Typical Limiting Values of SubStation Equipments.


Typical Limiting Values of Sub Station Electrical Equipments.

1. Transformer / Reactor:

Sr. No. Equipment / test data Permissible limits Reference
A) Transformer oil    
a) BDV    
-At the time of first charging 600 kV (Gap – 2.5 mm) – Minimum IS – 1866
-During O&M 50 kV (Gap – 2.5 mm) – Minimum IS – 1867
b) Moisture content   IS – 1868
-At the time of first charging 15 PPM (Max.) IS – 1869
-During O&M 25 PPM (Max.) IS – 1870
c) Resistivity at 90 degree C 0.1-1012 Ohm-CM (Min.) IS – 1871
d) Acidity 0.2 mg KOH/gm (Max.) IS – 1872
e) IFT at 27 degree C 0.018 N/M (Min.) IS – 1873
f) Tan delta at 90 degree C 0.20 (Max.) IS – 1874
g) Flash point 126 Deg. C (Min.) IS – 1875
B) Vibration level for reactors 200 Microns (Peak to Peak) IS – 1876
  60 Microns (Average) IS – 1877
C) Tan delta for bushing at 20 Deg. C 0.007* IEC – 137
D) Capacitance for bushing + 5% variation IEC – 138
E) IR value for winding 1000 M-Ohm By 5.0/10.0 kV Megger IEC – 139
F) Tan delta for windings at 20 Deg. C 0.007* IEEE/C57.12.90.1980
G) Contact resistance of bushing terminal connectors 10 M. Ohm / Connector NGC.UK Recommendations
H) Turret Neutral CT ratio errors 39 IS – 2705

 2. Circuit Breakers 

Sr. No. Equipment / test data Permissible limits Reference
A) Dew point of SF6 gas Dew point values as per Annexure – II  
B) Dew point of operating air -45 Deg. C at ATM. Pressure  
C) CB Operating timings 400 kV               220KV  
a) Closing time (Max.) 150 MS              200MS  
b) Trip time (Max.) 25 MS                35MS  
c) Close/trip time, Pole discrepancy    
– Phase to Phase (Max.) 3.33 MS            3.33MS  
-Break to break (Max.) of same phase 2.5 MS              2.5MS  
D) PIR time    
BHEL make 12-16 MS Manufacturers Recommendations
     
ABB make 8-12 MS Manufacturers Recommendations
NGEF make 8-12 MS Manufacturers Recommendations
M&G make 8-12 MS Manufacturers Recommendations
TELK make 8-12 MS Manufacturers Recommendations
ABB make (HVDC) 8-12 MS Manufacturers Recommendations
E) PIR opening time prior to opening of main contacts (ABB, CGL, NGEF make CBs) 5 MS (Min.) at rated pressure Manufacturers Recommendations
F) Pir and main contacts overlap time [BHEL, M&G, ABB (imported) make CBs] 6 MS (Min.) at rated pressure Manufacturers Recommendations
G) Tan delta of grading capacitors 0.007 at 20 Deg. C  
H) Capacitance of grading capacitors Within + 10% / -5% of the rated value IEC 359
I) Contact resistance of CB 150 M. Ohm  
J) Contact resistance of CB terminal connector 10 M. Ohm per connector NGC, UK recommendations
K) IR value:    
1. Phase – earth 1000 M Ohm (Min.) by 5.0 / 10.0 kV Megger  
2. Across open contacts 1000 M Ohm (Min.) by 5.0/10.0 kV Megger  
3. Control cables 50 M Ohm (Min.) by 0.5 kV Megger  
L) Pressure switch settings    
-SF6 gas pressure switches Within + 0.1 Bar of set value  
-Operating air pr. Switches Within + 0.1 Bar of set value  
-Operating oil pr. Switches Within + 0.1 Bar of set value  
M) BDV of oil used for MOCB    
-At the time of filling 40 kV at 2.5 mm Gap (Min.) Mfgs. Recommendation
-During O&M 20 kV at 2.5 mm Gap. (Min.) Mfgs. Recommendation

 3. Current Transformer

Sr. No. Equipment / test data Permissible limits Reference
A) IR value    
1. Primary – earth 1000 M – Ohm (Min.) by 5.0/10.0 kV Megger  
2. Secondary – earth 50 M – Ohm (Min.) by 0.5 kV Megger  
3. Control cables 50 M-Ohm (Min.) by 0.5 kV Megger  
B) Tan delta value 0.007* at 20 Deg. C  
C) Terminal Connector 10 M-Ohm per connector NGC, UK Recommendations
D) CT ratio errors + 3% -Protection cores IS – 2705
D) CT ratio errors + 1% -Metering cores IS – 2706

 4. Capacitive Voltage

Sr. No. Equipment / test data Permissible limits Reference
A) Tan Delta 0.007* at 20 Deg. C  
B) Capacitance Within +10%/-5% of the rated value IEC – 358
C Contact resistance of terminal connector 10 M-Ohm per connector NGC, UK Recommendations
D) IR Value IR Value  
1. Primary – earth 1000 M – Ohm (Min.) by 5.0/10.0 kV Megger  
2. Secondary – earth 50 M – Ohm (Min.) by 0.5 kV Megger  
3. Control cables 50 M-Ohm (Min.) by 0.5 kV Megger  
E) EMU tank oil parameters EMU tank oil parameters  
a) BDV (Min.) 30 kV (Gap. –2.5 mm) IS – 1866
b) Moisture content (Max.) 35 ppm -do-
c) Resistivity at 90 Deg. C 0.1 – 1012 Ohm. – CM -do-
d) Acidity 0.5 mg kOH /gm (Max.) -do-
e) IFT at 27 Deg. C 0.018 N/M (Min.) -do-
f) Tan delta at 90 Deg. C 1.0 Max. -do-
g) Flash point 125 Deg. C (Min.) -do-
F) CVT voltage ratio errors + 5% protection cores IEEE/C93.1.1990
F) CVT voltage ratio errors + 0.5% metering cores IEC 186

 5. Isolators

Sr. No. Equipment / test data Permissible limits Reference
A) Contact resistance 300 M-Ohm. (Max.)  
B) Contact resistance of terminal connector 10 M – Ohm per connector NGC, UK Recommendations
C) IR value    
1. Phase – earth 1000 M – Ohm (Min.) by 5.0/10.0 kV Megger NGC, UK Recommendations
2. Across open contacts 1000 M – Ohm (Min.) by 5.0/10.0 kV Megger NGC, UK Recommendations
3. Control cables 50 M-Ohm (Min.) by 0.5 kV Megger  

 6. Surge Arrester

Sr. No. Equipment / test data Permissible limits Reference
A) Leakage current 500 M-Amp. (Resistive) Hitachi, Japan Recom.
B) IR value 1000 M-Ohm. (Min.) Hitachi, Japan Recom.

 7. Miscellaneous

Sr. No. Equipment / test data Permissible limits Reference
A) Station earth resistance 1.0 Ohm (Max.)  
B) Thermo vision scanning    
Temp. up to 15 Deg. C (above ambient) Normal  
Temp. above 15-50 Deg. C (above ambient) Alert  
Temp. above 50 Deg. C (above ambient) To be immediately attended  
C) Terminal connectors – Contact resistance 10 M-Ohm per connector HGC, UK Recommendations
D) IR values    
1. All electrical motors 50 M-Ohm (Min.) by 0.5 kV Megger  
2. Control cables 50 M-Ohm (Min) by 0.5 kV Megger  
3. Lt. Transformers 100 M.-Ohm (Min.) by Megger  
4. Lt. Switchgears 100 M – Ohm (Min.) by 0.5 kV Megger  

 8. Batteries

Sr. No. Equipment / test data Permissible limits Reference
A) Terminal connector resistance 10 M – Ohm + 20% ANSI/IEEE – 450 1989
B) Specific gravity 1200 + 5 GM/L at 27 Deg. C  

 Temperature Correction Factor for Tan Delta Measurement

 Sr. No.  Oil temperature Deg. C  Correction factor(K)
 1  10  0.8
 2  15  0.9
 3  20  1.0
 4  25  1.12
 5  30  1.25
 6  35  1.40
 7  40  1.55
 8  45  1.75
 9  50  1.95
 10  55  2.18
 11  60  2.42
 12  65  2.70
 13  70  3.00

If Tan Delta of bushing/winding/CVT/CT is measured at oil temperature T Deg. C. Then Tan Delta at 20 Deg. C shall be as given below:

Tan Delta at 20 Deg. C = Tan Delta at Temp T Deg. C / Factor K.

Dew Point Limits for SF6 Gas in EHV Circuit Breakers

Sr. No Make of C.B Dew point at rated Pr. Deg. C  Corresponding dew point at Atmospheric .Pr.  Remarks
1 BHEL   -15  -36  At the time of commissioning
 -7  -29  During O&M
 -5  -27  Critical
2 M&G  —  -39  At the time of commissioning
   -32  During O&M
3 CGL  -15  -35  At the time of commissioning
 -10  -31  During O&M
4 ABB  -15  -35  At the time of commissioning
 -5  -26 During O&M
5 NGEF  -15  -36 At the time of commissioning
 -7  -29  During O&M

 Note: Dew point of SF6 gas varies with pressure at which measurement is carried out. So it is to be ensured that if measurement is done at pressure other than atmospheric pressure, it needs to be converted to the atmospheric pressure.

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Lighting Arrester


Lighting and Voltage Surge

  • Lightning can create voltage surges in several of the following ways. Lightning can score a direct hit on your house. It can strike the overhead power line which enters your house, or a main power line that is blocks away from your home. Lightning can strike branch circuitry wiring in the walls of your house. Lightning can strike an object near your home such as a tree or the ground itself and cause a surge. Voltage surges can be created by cloud to cloud lightning near your home. A highly charged cloud which passes over your home can also induce a voltage surge.
  • Voltage surges can also be caused by standard on and off switching activities of large electric motors or pieces of equipment. These surges can be created by a neighbor, or by a business or manufacturing facility some distance from your house. These surges are insidious and for the most part are silent. They can occur with little or no warning.

Method to Suppress Lighting and Voltage Surge:

  • When a voltage surge is created, it wants to equalize itself and it wants to do it as quickly as possible. These things seem to have very little patience. The surges will do whatever it takes to equalize or neutralize themselves, even if it means short circuiting all of your electronic equipment.
  • The method of providing maximum protection for equipment is quite simple. Create a pathway for the voltage surge (electricity) to get to and into the ground outside your house as quickly as possible. This is not, in most cases, a difficult task.
  • The first step is simple. Create an excellent grounding system for your household electrical system. The vast majority of homes do not have an excellent grounding system. Many homes have a single grounding rod and /or a metallic underground water pipe which are part of the electrical grounding system. In most cases, this is inadequate. The reason is somewhat easy to explain. Imagine putting a two inch fire hose into your kitchen sink and opening the nozzle to the full on position. I doubt that the drain in your sink could handle all of the water. Your grounding system would react in the same way to a massive voltage surge. Just as the water jumps out of the sink, the electricity jumps from the grounding system and looks for places to go. Frequently it looks for the microchips in your electronic devices. They are an easy target. They offer a path of least resistance.
  • Voltage surges want to be directed to the grounding system, and when they do, they want to get into the ground around your house in a hurry. You can achieve this by driving numerous grounding rods into virgin soil around your house. These rods should be UL approved and connected by a continuous heavy solid copper wire which is welded to each grounding rod. This solid copper wire begins on the grounding bar inside of your electrical panel and terminates at the last grounding rod. Avoid using clamps if at all possible. Over time, the connection at the clamp can corrode or become loose creating tremendous resistance. This will act as a roadblock to the electricity trying to get into the ground around your home.
  • The grounding rods should be at least ten feet apart from one another. They should be located in soil which readily accepts electricity. Moist clay soils are very desirable. Rocky, sandy, or soils with gravel generally have high resistance factors. Electricity has a tough time dissipating into them. Resistance readings should be in the range of 10 to 30 ohms. The lower the better.
  • The second step in household surge protection is to install a lightning arrester inside of your electric service panel. These devices can be extremely effective in intercepting large voltage surges which travel in the electric power lines. These devices capture the voltage surges and ‘bleed’ them off to the grounding wire which we just spoke of. If for some reason you do not have a large enough grounding wire, or enough ground rods, the arrester cannot do its job. It must be able to send the surge quickly to the ground outside of your house. Almost every manufacturer of circuit breakers makes one to fit inside their panel. They can be installed by a homeowner who is experienced in dealing with high voltage panels. If you do not have this capability, have an experienced electrician install it for you.
  • The final step in the protection plan is to install ‘point of use’ surge suppression devices. Often you will see these called ‘transient voltage surge suppressors’. These are your last line of defense. They are capable of only stopping the leftover voltage surge which got past the grounding system and the lightning arrester. They cannot protect your electronic devices by themselves. They must be used in conjunction with the grounding system and the lightning arresters. Do not be lulled into a false sense of security if you merely use one of these devices!
  • The ‘point of use’ surge suppression devices are available in various levels of quality. Some are much better than others. What sets them apart are several things. Generally speaking, you look to see how fast their response time is. This is often referred to as clamping speed. Also, look to see how high of a voltage surge they will suppress. Make sure that the device has a 500 volt maximum UL rated suppression level. Check to see if it has an indicator, either visual or audio, which lets you know if it is not working. The better units offer both, in case you install the device out of sight. Check to see if it offers a variety of modes with respect to protection. For example, does the device offer protection for surges which occur between the ‘hot’ and neutral, between ‘hot’ and ground, as well as between neutral and ground. There is a difference! Check to see if it monitors the normal sine waves of regular household current. Surges can cause irregularities in these wave patterns. Good transient surge suppression devices ‘devour’ these voltage spikes. Finally, check the joule rating. Attempt to locate a device which has a joule rating of 140 or higher. Electrical supply houses often are the best place to look for these high quality devices.
  • Some devices can also protect your phone equipment at the same time. This is very important for those individuals who have computer modems. Massive voltage surges can come across phone lines as well. These surges can enter your computer through the telephone line! Don’t forget to protect this line as well. Also, be sure the telephone ground wire is tied to the upgraded electrical grounding system.

What is a surge arrester?

  • Surge arresters are devices that help prevent damage to apparatus due to high voltages. The arrester provides a low-impedance path to ground for the current from a lightning strike or transient voltage and then restores to a normal operating conditions.
  • A surge arrester may be compared to a relief valve on a boiler or hot water heater. It will release high pressure until a normal operating condition is reached. When the pressure is returned to normal, the safety valve is ready for the next operation.
  • When a high voltage (greater than the normal line voltage) exists on the line, the arrester immediately furnishes a path to ground and thus limits and drains off the excess voltage. The arrester must provide this relief and then prevent any further flow of current to ground. The arrester has two functions; it must provide a point in the circuit at which an over-voltage pulse can pass to ground and second, to prevent any follow-up current from flowing to ground.

Causes of over voltages

  • Internal causes
  • External causes

Internal causes

  • Switching surge
  • Insulation failure
  • Arcing ground
  • Resonance
  • Switching surge: The over voltages produced on the power system due to switching are known as switching surge.
  • Insulation failure: The most common case of insulation failure in a power system is the grounding of conductors (i.e. insulation failure between line and earth) which may cause overvoltage in the system.
  • Arcing ground: The phenomenon of intermittent arc taking place in line to ground fault of a 3phase system with consequent production of transients is known as arcing ground.
  • Resonance: It occurs in an electrical system when inductive reactance of the circuit becomes equal to capacitive reactance. under resonance , the impedance of the circuit is equal to resistance of the circuit and the p.f is unity.

Types of lightning strokes

  • Direct stroke
  • Indirect stroke

(1) Direct stroke

  • In direct stroke, the lightning discharge is directly from the cloud to the subject equipment. From the line, the current path may be over the insulator down the pole to the ground.

(2) Indirect stroke

  • Indirect stroke results from the electro statically induced charges on the conductors due to the presence of charge clouds.

Harmful effects of lightning

  • The traveling waves produced due to lightning will shatter the insulators.
  • If the traveling waves hit the windings of a transformer or generator it may cause considerable damage.

Protection against lightning

  • Different types of protective devices are:-
  • Earthing screen
  • Overhead ground wires
  • Lightning arresters

(1)The Earthing screen

  • The power station & sub-station can be protected against direct lightning strokes by providing earthing screens.
  • On occurrence of direct stroke on the station ,screen provides a low resistance path by which lightning surges are conducted to ground.
  • Limitation:
  • It does not provide protection against the traveling waves which may reach the equipments in the station.

(2)Overhead ground wires

  • It is the most effective way of providing protection to transmission lines against direct lightning strokes.
  • It provides damping effect on any disturbance traveling along the lines as it acts as a short-circuited secondary.
  • Limitation:
  • It requires additional cost.
  • There is a possibility of its breaking and falling across the line conductors, thereby causing a short-circuit fault.

(3)Lightning Arresters

  • It is a protective device which conducts the high voltage surge on the power system to ground
  • The earthing screen and ground wires fail to provide protection against traveling waves. The lightning arrester provides protection against surges.

AC Power Surge Arrester

Type 1 Surge Protectors

  • Type 1 surge protectors are designed to be installed where a direct lightning strike risk is high, especially when the building is equipped with external lightning protection system (LPS or lightning rod).
  • In this situation IEC 61643-11 standards require the Class I test to be applied to surge protectors : this test is characterized by the injection of 10/350 μs impulse current in order to simulate the direct lightning strike consequence. Therefore these Type 1 surge protectors must be especially powerful to conduct this high energy impulse current.

Type 2 surge protectors

  • Type 2 surge protectors are designed to be installed at the beginning of the installation, in the main switchboard, or close to sensitive terminals, on installations without LPS (lightning rods).
  • These protectors are tested following the Class II test from IEC61643-11 based on 8/20 μs impulse current injection.

Type 3 surge protectors

  • In case of very sensitive or remote equipment, secondary stage of surge protectors is required : these low energy SPDs could be Type 2 or Type 3. Type 3 SPDs are tested with a combination waveform (1,2/50  μs – 8/20 μs) following Class III test.

Types of Lightning Arrestors according to Class:

1.     Station Class

  • Station class arrestors are typically used in electrical power stations or substations and other high voltage structures and areas.
  • These arrestors protect against both lightning and over-voltages, when the electrical device has more current in the system than it is designed to handle.
  • These arrestors are designed to protect equipment above the 20 mVA range.

2.     Intermediate Class

  • Like station class arrestors, intermediate class arrestors protect against surges from lightning and over-voltages, but are designed to be used in medium voltage equipment areas, such as electrical utility stations, substations, transformers or other substation equipment.
  • These arrestors are designed for use on equipment in the range of 1 to 20 mVA.

3.     Distribution Class

  • Distribution class arrestors are most commonly found on transformers, both dry-type and liquid-filled.
  • These arrestors are found on equipment rated at 1000 kVA or less.
  • These arrestors are sometimes found on exposed lines that have direct connections to rotating machines.

4.     Secondary Class

  • Secondary class lightning arrestors are designed to protect most homes and businesses from lightning strikes, and are required by most electrical codes, according to, Inc., an electrical power protection company.
  • These arrestors cause high voltage overages to ground, though they do not short all the over voltage from a surge. Secondary class arrestors offer the least amount of protection to electrical systems, and typically do not protect solid state technology, or anything that has a microprocessor.

Choosing the right AC Power Surge Arrester

  • AC power surge protectors is designed to cover all possible configurations in low voltage installations. They are available in many versions, which differ in:
  • Type or test class (1 , 2 or 3)
  • Operating voltage (Uc)
  • AC network configuration (Single/3-Phase)
  • Discharge currents (Iimp, Imax, In)
  • Protection level (Up)
  • Protection technology (varistors, gas tube-varistor, filter)
  • Features (redundancy, differential mode, plug-in, remote signaling…).
  • The surge protection selection must be done following the local electrical code requirements (i.e.: minimum rating for In) and specific conditions (i.e. : high lightning density).

Working Principle of LA:

  • The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves, which may reach the terminal apparatus. The lightning arresters or surge diverts provide protection against such surges. A lightning arrester or a surge diverted is a protective device, which conducts the high voltage surges on the power system to the ground.
  • The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves, which may reach the terminal apparatus. The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverted is a protective device, which conducts the high voltage surges on the power system to the ground.

  • Fig shows the basic form of a surge diverter. It consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc. The property of the non-linear resistance is that its resistance increases as the voltage (or current) increases and vice-versa. This is clear from the volt/amp characteristic of the resistor shown in Fig
  • The action of the lightning arrester or surge divert er is as under:
  • (i) Under normal operation, the lightning arrester is off the line i.e. it conducts no current to earth or the gap is non-conducting
  • (ii) On the occurrence of over voltage, the air insulation across the gap breaks down and an arc is formed providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.
  • (iii) It is worthwhile to mention the function of non-linear resistor in the operation of arrester. As the gap sparks over due to over voltage, the arc would be a short-circuit on the power system and may cause power-follow current in the arrester. Since the characteristic of the resistor is to offer low resistance to high voltage (or current), it gives the effect of short-circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting.

Type of LA for Outdoor Applications:

  • There are several types of lightning arresters in general use. They differ only in constructional details but operate on the same principle, providing low resistance path for the surges to the round.
  • 1. Rod arrester
  • 2. Horn gap arrester
  • 3. Multi gap arrester
  • 4. Expulsion type lightning arrester
  • 5. Valve type lightning arrester

(1) Rod Gap Arrester

  • It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in between as shown in Fig.
  • One rod is connected to the line circuit and the other rod is connected to earth. The distance between gap and insulator (i.e. distance P) must not be less than one third of the gap length so that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators.
  • The string of insulators for an overhead line on the bushing of transformer has frequently a rod gap across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly conducted to earth

Limitations:

  • (i) After the surge is over, the arc in the gap is maintained by the normal supply voltage, leading to short-circuit on the system.
  • (ii) The rods may melt or get damaged due to excessive heat produced by the arc.
  • (iii) The climatic conditions (e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester.
  • (iv) The polarity of the f the surge also affects the performance of this arrester.
  • Due to the above limitations, the rod gap arrester is only used as a back-up protection in case of main arresters.

(2) Horn Gap Arrester:

  • Fig shows the horn gap arrester. It consists of a horn shaped metal rods A and B separated by a small air gap. The horns are so constructed that distance between them gradually increases towards the top as shown.
  • The horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance and choke coil L while the other end is effectively grounded.
  • The resistance R helps in limiting the follow current to a small value. The choke coil is so designed that it offers small reactance at normal power frequency but a very high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be protected.
  • The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc across the gap.

  • Under normal conditions, the gap is non-conducting i.e. normal supply voltage is insufficient to initiate the arc between the gap. On the occurrence of an over voltage, spark-over takes place across the small gap G. The heated air around the arc and the magnetic effect of the arc cause the arc to travel up the gap. The arc moves progressively into positions 1, 2 and 3.
  • At some position of the arc (position 3), the distance may be too great for the voltage to maintain the arc; consequently, the arc is extinguished. The excess charge on the line is thus conducted through the arrester to the ground.

(3) Multi Gap Arrester:

  • Fig shows the multi gap arrester. It consists of a series of metallic (generally alloy of zinc) cylinders insulated from one another and separated by small intervals of air gaps. The first cylinder (i.e. A) in the series is connected to the line and the others to the ground through a series resistance. The series resistance limits the power arc. By the inclusion of series resistance, the degree of protection against traveling waves is reduced.
  • In order to overcome this difficulty, some of the gaps (B to C in Fig) are shunted by resistance. Under normal conditions, the point B is at earth potential and the normal supply voltage is unable to break down the series gaps. On the occurrence an over voltage, the breakdown of series gaps A to B occurs.
  • The heavy current after breakdown will choose the straight – through path to earth via the shunted gaps B and C, instead of the alternative path through the shunt resistance.

  • Hence the surge is over, the arcs B to C go out and any power current following the surge is limited by the two resistances (shunt resistance and series resistance) which are now in series. The current is too small to maintain the arcs in the gaps A to B and normal conditions are restored. Such arresters can be employed where system voltage does not exceed 33kV.

(4) Expulsion Type Arrester:

  • This type of arrester is also called ‘protector tube’ and is commonly used on system operating at voltages up to 33kV. Fig shows the essential parts of an expulsion type lightning arrester.
  • It essentially consists of a rod gap AA’ in series with a second gap enclosed within the fiber tube. The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth. One expulsion arrester is placed under each line conductor. Fig shows the installation of expulsion arrester on an overhead line.

  • On the occurrence of an over voltage on the line, the series gap AA’ spanned and an arc is stuck between the electrodes in the tube. The heat of the arc vaporizes some of the fiber of tube walls resulting in the production of neutral gas. In an extremely short time, the gas builds up high pressure and is expelled through the lower electrode, which is hollow. As the gas leaves the tube violently it carries away ionized air around the arc. This de ionizing effect is generally so strong that the arc goes out at a current zero and will not be re-established.

Advantages:

  • (i) They are not very expensive.
  • (ii)They are improved form of rod gap arresters as they block the flow of power frequency follow currents
  • (iii)They can be easily installed.

Limitations:

  • (i)An expulsion type arrester can perform only limited number of operations as during each operation some of the fiber material is used up.
  • (ii) This type of arrester cannot be mounted on enclosed equipment due to discharge of gases during operation.
  • (iii)Due to the poor volt/am characteristic of the arrester, it is not suitable for protection of expensive equipment

(5) Valve Type Arrester:

  • Valve type arresters incorporate non linear resistors and are extensively used on systems, operating at high voltages. Fig shows the various parts of a valve type arrester. It consists of two assemblies (i) series spark gaps and (ii) non-linear resistor discs in series. The non-linear elements are connected in series with the spark gaps. Both the assemblies are accommodated in tight porcelain container.
  • The spark gap is a multiple assembly consisting of a number of identical spark gaps in series. Each gap consists of two electrodes with fixed gap spacing. The voltage distribution across the gap is line raised by means of additional resistance elements called grading resistors across the gap. The spacing of the series gaps is such that it will withstand the normal circuit voltage. However an over voltage will cause the gap to break down causing the surge current to ground via the non-linear resistors.
  • The non-linear resistor discs are made of inorganic compound such as thyrite or metrosil. These discs are connected in series. The non-linear resistors have the property of offering a high resistance to current flow when normal system voltage is applied, but a low resistance to the flow of high surge currents. In other words, the resistance of these non-linear elements decreases with the increase in current through them and vice-versa.

Working.

  • Under normal conditions, the normal system voltage is insufficient to cause the break down of air gap assembly. On the occurrence of an over voltage, the breakdown of the series spark gap takes place and the surge current is conducted to earth via the non-linear resistors. Since the magnitude of surge current is very large, the non-linear elements will offer a very low resistance to the passage of surge. The result is that the surge will rapidly go to earth instead of being sent back over the line. When the surge is over, the non-linear resistors assume high resistance to stop the flow of current.

(6) Silicon carbide arresters:

  • A great number of silicon carbide arresters are still in service. The silicon carbide arrester has some unusual electrical characteristics. It has a very high resistance to low voltage, but a very low resistance to high-voltage.
  • When lightning strikes or a transient voltage occurs on the system, there is a sudden rise in voltage and current. The silicon carbide resistance breaks down allowing the current to be conducted to ground. After the surge has passed, the resistance of the silicon carbide blocks increases allowing normal operation.
  • The silicon carbide arrester uses nonlinear resistors made of bonded silicon carbide placed in series with gaps. The function of the gaps is to isolate the resistors from the normal steady-state system voltage. One major drawback is the gaps require elaborate design to ensure consistent spark-over level and positive clearing (resealing) after a surge passes. It should be recognized that over a period of operations that melted particles of copper might form which could lead to a reduction of the breakdown voltage due to the pinpoint effect. Over a period of time, the arrester gap will break down at small over voltages or even at normal operating voltages. Extreme care should be taken on arresters that have failed but the over pressure relief valve did not operate. This pressure may cause the arrester to

(7) Metal Oxide Arrestor:

  • The MOV arrester is the arrester usually installed today
  • The metal oxide arresters are without gaps, unlike the SIC arrester. This “gap-less” design eliminates the high heat associated with the arcing discharges.
  • The MOV arrester has two-voltage rating: duty cycle and maximum continuous operating voltage, unlike the silicon carbide that just has the duty cycle rating. A metal-oxide surge arrester utilizing zinc-oxide blocks provides the best performance, as surge voltage conduction starts and stops promptly at a precise voltage level, thereby improving system protection. Failure is reduced, as there is no air gap contamination possibility; but there is always a small value of leakage current present at operating frequency.
  • It is important for the test personnel to be aware that when a metal oxide arrester is disconnected from an energized line a small amount of static charge can be retained by the arrester. As a safety precaution, the tester should install a temporary ground to discharge any stored energy.
  • Duty cycle rating: The silicon carbide and MOV arrester have a duty cycle rating in KV, which is determined by duty cycle testing. Duty cycle testing of an arrester is performed by subjecting an arrester to an AC rms voltage equal to its rating for 24 minutes. During which the arrester must be able to withstand lightning surges at 1-minute intervals.
  • Maximum continuous operating voltage rating: The MCOV rating is usually 80 to 90% of the duty cycle rating.

Installation of LA:

  • The arrester should be connected to ground to a low resistance for effective discharge of the surge current.
  • The arrester should be mounted close to the equipment to be protected & connected with shortest possible lead on both the line & ground side to reduce the inductive effects of the leads while discharging large surge current.

Maintenance of LA:

  • Cleaning the outside of the arrester housing.
  • The line should be de-energized before handling the arrester.
  • The earth connection should be checked periodically.
  • To record the readings of the surge counter.
  • The line lead is securely fastened to the line conductor and arrester
  • The ground lead is securely fastened to the arrester terminal and ground.

Standard Makes for Electrical Equipments


Standard Makes for Electrical Equipments:

Sr. No. Item IS No. Makes
1 Transformer IS: 2026/1977 Crompton Greaves, NGEF, Kirloskar, BHEL, Andrewyule, Bharat Bijlee, Alsthom, ABB, Voltamp, Siemens, GEC, Voltas, TELK
2 11 kV/HT Vacuum Circuit Breaker, SF-6/11kV gas filled Circuit Breaker IS: 3427 for VCB GEC, Siemens, Andrew yule, Crompton Greave, Alsthom(Areva), Jyoti, ABB, BHEL, Alind, L&T,  Schneider, Biecco Lawrie.
3 ACB(11kV) IS: 13118/1991 Siemens, L&T, Crompton Greave, Schneider, Jyoti, GEC, MEI, ABB, Merlin Gerin, Moeller, Biecco Lawrie.
IS: 13947/1993
4 PSS/CSS with HT/LT switch gear, transformer and connected accessories IS:11171/85 for dry type transformer ABB, Siemens, L&T, Crompton Greave, BHEL, GEC, Kirloskar, Alsthom(Areva), Schneider
5 MCCBs, MCBs, ELCBS, RCCBs, DB, ICTPN, TP, HRC fuse, Changing over switch, Switch Fuse Unit IS: 8828/96 for MCB L&T, Crompton Greave, Siemens, Legrand, Jyoti, GEC, Andrew yule, BCH, Schneider, ABB, Moeller, Merlin Gerin
IS: 13947(Part-1&5/Sec1)/93for MCCB
IS: 12640(Part-1)/2000 for RCCB
IS: 13703(Part-2/Sec1)/93 for HRC fuse
IS: 13947(Part-3)/93 for SFU
6 XLPE Cable 11/33kV grade IS:7098(Part-2)/1985 Asian, NICCO, Universal, CCI, Torrent, Fort Gloster, INCAB, Industrial Cable, Polycab
7 PVC/XLPE Power Cables up to 1.1kV grade IS: 694/1990 for PVC cable CCI, Universal Cable,  Polycab, NICCO, Torrent, Asian, Fort Gloster, Finolex, Incab, Industrial Cable
IS: 1554(Part1)/1988 for heavy duty PVC cable
IS:7098(Part-1)/1988 for XLPE cable
8 Instrument Voltmeter, Ammeter, PF meter IS:1248 for Analog Automatic Electric, Meco, Industrial Meter, Motwani, Toshniwal, L&T, Siemens, Rishab, IMP, Shanti, Moeller(HPL).
9 11kV Cable End Termination & Jointing kits IS: 13573/92 Raychem, M-Seal, Safe system, Xicon brand of CCI, Hari consolidated(Cable seal brand), Densons(Yamuna)
10 Relays IS: 3231/65 Siemens, L&T, Alsthom, EA SUN REY Roll, ABB, BHEL, Jyoti, Alind, GE, BCH, Minilec, Enercon
11 Luminaries, MH, HPSV, T-5 fittings, CFL & related accessories IS: 9974(Part-1)/83 for HPSV Phillips, Crompton, Bajaj, GE, Osram, Wipro.
IS:10322(Part-5), 10322(Part2&3)/84 for Luminaries
IS:15111 for CFL
12 PVC insulated Elect. Wires Sheathed/ unsheathed, PVC flexible LT cable, multicore, single core, Flat cable for submersible pumps IS: 694/1990 for PVC cable Finolex, Asian,  Fort Gloster, CCI, NICCO, Universal,  Incab, Torrent, Uniflex, RPG, Polycab, ICL, Unistar
13 Current Transformer IS: 2705(Part3)/92 Automatic Electric, CGL, C&S, MECO, KAPPA, Siemens, L&T, Schneider
14 On line UPS, Servo Stabilizer, Inverter, CVT IS:13314/92 for Inverter AEI, BHEL, Hind Rectifier, L&T, NGEF, Siemens, Hi-rel, Autometer,  Enertech, Pyramid, APC, Dubas, Luminous, Microtech , TATA Libert
IS:11260/85 for voltage Stabilizer
15 Rotary Switches. Selector Switches Relevant IS Kaycee, L&T, Salzer, GE, ABB, C&S, Siemens, HPL, Moeller
16 Exhaust fan/Air Circulator/ Bracket & Pedestal fans/Ceiling fan IS: 374/79 for ceiling fan Crompton, Khaitan, GEC, Usha, Philips, Bajaj, Polar, Orient, Almonard.
IS: 2312/63 for Exhaust fan
IS:875(Part3), BSTN-10025/1993, CPE III TRT/1996 of ILE UK.
Octagonal Pole
S355JO.
Galvanization
IS: 2629
BSEN ISO-1461
18 Electronic Energy Meter IS:13779/1999 L&T, IMP,HPL, Secure, ABB, Enercon,
IEC:62053-21
19 Central Air Conditioning Plants & Package type plant IS:8148 for package type. Voltas, Blue Star, Sidwal, Fedder Lloyd, Videocon, Amtrex, Carrier, Frick, Shriram, Hitachi, O General, Mitsubhisi
IS:1391 for Room Air Conditioners.
20 Capacitors- PF correction for Electrical General Services IS:13340/93 ABB, BHEL, Indian Capacitors, Khatau Junker, Shreem, Unistar, WS Insulators, L&T, Hind Rectifier, Voltas, Siemens, Schneider, Indian Condenser, EPCOS
IS:13341/92
21 DG Sets- Portable IS: 13364(Part-1)/92 for Alternator Birla Yamaha, CGL, Shriram Honda
IS:1001/91 for Diesel Engine
22 DG Engine IS:13364 Cummins, Kirloskar, Caterpillar, Ashok Leylend, Penta-volvo
23 Alternator for DG set IS:4722/2001 KEC, CGL, Stampford, Leroy-somer, Kirloskar-Green
IS:4728/1975
24 Induction Motor IS:235/96 Bharat Bijlee, BHEL, CGL, GE, Jyoti, Kirloskar, Siemens, ABB, NGEF, Alsthom
IS:12615/2004
25 LT Switchgear & control gears- Contactors & motor starters, Energy Efficient Soft Starter panel/ Earthing Switch, Single phase preventer IS:13947(Part1)/1993 ABB, CGL, Jyoti, L&T, MEI,  NGEF,  Siemens, Telemecanique & control (India) (TC), Legrand(MDS), BCH, Standard, GEC, BHEL, Minilec, Enercon, Andrewyule, C&S, N.N. Planner, Power Boss, Schneider.
IS:13947(Part4)/1993
26 Pumps- Submersible IS: 8034/2002 for submersible pumpsets IS: 9283/1995 for motors of submersible pumpsets Calama, CGL, Jyoti, Kirloskar, KSB, TEXMO, SABAR
IS: 14220/1994 for open well submersible pumpsets
27 Timers- electronic solid state Relevant IS ABB, BHEL, GE, Jyoti, L&T, BCH, Siemens, Minilec, Legrand
28 Water Coolers IS: 1475/2001 Blue Star, Fedders, Kelvinator, Shriram, Sidwal, Voltas
29 Electrical accessories (Piano switch, Plugs & sockets, ceiling rose, Angle holder, holders,) IS: 3854/88 for switches SSK(Top line),  Anchor(Penta-ornet), Precision(Prime), Vinay(Clair-30), CONA(Nice-Indian), Leader, Legrand, ABB
IS: 1293/88 for plugs& sockets
IS: 371/79 for ceiling rose
IS: 1258/79 for lamp holder bakelite
30 Bell Buzzer IS:2268/1988 or latest CONA, MAX, Anchor, Leader, SSK
31 Electronic fan regulator IS:11037/1984 Anchor, Usha, ERIK, Rider
32 Solar cell/Module system Relevant IS TATA BP, BEL, BHEL, REIL, MOSER BEAR, CEL, Sharp Business Systems (India) Ltd.
33 Solar Lighting system Relevant IS DGS&D approved vendors on RC
34 GI/MS Pipe IS: 1239(Part-1)/90 TATA, Jindal, TT Swastik, Prakash Surya
35 Geysers IS:2082/93 Bajaj, Usha, Crompton , Spherehot, Recold, Venus.
36 Lifts & Escalators IS:14665/2000 OTIS, Thysson Krup, Shindler, KONE, Mitsubhisi.
37 LEDs Relevant IS BHEL, Indo Asian Fusegear Ltd., Switch control (India), Siemens, GE, ABB, L&T, Moeller.
38 Solar Water Heaters Relevant IS As per MNES approved sources.
39 Solar Distilled Water Plants Relevant IS Makes can be approved on the recommendation of divisions.
41 Air Cooling Plants Relevant IS for its concern equipments Voltas, Blue Star, Carrier
42 Battery Charger for other than battery room for Train Lighting IS:2026 Hind Rectifier, Usha Rectifier, Suresh Electrical, Pyramid, Automatic Electric, Delta Elect., Trinity Elect., Universal Ind. Products, Venus Engg., RS Power.
IS:3895
43 Battery Charger for battery room As per RDSO specification having re-generation facility Amar Raja, Exide, RS Power
44 PVC Conduit pipe & Casing capping for electrical wiring IS:9537/93 Precision, A.K.G., Polycab, Finolex , Prestoplast
45 Aluminum Ladders IS:4571/1977 Sumer, Beatfire
46 LT Panels As per Relevant IEC From CPRI tested firms.
47 Air Curtain Relevant IS Aircon, ALMONARD, Technocrate, Thermadyne, Mitzwak
48 Lighting Arrestor IS:5621/1980 with latest amendment/ revision (specification for hollow insulators for use in electrical equipment) (a) Jayshree Make – JSI (b) WSI (c) Shesashayee (d) BHEL (e) Luster Ceramics (f) Shakti make (g) Oblum make. Any other make approved by the PGVCL/GUVNL.
Creepage Distance : 11KV – 320 mm. 22KV – 600 mm.
49 Power transformer   BHEL, Bharat Bijlee, EMCO, Kanohar
50 66KV Circuit Breaker   ABB, Siemens, AREVA
51 66KV Isolator   ABB, AREVA.
52 66KV Current Transformer   ABB, CGL, BHEL, TELK
53 66KV CVT   ABB, TELK, CGL, BHEL
54 66KV LA   ABB, CGL
55 Control and Relay Panel   ABB, AREVA, Siemens
56 1.1KV Grade control and power Cable   Universal, KEI, GEMSCAB, Polycab, Paramount, CCI, Torrent, Krishna, HVPL
57 Batteries   Exide, Amco, Amaraja, HBL nife
58 Battery Charger   Chhabi, HBL, Caldyne, Masstech, DUBAS, Statcon
59 Station Transformer Transformer   CGL, Kotson, Voltamp, Kanohar, EMCO, AREVA,
60 Fire Protection system   Vijay, unitech, Technofab, Mather & Platt, Steelage.
61 11KV switchgear   ABB, AREVA, Siemens
       

Power Quality


Power Quality:

  • In the present scenario of power utilization and consumption, the importance of power quality is vital for a continuous and effective power supply. The features of power quality play a major role in the effective power utilization along with the control & improvement measures for various factors affecting it.
  • Power quality is defined as the ability of a system to

1.     Deliver electric power service of sufficiently high quality so that the end-use equipment will operate within their design specifications and

2.     It should be of sufficient reliability so that the operator of end-use equipment will be continuous.

  • In other words it may be defined as the concept of powering, grounding and protecting electric equipment in a manner that is suitable to the operation of that equipment.

Why is it a concern?

  • Power Quality has been a problem since the conception of electricity, but only over the last 2 decades has it gotten considerable attention with the introduction of large numbers of computers & microprocessors in business and homes; and the network revolution and ever increasing equipment capability and speed.
  • There are various factors that really make us think about it.

1.     Power quality problems can cause equipment malfunctions, excessive wear or premature, failure of equipment, increased costs, increased maintenance, repair time and expense & outside consultant expense.

2. Electronic equipments are more sensitive to minor fluctuations. We rely on the equipment more and have higher expectations. New electronic devices are more sensitive than the equipment being replaced as well.

Power Quality Affecting Factor:

  • Many electronic devices are susceptible to power quality problems and a source of power quality problems. Some of the important concerns are

1.     Waveform Distortions like Harmonics

2.     Transients

3.     Voltage Fluctuations such as Voltage Sags & Swells

4.     Interruptions e.g. Outages & Blinks

1. Waveform Distortions -Harmonics

  • Due to substantial increase of non-linear loads such as the use of power electronics circuits and devices, the ac power system suffers from harmonic problems. In general, we may classify sources of harmonics into three categories i.e.

1.     Domestic loads,

2.     Industrial loads,

3.     Control devices.

  • A harmonic is “a sinusoidal component of a periodic wave or quantity having a frequency i.e. an integral multiple of fundamental frequency”. Pure or clean power is referred as those without harmonics. But this only exists in laboratories. The frequencies of the harmonics are different, depending on the fundamental frequency. Due to high harmonic voltage and/or current levels, there are a number of equipments that can have miss operation or failures.
  • The main sources of harmonic current are the phase angle controlled rectifiers and inverters.
  • Although the applied voltage to a transformer is sinusoidal, the magnetization current related to the flux through the lamination magnetization curve is non-sinusoidal. These harmonics have their maximum effect during the first hours of the day (when the system is lightly loaded and the voltage is higher).

2. Transients

  • Transients occur in Distribution System due to factors like Lightning, Switching Operations, and Fault Clearing/Breaker Operations etc. The various causes of transients in Customer System are Lightning, Arcing Devices, Starting & Stopping Motors, Breaker Operations, and Capacitor Switching etc.

3. Voltage Fluctuations such as Voltage Sags & Swells

  • In Sags, Voltage falls below 90% of normal but stays above 10% of normal for any amount of time. In Swells, Voltage rises above 110% of normal but below 180% of normal for any amount of time. If it’s long enough, you notice lights dimming or getting brighter. Sags are much more common than swells.

4. Interruptions e.g. Outages and Blinks

  • Interruptions may be defined as the interrupts that hampers the normal flow of voltage or power quality. When Voltage falls below 10% of normal circuit voltage for any length of time the power supply is off. The outages can be of microseconds to hours or days. When interruptions occur there is a chance of blinking as well.

Control & improvement of The System:

  • In order to overcome the various affecting factors, we need to implement some control and improvement measures. They are discussed as follows:

1. Harmonics

  • Several techniques are adopted to minimize harmonic effects like increasing pulse number, passive filters and active filters. By use of these techniques we get higher pulse, trap the harmonics and convert the non-linear ac line current into a sinusoidal wave respectively.
  • Power quality analysis is really a matter of concern as it is quite evident how important supply of power is especially in organizations where critical loads need continuous supply of clean power and that too without any disruption.
  • Technological advancements are developing in this sector in order to manage the advanced and sophisticated power systems with utmost proficiency.

2. Transients

  • We can use power enhancers like Surge Suppressors, Lightning Protection/Arrestors, Power Conditioning, Line Reactors/Chokes etc. Power Synthesizers such as Standby Power Systems, UPS & Motor Generator Set can be utilized. Simplest, least expensive way to condition power by clamping voltage when it exceeds a certain level and sending it away from the equipment it protects.
  • Transient voltage surge suppressors (TVSS) can be installed at the terminals of the sensitive electronic loads. Power line filters limit noise and transients to a safe level by slowing down the rate of change of these problems and keeping electronic systems safer than surge protectors can.

3. Voltage Fluctuations such as Voltage Sags & Swells

  • Use of Power Enhancers like Reduced Voltage Starters on large offending motors, Voltage Regulators, Constant Voltage Transformers (CVTs), Power Conditioners; as well as Power Synthesizers like UPS, Motor-Generator Sets can minimize voltage fluctuations.
  • Voltage Regulators can be utilized to maintain voltage output within a desired limit or tolerance regardless how much input voltage varies. They can also be utilized for protection against swells or noise and limited protection from fast voltage changes depending upon the response time of the regulator. Voltage regulators respond best to slow changes in voltage.
  • Constant Voltage Transformers (CVT’s), also known as Ferro resonant transformers are used for sags, swells, longer term over and under-voltages, especially attractive for constant, low-power loads like electronic controllers (PLC’s) where they provide ride-through capability. Variable loads, especially those with high inrush currents, (Drives) present more of a problem for CVT’s.

Electrical Motor Connection


ELECTRICAL MOTOR CONNECTION:

 

How to Change Rotation of Motor in Clockwise Direction

No Present Motor Connection: Change Direction in Clockwise
1 R Phase Connected to U1 W2 R Phase Connected to U1 V2
Y Phase Connected to V1 U2 Y Phase Connected to V1 W2
B Phase Connected to W1 V2 B Phase Connected to W1 U2
2 R Phase Connected to W1 V2 R Phase Connected to W1 U2
Y Phase Connected to U1 W2 Y Phase Connected to U1 V2
B Phase Connected to V1 U2 B Phase Connected to V1 W2
3 R Phase Connected to V1 U2 R Phase Connected to V1 W2
Y Phase Connected to W1 V2 Y Phase Connected to W1 U2
B Phase Connected to U1 W2 B Phase Connected to U1 V2

Change Rotation in Anticlockwise Direction

No Present Motor Connection: Change Direction in Anticlockwise
1 R Phase Connected to U1 V2 R Phase Connected to U1 W2
Y Phase Connected to W1 U2 Y Phase Connected to W1 V2
B Phase Connected to V1 W2 B Phase Connected to V1 U2
2 R Phase Connected to W1 U2 R Phase Connected to W1 V2
Y Phase Connected to V1 W2 Y Phase Connected to V1 U2
B Phase Connected to U1 V2 B Phase Connected to U1 W2
3 R Phase Connected to V1 W2 R Phase Connected to V1 U2
Y Phase Connected to U1 V2 Y Phase Connected to U1 W2
B Phase Connected to W1 U2 B Phase Connected to W1 V2

Thumb Rule :

Check Phase Winding Starting Phase and Connected ending Connection of That Phase winding to the one Phase after the Phase where Phase winding Starting lead is connected. (Ex If U1 is connected to R Phase than Connect U2 to B Phase, If V1 is connected to Y Phase than V2 should be connected to R Phase)

Ferranti Effect


What is Ferranti Effect

  • A long transmission line draws a substantial quantity of charging current. If such a line is open circuited or very lightly loaded at the receiving end, Receiving end voltage being greater than sending end voltage in a transmission line is known as Ferranti effect. All electrical loads are inductive in nature and hence they consume lot of reactive power from the transmission lines. Hence there is voltage drop in the lines. Capacitors which supply reactive power are connected parallel to the transmission lines at the receiving end so as to compensate the reactive power consumed by the inductive loads.
  • As the inductive load increases more of the capacitors are connected parallel via electronic switching. Thus reactive power consumed by inductive loads is supplied by the capacitors thereby reducing the consumption of reactive power from transmission line. However when the inductive loads are switched off the capacitors may still be in ON condition. The reactive power supplied by the capacitors adds on to the transmission lines due to the absence of inductance. As a result voltage at the receiving end or consumer end increases and is more than the voltage at the supply end. This is known as Ferranti effect.

Why does voltage rise on a long, unloaded transmission line?

  • The Ferranti Effect occurs when current drawn by the distributed capacitance of the transmission line itself is greater than the current associated with the load at the receiving end of the line. Therefore, the Ferranti effect tends to be a bigger problem on lightly loaded lines, and especially on underground cable circuits where the shunt capacitance is greater than with a corresponding overhead line. This effect is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. As this voltage drop affects the sending end voltage, the receiving end voltage becomes greater. The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied.
  • The Ferranti Effect is not a problem with lines that are loaded because line capacitive effect is constant independent of load, while inductance will vary with load. As inductive load is added, the VAR generated by the line capacitance is consumed by the load.

How to Reduce Ferranti Effect:

Shunt Reactors and Series Capacitors:

  • The need for large shunt reactors appeared when long power transmission lines for system voltage 220 kV & higher were built. The characteristic parameters of a line are the series inductance (due to the magnetic field around the conductors) & the shunt capacitance (due to the electrostatic field to earth).

  • Both the inductance & the capacitance are distributed along the length of the line. So are the series resistance and the admittance to earth. When the line is loaded, there is a voltage drop along the line due to the series inductance and the series resistance. When the line is energized but not loaded or only loaded with a small current, there is a voltage rise along the line (the Ferranti-effect)
  • In this situation, the capacitance to earth draws a current through the line, which may be capacitive. When a capacitive current flows through the line inductance there will be a voltage rise along the line.
  • To stabilize the line voltage the line inductance can be compensated by means of series capacitors and the line capacitance to earth by shunt reactors. Series capacitors are placed at different places along the line while shunt reactors are often installed in the stations at the ends of line. In this way, the voltage difference between the ends of the line is reduced both in amplitude and in phase angle.
  • Shunt reactors may also be connected to the power system at junctures where several lines meet or to tertiary windings of transformers.
  • Transmission cables have much higher capacitance to earth than overhead lines. Long submarine cables for system voltages of 100 KV and more need shunt reactors. The same goes for large urban networks to prevent excessive voltage rise when a high load suddenly falls out due to a failure.
  • Shunt reactors contain the same components as power transformers, like windings, core, tank, bushings and insulating oil and are suitable for manufacturing in transformer factories. The main difference is the reactor core limbs, which have non-magnetic gaps inserted between packets of core steel.
  • 3-phase reactors can also be made. These may have 3- or -5-limbed cores. In a 3-limbed core there is strong magnetic coupling between the three phases, while in a 5-limbed core the phases are magnetically independent due to the enclosing magnetic frame formed by the two yokes and the two unwound side-limbs.
  • The neutral of shunt reactor may be directly earthed, earthed through an Earthing-reactor or unearthed.
  • When the reactor neutral is directly earthed, the winding are normally designed with graded insulation in the earthed end. The main terminal is at the middle of the limb height, & the winding consists of two parallel-connected halves, one below & one above the main terminal. The insulation distance to the yokes can then be made relatively small. Sometimes a small extra winding for local electricity supply is inserted between the main winding & yoke.
  • When energized the gaps are exposed to large pulsation compressive forced with a frequency of twice the frequency of the system voltage. The peak value of these forces may easily amount to 106 N/m2 (100 ton /m2). For this reason the design of the core must be very solid, & the modulus of elasticity of the non-magnetic (& non-metallic) material used in gaps must be high (small compression) in order to avoid large vibration amplitudes with high sound level consequently. The material in the gaps must also be stable to avoid escalating vibration amplitudes in the end.
  • Testing of reactors requires capacitive power in the test field equal to the nominal power of the reactor while a transformer can be tested with a reactive power equal to 10 – 20% of the transformer power rating by feeding the transformer with nominal current in short –circuit condition.
  • The loss in the various parts of the reactor (12R, iron loss & additional loss) cannot be separated by measurement. It is thus preferable, in order to avoid corrections to reference temperature, to perform the loss measurement when the average temperature of the winding is practically equal to the reference temperature.

How does a phase shifting transformer help operators load and unload transmission lines?

  • Power flow between two buses can be expressed as:
  • Power Flow = (Vs*Vr / X) * Sine of the Power Angle.
  • In other words: power flow (in watts) between two buses will be equal to the voltage on the sending bus multiplied by the voltage on the receiving bus divided by the line reactance, multiplied by the sine of the power angle between the two buses.
  • This leaves grid operators with at least two options for making a path more conducive to power flow, or if desired, making a path look less conducive to power flow. The two options are to (1) adjust line reactance and (2) adjust power angle. The Phase Shifting Transformer (PST) affects the second option, i.e. adjusting power angle.
  • The physical appearance of the PST device is noteworthy, being one of the few transformer types where the physical height and construction of the primary bushings is the same as the secondary bushings. This makes sense since both bushing sets are at the same potential. Internally, the primary voltage of a PST is bussed directly to the secondary bushings, with one important addition. The primary voltage is applied to a delta-wound transformer primary that has adjustable taps that inject “opposing phase” signals. For instance the A-B primary winding has a C phase injection, the B-C winding is injected with A, and the C-A winding is injected with B. These injection points are simultaneously adjustable taps that result in an adjustable shift of power angle.
  • Since power angle is a direct contributor to the Power Flow formula provided above (in the numerator, not the denominator), changing the PST tap settings can increase power angle making the path more conducive to power flow. The PST tap settings can also decrease power angle making the path less conducive to power flow. (Remember that “power flows downhill on angle”.)
  • Why is this important? Many transmission paths naturally have less impedance by virtue of their construction and length, and these paths can carry scheduled flow as well as unscheduled flow from parallel (but higher impedance) paths. In some cases these low impedance paths become congested and PST devices and other devices and techniques may be used to relieve the congestion. This is particularly the case in regions where transmission paths are less densely developed.

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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