcing?

  • Flash-over is an instantaneous event where the voltage exceeds the breakdown potential of the air but does not have the current available to sustain an arc, an arc can have the grid fault current behind it and sustain until the voltage decreases below 50% or until a protective device opens.
  • Flash-over can also occur due to induced voltages in unbounded (loose bolts, washers, etc) power pole or substation hardware, this can create RFI/TVI or radio/TV interference. Arcing can begin at 5 volts on a printed circuit board or as the insulation increases it may require 80kVAC to create flash-over on a good cap and pin insulator.

33) How to Minimizing Corona Effects

  • Installing corona rings at the end of transmission lines.
  • A corona ring, also called anti-corona ring, is a toroid of (typically) conductive material located in the vicinity of a terminal of a high voltage device. It is electrically insulated.
  • Stacks of more spaced rings are often used. The role of the corona ring is to distribute the electric field gradient and lower its maximum values below the corona threshold, preventing the corona discharge.

34) What is BIL and how does it apply to transformers?

  • BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground. The Basic Impulse Level of a transformer is a method of expressing the voltage surge (lightning, switching surges, etc.) that a transformer will tolerate without breakdown.
  • All transformers manufactured in this catalog, 600 volts and below, will withstand the NEMA standard BIL rating, which is 10 KV.
  • This assures the user that he will not experience breakdowns when his system is properly protected with lightning arrestors or similar surge protection devices.

35) The difference between Ground and Neutral?

  • NEUTRAL is the origin of all current flow. In a poly-phase system, as its phase relationship with all the three phases is the same, (i.e.) as it is not biased towards any one phase, thus remaining neutral, that’s why it is called neutral.
  • Whereas, GROUND is the EARTH on which we stand. It was perceived to utilize this vast, omnipresent conductor of electricity, in case of fault, so that the fault current returns to the source neutral through this conductor given by nature which is available free of cost. If earth is not used for this purpose, then one has to lay a long. long metallic conductor for the purpose, thus increasing the cost.
  • Ground should never be used as neutral. The protection devices (eg ELCB, RCD etc) work basically on principle that the phase currects are balanced with neutral current. In case you use ground wire as the neutral, these are bound to trip if they are there – and they must be there. at least at substations. And these are kept very sensitive i.e. even minute currents are supposed to trip these.
  • One aspect is safety – when someone touches a neutral, you don’t want him to be electrocuted – do you? Usually if you see the switches at home are on the phase and not neutral (except at the MCB stage). Any one assumes the once the switch is off, it is safe (the safety is taken care of in 3 wire system, but again most of the fixtures are on 2 wire) – he will be shocked at the accidental touching of wire in case the floating neutral is floating too much.

36) What is impedance of a transformer?

  • If you mean the percentage impedance of the transformed it means the ratio of the voltage( that if you applied it to one side of the transformer while the other side of the transformer is short cuitcuted, a full load current shall flow in the short circuits side), to the full load current.
  • More the %Z of transformer, more Copper used for winding, increasing cost of the unit. But short circuit levels will reduce, mechanical damages to windings during short circuit shall also reduce. However, cost increases significantly with increase in %Z.
  • Lower %Z means economical designs. But short circuit fault levels shall increase tremendously, damaging the winding & core.
  • The high value of %Z helps to reduce short circuit current but it causes more voltage dip for motor starting and more voltage regulation (% change of voltage variation) from no load to full load.

37) How are transformers sized to operate Three Phase induction type squirrel cage motors?

  • The minimum transformer KVA rating required to operate a motor is calculated as follows:
  • Minimum Transformer KVA =Running Load Amperes x 1.73x Motor Operating Voltage / 1000
  • NOTE: If motor is to be started more than once per hour add 20% additional KVA. Care should be exercised in sizing a transformer for an induction type squirrel cage motor as when it is started, the lock rotor amperage is approximately 5 to 7 times the running load amperage. This severe starting overload will result in a drop of the transformer output voltage.
  • When the voltage is low the torque and the horsepower of the motor will drop proportionately to the square of the voltage.
  • For example: If the voltage  were to drop to 70% of nominal, then motor horsepower and torque would drop to 70 % squared or 49% of the motor nameplate rating.
  • If the motor is used for starting a high torque load, the motor may stay at approximately 50% of normal running speed The underlying problem is low voltage at the motor terminals. If the ampere rating of the motor and transformer over current device falls within the motor’s 50% RPM draw requirements, a problem is likely to develop. The over current device may not open under intermediate motor ampere loading conditions.
  • Overheating of the motor and/or transformer would occur, possibly causing failure of either component.
  • This condition is more pronounced when one transformer is used to power one motor and the running amperes of the motor is in the vicinity of the full load ampere rating of the transformer. The following precautions should be followed:
  • (1)When one transformer is used to operate one motor, the running amperes of the motor should not exceed 65% of the transformer’s full load ampere rating.
  • (2) If several motors are being operated from one transformer, avoid having all motors start at the same time. If this is impractical, then size the transformer so that the total running current does not exceed 65% of the transformer’s full load ampere rating.

38) Which Point need to be consider while Neutral Earthing of Transformer?

  • The following points need to check before going for Neutral Grounding Resistance.
  • Fault current passing through ground, step and touch potential.
  • Capacity of transformer to sustain ground fault current, w.r.t winding, core burning.
  • Relay co-ordination and fault clearing time.
  • Standard practice of limiting earth fault current. In case no data or calculation is possible, go for limiting E/F current to 300A or 500A, depending on sensivity of relay.

39) Why a neutral grounding contactor is needed in diesel generator?

  • There would not be any current flow in neutral if DG is loaded equally in 3 phases , if there any fault(earth fault or over load) in any one of the phase ,then there will be un balanced load in DG . at that time heavy current flow through the neutral ,it is sensed by CT and trips the DG. so neutral in grounded to give low resistance path to fault current.
  • An electrical system consisting of more than two low voltage Diesel Generator sets intended for parallel operation shall meet the following conditions:
  • (i) Neutral of only one generator needs to be earthed to avoid the flow of zero sequence current.
  • (ii) During independent operation, neutrals of both generators are required in low voltage switchboard to obtain three phases, 4 wire system including phase to neutral voltage.
  • (iii) required to achieve restricted earth fault protection (REF) for both the generators whilst in operation.
  • Solution:
  • Considering the requirement of earthing neutral of only one generator, a contactor of suitable rating shall be provided in neutral to earth circuit of each generator. This contactor can be termed as “neutral contactor”.
  • Neutral contactors shall be interlocked in such a way that only one contactor shall remain closed during parallel operation of generators. During independent operation of any generator its neutral contactor shall be closed.
  • Operation of neutral contactors shall be preferably made automatic using breaker auxiliary contacts.

40) Neutral grounded system vs solidly grounded system

  • In India, at low voltage level (433V) we MUST do only Solid Earthing of the system neutral.
  • This is by IE Rules 1956, Rule No. 61 (1) (a). Because, if we option for impedance earthing, during an earth fault, there will be appreciable voltage present between the faulted body & the neutral, the magnitude of this voltage being determined by the fault current magnitude and the impedance value.
  • This voltage might circulate enough current in a person accidentally coming in contact with the faulted equipment, as to harm his even causing death. Note that, LV systems can be handled by non-technical persons too. In solid earthing, you do not have this problem, as at the instant of an earth fault, the faulted phase goes to neutral potential and the high fault current would invariably cause the Over current or short circuit protection device to operate in sufficiently quick time before any harm could be done

 

Electrical Q&A Part-1


1)     Why ELCB cannot work if Neutral input of ELCB does not connect to ground?

  • ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that same current will have to return neutral so resultant current is zero.
  • Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.

2)     What is the difference between MCB & MCCB, Where it can be used?

  • MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit.
  • Normally it is used where normal current is less than 100A.
  • MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt.
  • Normally it is used where normal current is more than 100A.

3)     Why in a three pin plug the earth pin is thicker and longer than the other pins?

  • It depends upon R=ρL/A where area (A) is inversely proportional to resistance (R), so if  area (A) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the The First to make the connection and last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

4)     Why Delta Star Transformers are used for Lighting Loads?

  • For lighting loads, neutral conductor is must and hence the secondary must be star winding and this lighting load is always unbalanced in all three phases.
  • To minimize the current unbalance in the primary we use delta winding in the primary So delta / star transformer is used for lighting loads.

5)      What are the advantages of star-delta starter with induction motor?

  • The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting  Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.

6)     What is meant by regenerative braking?

  • When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as “Regenerative Breaking”.

7)     When voltage increases then current also increases then why we need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?

  • No. We cannot sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature).So, the over voltage protection & over current protection are completely different.
  • Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system. It should have both over voltage & over current relay.

8)     If one lamp connects between two phases it will glow or not?

  • If the voltage between the two phases is equal to the lamp voltage then the lamp will glow.
  • When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

9)     What are HRC fuses and where it is used?

  • HRC stand for “high rupturing capacity” fuse and it is used in distribution system for electrical transformers

10)  Mention the methods for starting an induction motor?

  • The different methods of starting an induction motor
  • DOL:direct online starter
  • Star delta starter
  • Auto transformer starter
  • Resistance starter
  • Series reactor starter

11)  What is the difference between earth resistance and earth electrode resistance?

  • Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition:
  • Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.
  • The resistance of the electrode has the following components:
    (A) the resistance of the metal and that of the connection to it.
    (B) The contact resistance of the surrounding earth to the electrode.

12)  Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

  • 4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation.
  • For example: A pressure instrument gives output 4mA to indicate 0 psi  up to 20 mA to indicate 100 psi or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

13)  Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?

  • Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the Bulb (P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

14)  What happen if we give 220 volts dc supply to bulb or tube light?

  • Bulbs or devices for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element

15)  What is meant by knee point voltage?

  • Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.

16)  What is reverse power relay?

  • Reverse Power flow relay are used in generating stations’ protection.
  • A generating station is supposed to feed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

17)  What will happen if DC supply is given on the primary of a transformer?

  • Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance, only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out
  • When AC current flow to primary winding it induced alternating flux which also link to secondary winding so secondary current flow in secondary winding according to primary current.Secondary current also induced emf (Back emf) in secondary winding which oppose induced emf of primary winding and thus control primary current also.
  • If DC current apply to Primary winding than alternating flux is not produced so no secondary emf induced in secondary winding  so primary current may goes high and burn transformer winding.

18)  Different between megger and contact resistance meter?

  • Megger used to measure cable resistance, conductor continuity, phase identification where as contact resistance meter used to measure low resistance like relays, contactors.

19)  When we connect the capacitor bank in series?

  • We connect capacitor bank in series to improve the voltage profile at the load end in transmission line there is considerable voltage drop along the transmission line due to impedance of the line. so in order to bring the voltage at the load terminals within its limits i.e (+ or – %6 )of the rated terminal voltage the capacitor bank is used in series

20)  What is Diversity factor in electrical installations?

  • Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Diversity factor is usually more than one.

21)  Why humming sound occurred in HT transmission line?

  • This sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.

22)  Why frequency is 50 hz only & why should we maintain the frequency constant?

  • We can have the frequency at any frequency we like, but then we must also make our own motors, transformers or any other equipment we want to use.
  • We maintain the frequency at 50 Hz or 60hz because the world maintains a standard at 50 /60hz and the equipments are made to operate at these frequency.

23)  If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?

  • As we know the Voltage & current relation for transformer-V1/V2 = I2/I1
    We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps.
    By putting these value on Relation-
    540/11000= I2/2334
    So,I2 = 114.5 Amps

24)  What are the points to be considered for MCB (miniature circuit breaker selection)?

  • I(L)x1.25=I(MAX) maximum current. Mcb specification is done on maximum current flow in circuit.

25)  How can we start-up the 40w tube light with 230v AC/DC without using any choke/Coil?

  • It is possible by means of Electronic choke. Otherwise it’s not possible to ionize the particles in tube. Light, with normal voltage.

26)  What is “pu” in electrical engineering?

  • Pu stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (p.u=actual value/base value).

27)  Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

  • Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps.
  • But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off.
  • If Fuse is provided in Neutral and if it is blowout and at the same time Supply is on than due to open or break Neutral Voltage is increase and equipment may be damage.

28)  If 200w, 100 w and 60 w lamps connected in series with 230V AC , which lamp glow brighter? Each lamp voltage rating is 230V.

  • Each bulb when independently working will have currents (W/V= I)
  • For 200 Watt Bulb current (I200) =200/230=0.8696 A
  • For 100 Watt Bulb current (I100) =100/230=0.4348 A
  • For 60 Watt Bulb current (I60) =60/230=0.2609 A
  • Resistance of each bulb filament is (V/I = R)
  • For 200 Watt Bulb R200= 230/0.8696= 264.5 ohms
  • For 100 Watt Bulb R100= 230/0.4348 = 528.98 ohms and
  • For 60 Watt Bulb R60= 230/0.2609=881.6 ohms respectively
  • Now, when in series, current flowing in all bulbs will be same. The energy released will be I2R
  • Thus, light output will be highest where resistance is highest. Thus, 60 watt bulb will be brightest.
  • The 60W lamp as it has highest resistance & minimum current requirement.
  • Highest voltage drop across it X I [which is common for all lamps] =s highest power.
  • Note to remember:
  • Lowest power-lamp has highest element resistance.
  • And highest resistance will drop highest voltage drop across it in a Series circuit
  • And highest resistance in a parallel circuit will pass minimum current through it. So minimum power dissipated across it as min current X equal Voltage across =s min power dissipation

29)  How to check Capacitor with use of Multi meter.

  • Most troubles with Capacitors either open or short.
  • An ohmmeter (multi meter) is good enough. A shorted Capacitor will clearly show very low resistance. A open Capacitor will not show any movement on ohmmeter.
  • A good capacitor will show low resistance initially, and resistance gradually increases. This shows that Capacitor is not bad. By shorting the two ends of Capacitor (charged by ohmmeter) momentarily can give a weak spark. To know the value and other parameters, you need better instruments

30)  What is the difference between Electronic regulator and ordinary rheostat regulator for fans?

  • The difference between the electronic and ordinary regulator is that in electronic regulator power losses are less because as we decrease the speed the electronic regulator give the power needed for that particular speed .But in case of ordinary rheostat type regulator the power wastage is same for every speed and no power is saved. In electronic regulator triac is employed for speed control. by varying the firing angle speed is controlled but in rheostat control resistance is decreased by steps to achieve speed control.

31)  What will happen when power factor is leading in distribution of power?

  • If there is high power factor, i.e if the power factor is close to one:
  • Losses in form of heat will be reduced,
  • Cable becomes less bulky and easy to carry, and very cheap to afford.
  • It also reduces over heating of transformers.

32)  What the main difference between UPS & inverter?

  • Uninterrupted power supply is mainly use for short time. Means according to ups VA it gives backup. Ups is also two types: on line and offline. Online ups having high volt and amp for long time backup with high dc voltage. But ups start with 12v dc with 7 amps. but inverter is start with 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup

33)  Which type of A.C motor is used in the fan?

  • It is Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

34)  What is the difference between synchronous generator and asynchronous generator?

  • In simple, synchronous generator supplies’ both active and reactive power but asynchronous generator (induction generator) supply’s only active power and observe reactive power for magnetizing. This type of generators is used in windmills.

35)  What is the Polarization index value?

  • Its ratio between insulation resistance (IR)i.e meager value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5 .

36)  What is Automatic Voltage regulator (AVR)?

  • AVR is an abbreviation for Automatic Voltage Regulator.
  • It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

37)  Difference between a four point starter and three point starters?

  • The shunt connection in four point starter is provided separately from the line where as in three point starter it is connected with line which is the drawback in three point starter

38)  What happens if we connect a capacitor to a generator load?

  • Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf.
  •  Don’t connect a capacitor across an alternator while it is picking up or without any other load

39)  Why the capacitors work on ac only?

  • Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac components to pass through.

40)  Why the up to dia 70mm² live conductor, the earth cable must be same size but above dia 70mm² live conductor the earth conductor need to be only dia 70mm²?

  • The current carrying capacity of a cable refers to it carrying a continuous load.
  • An earth cable normally carries no load, and under fault conditions will carry a significant instantaneous current but only for a short time most Regulations define 0.1 to 5 sec before the fuse or breaker trips. Its size therefore is defined by different calculating parameters.
  • The magnitude of earth fault current depends on:
  • (a) the external earth loop impedance of the installation (i.e. beyond the supply terminals)
  • (b) the impedance of the active conductor in fault
  • (c) the impedance of the earth cable.
  • i.e. Fault current = voltage / a + b + c
  • Now when the active conductor (b) is small, its impedance is much more than (a), so the earth (c) cable is sized to match. As the active conductor gets bigger, its impedance drops significantly below that of the external earth loop impedance (a); when It is quite large its impedance can be ignored. At this point there is no merit in increasing the earth cable size
  • i.e. Fault current = voltage / a + c
  • (c) is also very small so the fault current peaks out.
  • The neutral conductor is a separate issue. It is defined as an active conductor and therefore must be sized for continuous full load. In a 3-phase system,
  • If balanced, no neutral current flows. It used to be common practice to install reduced neutral supplies, and cables are available with say half-size neutrals (remember a neutral is always necessary to provide single phase voltages). However the increasing use of non-linear loads which produce harmonics has made this practice dangerous, so for example the current in some standard require full size neutrals. Indeed, in big UPS installations I install double neutrals and earths for this reason.

 

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