Current Transformer


Current transformers

Principle of operation of CT

  • A current transformer is defined as “as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections.”
  • Current transformers are usually either “measuring” or “protective” types.

Some Definitions used for CT:

1)    Rated primary current:

  • The value of primary current which appears in the designation of the transformer and on which the performance of the current transformer is based.

2)    Rated secondary current:

  • The value of secondary current which appears in the designation of the transformer and on which the performance of the current transformer is based.
  • Typical values of secondary current are 1 A or 5 A. In the case of transformer differential protection, secondary currents of 1/ root 3 A and 5/ root 3 A are also specified.

3)    Rated burden:

  • The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards)

4)    Rated output:

  • The value of the apparent power (in volt-amperes at a specified power (factor) which the current transformer is intended to supply to the secondary circuit at the rated secondary current and with rated burden connected to it.

5)    Accuracy class:

  • In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3.
  • This means that the errors have to be within the limits specified in the standards for that particular accuracy class.
  • The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor.
  • In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT.

6)    Current Ratio Error:

  • The error with a transformer introduces into the measurement of a current and which arises from the fact that actual transformation ratio is not equal to the rated transformer ratio. The current error expressed in percentage is given by the formula:
  • Current error in % = (Ka(Is-Ip)) x 100 / Ip
  • Where Ka= rated transformation ratio ,Ip= actual primary current, Is= actual secondary current when Ip is flowing under the conditions of measurement

7)    Accuracy limit factor:

  • The value of primary current up to which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated primary current.

8)    Short time rating:

  • The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds.

9)    Instrument security factor (factor of security):

  • This typically takes a value of less than 5 or less than 10 though it could be much higher if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error of the metering CT at 5 times the rated primary current is equal to or greater than 10%. This means that heavy currents on the primary are not passed on to the secondary circuit and instruments are therefore protected. In the case of double ratio CT’s, FS is applicable for the lowest ratio only.

10) Class PS X CT:

  • In balance systems of protection, CT s with a high degree of similarity in their characteristics is required. These requirements are met by Class PS (X) CT s. Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Imag) at the knee point voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is defined in terms of the turn’s ratio.

11) Knee point voltage:

  • That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current).
  • The ‘Knee Point Voltage’ (Vkp) is defined as the secondary voltage at which an increase of 10% produces an increase in magnetizing current of 50%. It is the secondary voltage above which the CT is near magnetic saturation.

12) Core balance CT (CBCT):

  • The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting and the primary operating current need to be furnished.

13) Phase displacement:

  • The difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for the perfect transformer. The phase displacement is said to be positive when the secondary current vector leads the primary current vector. It is usually express in minutes

14) Highest system voltage:

  • The highest rms line to line voltage which can be sustained under normal operating conditions at any time and at any point on the system. It excludes temporary voltage variations due to fault condition and the sudden disconnection of large loads.

15) Rated insulation level:

  • That combination of voltage values (power frequency and lightning impulse, or where applicable, lightning and switching impulse) which characterizes the insulation of a transformer with regard to its capability to withstand by dielectric stresses. For low voltage transformer the test voltage 4kV, at power-frequency, applied during 1 minute.

16) Rated short-time thermal current (Ith):

  • The rms value of the primary current which the current transformer will withstand for a rated time, with their secondary winding short circuited without suffering harmful effects.

17) Rated dynamic current (Idyn):

  • The peak value of the primary current which a current transformer will withstand, without being damaged electrically for mechanically by the resulting electromagnetic forces, the secondary winding being short-circuited.

18) Rated continuous thermal current (Un)

  • The value of current which can be permitted to flow continuously in the primary winding, the secondary windings being connected to the rated burdens, without the temperature rise exceeding the specified values.

19) Instrument security factor (ISF or Fs):

  • The ratio of rated instrument limits primary current to the rated primary current. The times that the primary current must be higher than the rated value, for the composite error of a measuring current transformer to be equal to or greater than 10%, the secondary burden being equal to the rated burden. The lower this number is, the more protected the connected instrument are against. 

20) Sensitivity

  • Sensitivity is defined as the lowest value of primary fault current, within the protected zone, which will cause the relay to operate. To provide fast operation on an in zone fault, the current transformer should have a ‘Knee Point Voltage’ at least twice the setting voltage of the relay.

21) Field Adjustment of Current Transformer Ratio:

  • The ratio of current transformers can be field adjusted to fulfil the needs of the application.  Passing

more secondary turns or more primary turns through the window will increase or decrease the turns ratio.  

Actual Turns Ratio = (Name Plate Ration- Secondary Turns Added) / Primary Turns.

Types of Current transformers (CT’s)

 According to Construction of CT:

1)    Bar Type:

  • Bar types are available with higher insulation levels and are usually bolted to the current caring device.

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  • Bar type current transformers are insulated for the operating voltage of the system.
  • Bar-type CTs operate on the same principle of window CTs but have a permanent bar installed as a primary conductor

2)    Wound CT’s:

  • Capacity: There are designed to measure currents from 1 amp to 100 amps.
  • the most common one is the wound type current transformer. The wound type provides excellent performance under a wide operating range. Typically, the wound type is insulated to only 600 volts.

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  • Since the load current passes through primary windings in the CT, screw terminals are provided for the load and secondary conductors. Wound primary CT’s are available in ratios from 2.5:5 to 100:5.
  • Wound CTs have a primary and secondary winding like a normal transformer. These CTs are rare and are usually used at very low ratios and currents, typically in CT secondary circuits to compensate for low currents, to match different CT ratios in summing applications, or to isolate different CT circuits. Wound CTs have very high burdens, and special attention to the source CT burden should be applied when wound CTs are used.

3)    Window:

  • Window CTs are the most common. They are constructed with no primary winding and are installed around the primary conductor. The electric field created by current flowing through the conductor interacts with the CT core to transform the current to the appropriate secondary output. Window CTs can be of solid or split core construc­tion. The primary conductor must be disconnected when installing solid window CTs. However, split core CTs can be installed around the primary conductor without disconnecting the primary conductor

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  • Ring Core CT’s :
  • Capacity: There are available for measuring currents from 50 to 5000 amps

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  • Size: with windows (power conductor opening size) from 1″ to 8″ diameter.
  • Split Core CT’s:
  • Capacity: There are available for measuring currents from 100 to 5000 amps.
  • Size:  with windows in varying sizes from 1″ by 2″ to 13″ by 30″.
  • Split core CT’s have one end removable so that the load conductor or bus bar does not have to be disconnected to install the CT.

4)    Bushing

  • Bushing CTs are window CTs specially constructed to fit around a bush­ing. Usually they cannot be accessed, and their nameplates are found on the transformer or circuit-breaker control cabinets.
  • The bushing type is typically used around the bushing on circuit breakers and transformers and may not have a hard protective outside cover. 
  • Donut type current transformers are typically insulated for 600 volts. To ensure accuracy, the conductor should be positioned in the center of the current transformer opening.

 According to Application of CT:

1)    Measuring CT:

  • The principal requirements of a measuring CT are that, for primary currents up to 120% or 125% of the rated current, its secondary current is proportional to its primary current to a degree of accuracy as defined by its “Class” and, in the case of the more accurate types, that a specified maximum phase angle displacement is not exceeded.
  • A desirable characteristic of a measuring CT is that it should “saturate” when the primary current exceeds the percentage of rated current specified as the upper limit to which the accuracy provisions apply. This means that at these higher levels of primary current the secondary current is less than proportionate. The effect of this is to reduce the extent to which any measuring device connected to the CT secondary is subjected to current Overload.
  • On the other hand the reverse is required of the protective type CT, the principal purpose of which is to provide a secondary current proportional to the primary current when it is several, or many, times the rated primary current. The measure of this characteristic is known as the “Accuracy Limit Factor” (A.L.F.).
  •  A protection type CT with an A.L.F. of 10 will produce a proportional current in the secondary winding (subject to the allowable current error) with primary currents up to a maximum of 10 times the rated current.
  • It should be remembered when using a CT that where there are two or more devices to be operated by the secondary winding, they must be connected in series across the winding. This is exactly the opposite of the method used to connect two or more loads to be supplied by a voltage or power transformer where the devices are paralleled across the secondary winding.
  • With a CT, an increase in the burden will result in an increase in the CT secondary output voltage. This is automatic and necessary to maintain the current to the correct magnitude. Conversely, a reduction in the burden will result in a reduction in the CT secondary output voltage.
  • This rise in secondary voltage output with an increase in burden means that, theoretically, with infinite burden as is the case with the secondary load open circuit, an infinitely high voltage appears across the secondary terminals. For practical reasons this voltage is not infinitely high, but can be high enough to cause a breakdown in the insulation between primary and secondary windings or between either or both windings and the core. For this reason, primary current should never be allowed to flow with no load or with a high resistance load connected across the secondary winding.
  • When considering the application of a CT it should be remembered that the total burden imposed on the secondary winding is not only the sum of the burden(s) of the individual device(s) connected to the winding but that it also includes the burden imposed by the connecting cable and the resistance of the connections.
  • If, for example, the resistance of the connecting cable and the connections is 0.1 ohm and the secondary rating of the CT is 5A, the burden of the cable and connections (RI2) is 0.1 x 5 x 5 = 2.5VA. This must be added to the burden(s) of the connected device(s) when determining whether the CT has an adequately large burden rating to supply the required device(s) and the burden imposed by the connections.
  • Should the burden imposed on the CT secondary winding by the connected device(s) and the connections exceed the rated burden of the CT the CT may partly or fully saturate and therefore not have a secondary current adequately linear with the primary current.
  • The burden imposed by a given resistance in ohms [such as the resistance of a connecting cable] is proportional to the square of the rated secondary current. Therefore, where long runs of cable between CT and the connected device(s) are involved, the use of a 1A secondary CT and a 1A device rather than 5A will result in a 25-fold reduction in the burden of the connecting cables and connections. All burden ratings and calculations are at rated secondary current.
  • Because of the foregoing, when a relatively long [more than a very few meters] cable run is required to connect a CT to its burden [such as a remote ammeter] a calculation should be made to determine the cable burden. This is proportional to the “round trip” resistance, i.e. twice the resistance of the length of twin cable used. Cable tables provide information on the resistance values of different sizes of conductors at 20o C per unit length.

2)    Protective CT:

  • The calculated resistance is then multiplied by the square of the CT secondary current rating [25 for 5A, 1 for 1A]. If the VA burden as calculated by this method and added to the rated burden(s) of the device(s) to be driven by the CT exceeds the CT burden rating, the cable size must be increased [to reduce the resistance and thus the burden] or a CT with a higher VA burden rating must be used, or a lower CT secondary current rating [with matching change in the current rating of the device(s) to be driven] should be substituted

 Nomenclature of CT:

  1. Ratio: input / output current ratio
  2. Burden (VA): total burden including pilot wires. (2.5, 5, 10, 15 and 30VA.)
  3. Class: Accuracy required for operation (Metering: 0.2, 0.5, 1 or 3, Protection: 5, 10, 15, 20, 30).
  4. Accuracy Limit Factor:
  5. Dimensions: maximum & minimum limits
  6. Nomenclature of CT: Ratio, VA Burden, Accuracy Class, Accuracy Limit Factor.
  7. Example: 1600/5, 15VA 5P10  (Ratio: 1600/5, Burden: 15VA, Accuracy Class: 5P, ALF: 10)
  8. As per IEEE Metering CT: 0.3B0.1 rated Metering CT is accu­rate to 0.3 percent if the connected secondary burden if imped­ance does not exceed 0.1 ohms.
  9. As per IEEE Relaying (Protection) CT: 2.5C100 Relaying CT is accurate within 2.5 percent if the secondary burden is less than 1.0 ohm (100 volts/100A).

 1)   Current Ratio of CT:

  • The primary and secondary currents are expressed as a ratio such as 100/5. With a 100/5 ratio CT, 100A flowing in the primary winding will result in 5A flowing in the secondary winding, provided the correct rated burden is connected to the secondary winding. Similarly, for lesser primary currents, the secondary currents are proportionately lower.
  • It should be noted that a 100/5 CT would not fulfil the function of a 20/1 or a 10/0.5 CT as the ratio expresses the current rating of the CT, not merely the ratio of the primary to the secondary currents.
  • The rated secondary current is commonly 5A or 1A, though lower currents such as 0.5A are not uncommon. It flows in the rated secondary load, usually called the burden, when the rated primary current flows in the primary winding.
  • Increasing or Decreasing Turns Ratio of CT: 
  • Increasing Number of Turn: Increasing the number of primary turns can only decrease the turn’s ratio. A current transformer with a 50 to 5 turn’s ratio can be changed to a 25 to 5 turn’s ratio by passing the primary twice through the window. 
  • Increasing or Decreasing Turns Ratio:
  • The turn’s ratio can be either increased or decreased by wrapping wire from the secondary through the window of the current transformer.
  • Increasing the turn’s ratio with the secondary wire, turns on the secondary are essentially increased. A 50 to 5 current transformer will have a 55 to 5 ratio when adding a single secondary turn.
  • Decreasing the turn’s ratio with the secondary wire, turns on the secondary are essentially decreased.  A 50 to 5 current transformer will have a 45 to 5 ratio when adding a single secondary turn.
  • Decreasing the turn’s ratio with the primary, accuracy and VA burden ratings are the same as the original configuration.
  • Increasing the turn’s ratio with the secondary will improve the accuracy and burden rating.
  • Decreasing the turn’s ratio with the secondary will worsen the accuracy and burden rating.
  • When using the secondary of a current transformer to change the turn’s ratio, the right hand rule of magnetic fields comes into play.  Wrapping the white lead or the X1 lead from the H1 side of the transformer through the window to the H2 side will decrease the turn’s ratio.  Wrapping this wire from the H2 side to the H1 side will increase the turn’s ratio.
  • Using the black or X2 lead as the adjustment method will do the opposite of the X1(white) lead.  Wrapping from the H1 to the H2 side will increase the turns ratio, and wrapping from the H2 to the H1 side will decrease the turns ratio.

 2)   Burden of CT:

  • Common burden ratings of CT: 2.5, 5, 10, 15 and 30VA.
  • The external load applied to the secondary of a current transformer is called the “burden”.
  • The burden of CT is the maximum load (in VA) that can be applied to the CT secondary.
  • The burden can be expressed in two ways.
  • The burden can be expressed as the total impedance in ohms of the circuit or the total volt-amperes (VA) and power factor at a specified value of current or voltage and frequency.
  • Formerly, the practice was to express the burden in terms of volt-amperes (VA) and power factor, the volt-amperes being what would be consumed in the burden impedance at rated secondary current (in other words, rated secondary current squared times the burden impedance). Thus, a burden of 0.5Ωimpedance may be expressed also as “12.5 VA at 5 amperes,” if we assume the usual 5-ampere secondary rating. The VA terminology is no longer standard, but it needs defining because it will be found in the literature and in old data.

Burden for Measuring CT:

  • Total burden of Measuring CT = Sum of Meters Burden in VA (Ammeter, Wattmeter, Transducer etc.) connected in series to the CT secondary circuit + Connecting Secondary Circuit Cable Burden in VA.
  • Cable burden = I2 x R x2 L, where I = CT secondary current, R = cable resistance per length, 2L is the tro &fro distance of cable length L from CT to metering circuits. If the proper size and short length of wire is used, cable burden can be ignored.
  • The CT secondary circuit load shall not be more than the CT VA rating. If the load is less than the CT burden, all meters connected to the measuring CT should provide correct reading.
  • In the case of Measuring Current transformer, the burden depends on the connected meters and quantity of meters on the secondary i.e. no of Ammeters, KWh meters, Kvar meters, Kwh meters, transducers and also the connection cable burden (I2 x R x2 L) to metering shall be taken into account.
  • Note Meters burden can be obtained from manufacturer catalogue.
  • Selected CT burden shall be more than the calculated burden

Burden for Protecting CT:

  • In the case of Protection CTs the burden is calculated in the same way as above except the burden of individual protective relays burden shall be considered instead of meters. The connecting cable burden is calculated in the same way as metering CT
  • Total burden of Protection CT=Connecting cable Burden in VA + sum of Protective relays Burden in VA.
  • All manufacturers can supply the burden of their individual devices. Although not used very often these days, induction disk over-current devices always gave the burden for the minimum tap setting. To determine the impedance of the actual tap setting being used, First Square the ratio of minimum divide by the actual tap setting used and, second multiply this value by the minimum impedance.
  • Suppose an impedance of 1.47 + 5.34j at the 1A tap. To apply the relay at the 4A tap the engineer would multiply the impedance at the 1A taps setting by (1/4)2. The impedance at the 4A tap would be 0.0919 + 0.3338j or 0.3462 Z at 96.4 power factor.
  • The CT burden impedance decreases as the secondary current increases, because of saturation in the magnetic circuits of relays and other devices. Hence, a given burden may apply only for a particular value of secondary current. The old terminology of volt-amperes at 5 amperes is most confusing in this respect since it is not necessarily the actual volt amperes with 5 amperes flowing, but is what the volt-amperes would be at 5 amperes
  • If there were no saturation. Manufacturer’s publications give impedance data for several values of over current for some relays for which such data are sometimes required. Otherwise, data are provided only for one value of CT secondary current.
  •  If a publication does not clearly state for what value of current the burden applies, this information should be requested. Lacking such saturation data, one can obtain it easily by test. At high saturation, the impedance approaches the DC resistance. Neglecting the reduction in impedance with saturation makes it appear that a CT will have more inaccuracy than it actually will have. Of course, if such apparently greater inaccuracy can be tolerated, further refinements in calculation are unnecessary. However, in some applications neglecting the effect of saturation will provide overly optimistic results; consequently, it is safer always to take this effect into account.
  • It is usually sufficiently accurate to add series burden impedances arithmetically. The results will be slightly pessimistic, indicating slightly greater than actual CT ratio inaccuracy. But, if a given application is so borderline that vector addition of impedances is necessary to prove that the CTÕs will be suitable, such an application should be avoided.
  • If the impedance at pickup of a tapped over current-relay coil is known for a given pickup tap, it can be estimated for pickup current for any other tap. The reactance of a tapped coil varies as the square of the coil turns, and the resistance varies approximately as the turns. At pickup, there is negligible saturation, and the resistance is small compared with the reactance. Therefore, it is usually sufficiently accurate to assume that the impedance varies as the square of the turns. The number of coil turns is inversely proportional to the pickup current, and therefore the impedance varies inversely approximately as the square of the pickup current.
  • Whether CT is connected in wye or in delta, the burden impedances are always connected in wye. With wye-connected CT the neutrals of the CT and of the burdens are connected together, either directly or through a relay coil, except when a so-called zero phase-sequence-current shunt is used.
  • It is seldom correct simply to add the impedances of series burdens to get the total, whenever two or more CT are connected in such a way that their currents may add or subtract in some common portion of the secondary circuit. Instead, one must calculate the sum of the voltage drops and rises in the external circuit from one CT secondary terminal to the other for assumed values of secondary currents flowing in the various branches of the external circuit. The effective CT burden impedance for each combination of assumed currents is the calculated CT terminal voltage divided by the assumed CT secondary current. This effective impedance is the one to use, and it may be larger or smaller than the actual impedance which would apply if no other CTÕs were supplying current to the circuit.
  • If the primary of an auxiliary CT is to be connected into the secondary of a CT whose accuracy is being studied, one must know the impedance of the auxiliary CT viewed from its primary with its secondary short-circuited. To this value of impedance must be added the impedance of the auxiliary CT burden as viewed from the primary side of the auxiliary CT; to obtain this impedance, multiply the actual burden impedance by the square of the ratio of primary to secondary turns of the auxiliary CT. It will become evident that, with an auxiliary CT that steps up the magnitude of its current from primary to secondary, very high burden impedances, when viewed from the primary, may result.
  • Burden is depending on pilot lead length
  • For Metering Class CTs burden is expressed as ohms impedance. For Protection-class CTs burden is express as volt-amperes (VA).
VA Applications
1 To 2 VA Moving iron ammeter
1 To 2.5VA Moving coil rectifier ammeter
2.5 To 5VA Electrodynamics instrument
3 To 5VA Maximum demand ammeter
1 To 2.5VA Recording ammeter or transducer
  • Burden (VA) of copper wires between instrument & current transformer for 1A and 5A secondary’s
Cross Section (mm2)

CT  1 Amp Secondary Burden in VA (Twin Wire)

Distance

10 meter 20 meter 40 meter 60 meter 80 meter 100 meter

1.0

0.35

0.71

1.43

2.14

2.85

3.57

1.5

0.23

0.46

0.92

1.39

1.85

2.31

2.5

0.14

0.29

0.57

0.86

1.14

1.43

4.0

0.09

0.18

0.36

0.54

0.71

0.89

6.0

0.06

0.12

0.24

0.36

0.48

0.6

 

Cross Section (mm2)

CT  5 Amp Secondary Burden in VA (Twin Wire)

Distance

1 meter 2 meter 4 meter 6 meter 8 meter 10 meter

1.5

0.58

1.15

2.31

3.46

4.62

5.77

2.5

0.36

0.71

1.43

2.14

2.86

3.57

4.0

0.22

0.45

0.89

1.34

1.79

2.24

6.0

0.15

0.30

0.60

0.89

1.19

1.49

10.0

0.09

0.18

0.36

0.54

0.71

0.89

 CT Burden Calculation:

  • The Actual burden is formed by the resistance of the pilot conductors and the protection relay(s). The resistance of a conductor (with a constant cross-sectional area) can be calculated from the equation:
  • R =ƿxL / A
  • where ƿ  = resistivity of the conductor material (given typically at +20°C) ,L= length of the conductor , A = cross sectional area
  • If the resistivity is given in μΩm, the length in meters and the area in mm2, the equation 1 will give the resistance directly in ohms.
  • Resistivity: Copper 0.0178 µΩm at 20 °C and 0.0216 µΩm at 75 °C

Burden of CT for 4 or 6 wire connection:

  • If 6-wire connection is used, the total length of the wire, naturally, will be two times the distance between the CT and the relay.  However, in many cases a common return conductor is used as shown in figure then, instead of multiplying the distance by two, a factor of 1.2 is typically used. This rule only applies to the 3-phase connection only.  The factor 1.2 allows for a situation, where up to 20% of the electrical conductor length, including terminal resistances, uses 6-wire connection and at least 80% 4-wire connection.

  • Example: the distance between the CT and the relay is 5 meters the total length is 2 x 5 m = 10 meter for 6-wire connection, but only 1.2 x 5 m = 6.0 meter when 4-wire connection is used.

Burden of the relay:

  • Example: The Distance between the CTs and the protection relay is 15 meters, 4 mm2 Cu conductors in 4-wire connection are used. The burden of the relay input is less than 20 mΩ (5 A inputs). Calculate the actual burden of the CT at 75°C , the input impedance is less than 0.020 Ω for a 5 A input (i.e. burden less than 0.5 VA) and less than 0.100 Ω for a 1 A input (i.e. less than 0.1 VA):
  • Solution:
  • ƿ = 0.0216 µΩm (75°C) for copper conductor.
  • R =ƿxL / A ,R = 0.0216 µΩm x (1.2 x 15 m) / 4 mm2 = 0.097 Ω
  • Burden of CT = 0.097 Ω + 0.020 Ω = 0.117 Ω.
  • Using CTs of burden values higher than required, is unscientific since it leads to inaccurate reading (meter) or inaccurate sensing of fault / reporting conditions.
  • Basically, such high value of design burden extends saturation characteristics of CT core leading to likely damage to the meter connected across it under overload condition. e.g. When we expect security factor (ISF) to be 5, the secondary current should be restricted to less than 5 times in case primary current shoots to more than 5 times its rated value.
  • In such an overload condition, the core of CT is desired to go into saturation, restricting the secondary current thus the meter is not damaged. However, when we ask for higher VA, core doesn’t go into saturation due to less load (ISF is much higher than desired) which may damage the meter.
  • To understand the effect on Accuracy aspect, let’s take an example of a CT with specified burden of 15 VA, and the actual burden is 2.5 VA:15 VA CT with less than 5 ISF will have saturation voltage of 15 Volts (15/5×5), and actual burden of 2.5 VA the saturation voltage required shall be ( 2.5/5 x 5) 2.5 Volts against 15 Volts resulting ISF = 30 against required of 5.
  • Example: Decide  Whether 5A,20VA CT is sufficient for following circuit

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  • Total instrument burden = 2 + 2 + 3 + 2 + 4 = 13V A.
  • Total pilot load resistance = 2 x 0.1 = 0.2 ohm.
  • With 5A secondary current, volt-drop in leads is 5 x 0.2 = 1 V.
  • Burden imposed by both leads = 5A x 1 V = 5V A.
  • Total burden on CT = 13 + 5 = 18V A. 
  • As the CT is rated 20V A, it has sufficient margin.

3)   Accuracy Class of CT:

  • The CT accuracy is determined by its certified accuracy class which is stamped on nameplate. For example, CT accuracy class of 0.3 means that the CT is certified by the manufacturer to be accurate to within 0.3 percent of its rated ratio value for a primary current of 100 percent of rated ratio.
  • CT with a rated ratio of 200/ 5 with accuracy class of 0.3 would operate within 0.45 percent of its rated ratio value for a primary current of 100 amps. To be more explicit, for a primary current of 100A it is certified to produce a secondary current between 2.489 amps and 2.511 amps.
  • Accuracy is specified as a percentage of the range, and is given for the maximum burden as expressed in VA.  The total burden includes the input resistance of the meter and the loop resistance of the wire and connections between the current transformer and meter.
  • Example: Burden = 2.0 VA. Maximum Voltage drop = 2.0 VA / 5 Amps = 0.400 Volts.
  •  Maximum Resistance = Voltage / Current = 04.00 Volts / 5 Amps =0.080 Ohms.
  • If the input resistance of the meter is 0.010Ω, then 0.070Ω is allowed for loop resistance of the wire, and connections between the current transformer and the meter. The length and gauge of the wire must be considered in order to avoid exceeding the maximum burden.
  • If resistance in the 5 amp loop causes the burden to be exceeded, the current will drop. This will result in the meter reading low at higher current levels.
  • As in all transformers, errors arise due to a proportion of the primary input current being used to magnetize the core and not transferred to the secondary winding. The proportion of the primary current used for this purpose determines the amount of error.
  • The essence of good design of measuring current transformers is to ensure that the magnetizing current is low enough to ensure that the error specified for the accuracy class is not exceeded.
  • This is achieved by selecting suitable core materials and the appropriate cross-sectional area of core. Frequently in measuring currents of 50A and upwards, it is convenient and technically sound for the primary winding of a CT to have one turn only.
  • In these most common cases the CT is supplied with a secondary winding only, the primary being the cable or bus bar of the main conductor which is passed through the CT aperture in the case of ring CTs (i .e. single primary turn) it should be noted that the lower the rated primary current the more difficult it is (and the more expensive it is) to achieve a given accuracy.
  • Considering a core of certain fixed dimensions and magnetic materials with a secondary winding of say 200 turns (current ratio 200/1 turns ratio 1/200) and say it takes 2 amperes of the 200A primary current to magnetize the core, the error is therefore only 1% approximately. However considering a 50/1 CT with 50 secondary turns on the same core it still takes 2 amperes to magnetize to core. The error is then 4% approximately. To obtain a 1% accuracy on the 50/1 ring CT a much larger core and/or expensive core material is required
  • Accuracy Class of Metering CT:

Metering Class CT

Class Applications
0.1 To 0.2 Precision measurements
0.5 High grade kilowatt hour meters for commercial grade kilowatt hour meters
3 General industrial measurements
3 OR 5 Approximate measurements

  

Protective System CT Secondary VA Class
Per current for phase & earth fault 1A 2.5 10P20 Or 5P20
5A 7.5 10P20 Or 5P20
Unrestricted earth fault 1A 2.5 10P20 Or 5P20
5A 7.5 10P20 Or 5P20
Sensitive earth fault 1A or 5A   Class PX use relay manufacturers formula
Distance protection 1A or 5A   Class PX use relay manufacturers formula
Differential protection 1A or 5A   Class PX use relay manufacturers formula
High impedance differential impedance 1A or 5A   Class PX use relay manufacturers formula
High speed feeder protection 1A or 5A   Class PX use relay manufacturers formula
Motor protection 1A or 5A 5 5P10
  • Accuracy Class of  Letter of CT:

Metering Class CT

Accuracy Class Applications

B

Metering Purpose

Protection Class CT

C

CT has low leakage flux.

T

 CT can have significant leakage flux.

H

 CT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.)

L

 CT accuracy applies at the maximum rated secondary burden at 20 time rated only. The ratio accuracy can be up to four times greater than the listed value, depending on connected burden and fault current. (Typically window, busing, or bar-type CTs.)
  • Accuracy Class of Protection CT:
Class Applications
10P5 Instantaneous over current relays & trip coils: 2.5VA
10P10 Thermal inverse time relays: 7.5VA
10P10 Low consumption Relay: 2.5VA
10P10/5 Inverse definite min. time relays (IDMT) over current
10P10 IDMT Earth fault relays with approximate time grading:15VA
5P10 IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA
  •  Accuracy Class: Metering Accuracy as per IEEE C37.20.2b-1994

   

Ratio B0.1 B0.2 B0.5 B0.9 B1.8 Relaying Accuracy
50:5 1.2 2.4 C or T10
75:5 1.2 2.4 C or T10
100:5 1.2 2.4 C or T10
150:5 0.6 1.2 2.4 C or T20
200:5 0.6 1.2 2.4 C or T20
300:5 0.6 1.2 2.4 2.4 C or T20
400:5 0.3 0.6 1.2 1.2 2.4 C or T50
600:5 0.3 0.3 0.3 1.2 2.4 C or T50
800:5 0.3 0.3 0.3 0.3 1.2 C or T50
1200:5 0.3 0.3 0.3 0.3 0.3 C100
1500:5 0.3 0.3 0.3 0.3 0.3 C100
2000:5 0.3 0.3 0.3 0.3 0.3 C100
3000:5 0.3 0.3 0.3 0.3 0.3 C100
4000:5 0.3 0.3 0.3 0.3 0.3 C100

Important of accuracy & phase angle

  • Current error is an error that arises when the current value of the actual transformation ratio is not equal to rated transformation ratio.
  • Current error (%) = {(Kn x Is – Ip) x 100}/Ip
  • Kn = rated transformation ratio, Ip = actual primary current, Is = actual secondary current
  • Example: In case of a 2000/5A class 1 5VA current transformer
  • Kn = 2000/5 = 400 turn, Ip = 2000A, Is = 4.9A
  • Current error = ((400 x 4.9 – 2000) x100)/2000 = -2%
  • For protection class current transformer, the accuracy class is designed by the highest permissible percentage composite error at the accuracy limit primary current prescribed for the accuracy class concerned.
  • Accuracy class includes: 5P, 10P

By phase angle

  • Phase error is the difference in phase between primary & secondary current vectors, the direction of the vectors to be zero for a perfect transformer.
  • You will experience a positive phase displacement when secondary current vector lead primary current vector.
  • Unit of scale expressed in minutes / cent radians.
  • Circular measure = (unit in radian) is the ratio of the distance measured along the arc to the radius.
  • Angular measure = (unit in degree) is obtained by dividing the angle subtended at the center of a circle into 360 deg equal division known as “degrees”.
  • Limits of current error and phase displacement for measuring current transformer (Classes 0.1 To 1)

Accuracy

Class

+/- Percentage Current (Ratio) Error at % Rated Current

+/- Phase Displacement at % Rated Current

                  Minutes

Centi radians

5

20

100

120

5

20

100

120

5

20

100

120

0.1

0.4

0.2

0.1

0.1

15

8

5

5

0.45

0.24

0.15

0.15

0.2

0.75

0.35

0.2

0.2

30

15

10

10

0.9

0.45

0.3

0.3

0.5

1.5

0.75

0.5

0.5

90

45

30

30

2.7

1.35

0.9

0.9

1.0

3

1.5

1

1

180

90

60

60

5.4

2.7

1.8

1.8

  •  limits of current error and phase displacement for measuring current transformer For special application

Accuracy

Class

+/- Percentage Current (Ratio) Error at % Rated Current

+/- Phase Displacement at % Rated Current

Minutes

Centi radians

1

5

20

100

120

1

5

20

100

120

1

5

20

100

120

0.2S

0.75

0.35

0.2

0.2

0.2

30

15

10

10

10

0.9

0.4

0.3

0.3

0.3

0.5S

1.50

0.75

0.5

0.5

0.5

90

45

30

30

30

2.7

1.3

0.9

0.9

0.9

  •  limits of current error for measuring current transformers (classes 3 and 5)

Accuracy Class

+/- Percentage Current (Ratio) Error at % Rated Current

 
 

50

120

 

3

3

3

 

5

5

5

 

 Class X Current Transformer:

  • Class X current transformer is use in conjunction with high impedance circulating current differential protection relay, eg restricted earth fault relay. As illustrated in IEC60044-1, the class X current transformer is needed.
  • The following illustrates the method to size a class X current transformer.
  • Step 1: calculating knee point voltage Vkp
  • Vkp = {2 x Ift (Rct+Rw)}/ k
  • Vkp = required CT knee point voltage, Ift = max transformer through fault in ampere
  • Rct = CT secondary winding resistance in ohms, Rw = loop impedance of pilot wire between CT and the
  • K = CT transformation ratio
  • Step 2: calculate Transformer through fault Ift
  • Ift = (KVA x 1000)/ (1.732 x V x Impedance)
  • KVA = transformer rating in kVA , V = transformer secondary voltage, Impedance = transformer impedance
  • Step 3: How to obtain Rct
  • To measure when CT is produce
  • Step 4: How to obtain Rw
  • This is the resistance of the pilot wire used to connect the 5th class X CT at the transformer star point to the relay
  • In the LV switchboard. Please obtain this data from the Electrical contractor or consultant. We provide a table to Serve as a general guide on cable resistance.
  • Example:
  • Transformer Capacity: 2500kVA
    Transformer impedance: 6%
    Voltage system : 22kV / 415V 3phase 4 wire
    Current transformer ratio: 4000/5A
    Current transformer type: Class X PR10
    Current transformer Vkp : 185V
    Current transformer Rct  : 1.02½ (measured)
    Pilot wire resistance Rw : 25 meters using 6.0mm sq cable        
    = 2 x 25 x 0.0032 = 0.16½
    Ift = (kVA x 1000) / (1.732 x V x impedance) = (2500 x 1000) / (1.732 x 415 x 0.06)= 57,968 (Say 58,000A)
    Vkp = {2 x Ift (Rct+Rw)} / k= {2 x 58000 (1.02+0.16)} / 800= 171.1½.

 4)   Accuracy Limit Factor:

  • Standard Accuracy Limit Factors:  5, 10, 15, 20 and 30.
  • Accuracy of a CT is another parameter which is also specified with CT class. For example, if a measuring CT class is 0.5M (or 0.5B10), the accuracy is 99.5% for the CT, and the maximum permissible CT error is only 0.5%.
  • Accuracy limit Factor is defined as the multiple of rated primary current up to which the transformer will comply with the requirements of ‘Composite Error’. Composite Error is the deviation from an ideal CT (as in Current Error), but takes account of harmonics in the secondary current caused by non-linear magnetic conditions through the cycle at higher flux densities.
  • The electrical requirements of a protection current transformer can therefore be defined as :
  • Selection of Accuracy Class & Limit Factor.
  • Class 5P and 10P protective current transformers are generally used in over current and unrestricted earth leakage protection. With the exception of simple trip relays, the protective device usually has an intentional time delay, thereby ensuring that the severe effect of transients has passed before the relay is called to operate. Protection Current Transformers used for such applications are normally working under steady state conditions Three examples of such protection is shown. In some systems, it may be sufficient to simply detect a fault and isolate that circuit. However, in more discriminating schemes, it is necessary to ensure that a phase to phase fault does not operate the earth fault relay.
  • Calculation of the Accuracy limit factor
  • Fa=Fn X ( (Sin+Sn) / (Sin+Sa) )
  • Fn = Rated Accuracy Limit Factor, Sin = Internal Burden of CT secondary Coil
  • Sn= Rated Burden of CT (in VA), Sa= Actual Burden of CT (in VA)
  • Example: The internal secondary coil resistance of the CT(5P20) is 0.07 Ω, the secondary burden (including wires and relay) is 0.117 Ω and the CT is rated 300/5, 5P20, 10 VA. Calculate the actual accuracy limit factor.
  • Fn = 20 (CT data 5P20), Sin = (5A)2 × 0.07 Ω =1.75 VA, Sn = 10 VA (from CT data),
  • Sa = (5A)2 × 0.117 Ω = 2.925 VA
  • Accuracy limit factor ALF (Fa) = 20 X ((1.75+10) / (1.75+2.925)) =50.3

Selection of CT:

1)    Indoors or Out Door:

  • Determine where CT needs to be used. Indoor transformers are usually less costly than outdoor transformers. Obviously, if the current transformer is going to be enclosed in an outdoor enclosure, it need not be rated for outdoor use. This is a common costly error in judgment when selecting current transformers.

2)    What do We need:

  • The first thing we need to know that what degree of accuracy is required. Example, if you simply want to know if a motor is lightly or overloaded, a panel meter with 2 to 3% accuracy will likely suit for needs. In that case the current transformer needs to be only 0.6 to 1.2% accurate. On the other hand, if we are going to drive a switchboard type instrument with 1% accuracy, we will want a current transformer with 0.3 to 0.6 accuracy. We must keep in mind that the accuracy ratings are based on rated primary current flowing and per ANSI standards may be doubled (0.3 becomes 0.6%) when 10% primary current flows. As mentioned earlier, the rated accuracies are at stated burdens. We must take into consideration not only the burden of the load (instrument) but you must consider the total burden. The total burden includes the burden of the current transformers secondary winding, the burden of the leads connecting the secondary to the load, and the burden of the load itself. The current transformer must be able to support the total burden and to provide the accuracy required at that burden. If we are going to drive a relay you must know what relay accuracy the relay will require.

3)    Voltage Class:

  • You must know what the voltage is in the circuit to be monitored. This will determine what the voltage class of the current transformer must be as explained earlier.

4)    Primary Conductor:

  • If you have selected a current transformer with a window you must know the number, type and size of the primary conductor(s) in order to select a window size which will accommodate the primary conductors.

5)    Application:

  • The variety of applications of current transformers seems to be limited only by ones imagination. As new electronic equipment evolves and plays a greater role in the generation, control and application of electrical energy, new demands will be placed upon current transformer manufacturers and designers to provide new products to meet these needs

6)    Safety:

  • For personnel and equipment safety and measurement accuracy, current measurements on conductors at high voltage should be made only with a conducting shield cylinder placed inside the CT aperture. There should be a low electrical impedance connection from one end only to a reliable local ground. An inner insulating cylinder of adequate voltage isolation should be between the shield cylinder and the conductor at high voltage. Any leakage, induced or breakdown current between the high voltage conductor and the ground shield will substantially pass to local ground rather than through the signal cable to signal ground. Do not create a “current loop” by connecting the shield cylinder to ground from both ends. Current flowing in this loop will also be measured by the CT.

7)     CT output signal termination:

  • The CT output coaxial cable should preferably be terminated in 50 ohms. CT characteristics are guaranteed only when CT is terminated in 50 ohms.  The termination should present sufficient power dissipation capability.  When CT output is terminated in 50 ohms, its sensitivity is half that when terminated in a high-impedance load.

 Installing of CT:

  • Measurements must have the same polarity to keep the power factor and direction of power flow measurements accurate and consistent.
  • Most CTs are labelled that shows which side of the CT should face either the source or the load.

 

  • Primary Side : The Primary of CT is marked with H1 and H2 ( or only marking dot on one side)
  • The label “H1” or dot defines the direction as flowing current into the CT (H1 or the dot should face the Power source side). H2 side to load facing direction
  • Secondary Side: The Secondary (The output wires) of CT is marked with X1 and X2.
  •  X1 corresponds to H1, or the input side.The X1 secondary terminal is the polarity terminal. The polarity marks of a current transformer indicate that when a primary current enters at the polarity mark (H1) of the primary, a current in phase with the primary current and proportional to it in magnitude will leave the polarity terminal of the secondary (X1).
  •  Normally CT’s should not be installed on live services. The power should be disconnected when the CT’s are installed. Many times this is not possible because of critical loads such as computers, laboratories, etc. that cannot be shut down. Split core CT’s should not be installed on live un insulated bus bars under any conditions.

Modification of Primary & Secondary Turns Ratio:

  • The nameplate current ratio of the current transformer is based on the condition that the primary conductor will be passed once through the transformer opening. If necessary, this rating can be reduced in even multiples by looping this conductor two or more times through the opening.
  • A transformer having a rating of 300 amperes will be changed to 75 amperes if four loops or turns are made with the primary cable.
  • The ratio of the current transformer can be also modified by altering the number of secondary turns by forward or back-winding the secondary lead through the window of the current transformer.
  • By adding secondary turns, the same primary amperage will result in a decrease in secondary output.
  • By subtracting secondary turns, the same primary amperage will result in greater secondary output. Again using the 300:5 example, adding two secondary turns will require 310 amps on the primary to maintain the 5 amp secondary output or 62/1p = 310p/5s.
  • Subtracting two secondary turns will only require 290 amps on the primary to maintain the 5 amp secondary output or 58s/5p = 290p/5s. The ratio modifications are achieved in the following manner:
  • To add secondary turns, the white lead should be wound through the CT from the side opposite the polarity mark.
  • To subtract turns, the white lead should be wound through the CT from the same side as the polarity mark.

1)    Modifications in Primary Turns Ratio of CT:

  • The ratio of the current transformer can be modified by adding more primary turns to the transformer. By adding primary turns, the current required to maintain five amps on the secondary is reduced.
  • Ka = Kn X (Nn/Na)
  • Ka= Actual Turns Ration.
  • Kn=Name Plate T/C Ratio.
  • Nn=Name Plate Number of Primary Turns.
  • Na=Actual Number of Primary Turns.
  • Example: 100:5 Current Transformers.

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2)    Modifications in Secondary Turns Ratio of CT:

  • Formula  : Ip/Is = Ns/Np
  • Ip = Primary Current , Is = Secondary Current , Np = No of Primary Turns, Ns = No of Secondary Turns
  • Example: A 300:5 Current Transformer.
  • The ratio of the current transformer can be modified by altering the number of secondary turns by forward or back winding the secondary lead through the window of the current transformer.
  • By adding secondary turns, the same primary current will result in a decrease in secondary output. By subtracting secondary turns, the same primary current will result in greater secondary output.
  • Again using the 300:5 example adding five secondary turns will require 325 amps on the primary to maintain the 5 amp secondary output or:  325 p / 5s = 65s / 1p
  • Deducting 5 secondary turns will only require 275 amps on the primary to maintain the 5 amp secondary output or: 275p / 5s = 55s / 1p
  • The above ratio modifications are achieved in the following manner:

  • Current Transformer Ratio Modification:

CT Ratio

Number of Primary Turns

Modified Ratio

100:5A

2

50:5A

200:5A

2

100:5A

300:5A

2

150:5A

100:5A

3

33.3:5A

200:5A

3

66.6:5A

300:5A

3

100:5A

100:5A

4

25:5A

200:5A

4

50:5A

300:5A

4

75:5A

  • A primary turn is the number of times the primary conductor passes through the CT’s window. The main advantage of this ratio modification is you maintain the accuracy and burden capabilities of the higher ratio. The higher the primary rating the better the accuracy and burden rating.
  • You can make smaller ratio modification adjustments by using additive or subtractive secondary turns.
  •  For example, if you have a CT with a ratio of 100:5A. By adding one additive secondary turn the ratio modification is 105:5A, by adding on subtractive secondary turn the ratio modification is 95:5A.
  • Subtractive secondary turns are achieved by placing the “X1” lead through the window from the H1 side and out the H2 side. Additive secondary turns are achieved by placing the “X1” lead through the window from the H2 and out the H1 side.
  • So, when there is only one primary turn each secondary turn modifies the primary rating by 5 amperes. If there is more than one primary turn each secondary turn value is changed (i.e. 5A divided by 2 primary turns = 2.5A).
  •  The following table illustrates the effects of different combination of primary and secondary turns:

CT RATIO 100:5A

PRIMARY TURNS

SECONDARY TURNS

RATIO ADJUSTMENT

1

-0-

100:5A

1

1+

105:5A

1

1-

95:5A

2

-0-

50:5A

2

1+

52.5:5A

2

2-

45.0:5A

3

-0-

33.3:5A

3

1+

34.97:5A

3

1-

31.63:5A

Advantages of using a CT having 1A Secondary:

  • The standard CT secondary current ratings are 1A & 5A,The selection is based on the lead burden used for connecting the CT to meters/Relays.5A CT can be used where Current Transformer & protective’s device are located within same Switchgear Panel.
  • 1A CT is preferred if CT leads goes out of the Switchgear.
  • For Example if CT is located in Switch Yard & CT leads have to be taken to relay panels located in control room which can be away.1A CT is preferred to reduce the load burden. For CT with very High lead length, CT with Secondary current rating of 0.5 Amp can be used.
  • In large Generator Circuits, where primary rated current is of the order of few kilo-amperes only,5A CTs are used, 1A CTs are not preferred since the turns rations becomes very high & CT becomes unwieldy.

Danger with Current Transformer:

  • When a CT secondary circuit is closed, current flows through it, which is an exact proportion of the primary current, regardless of the resistance of the burden. In the CT have a ratio of 1OOO/5A and to have 1OOOA flowing in the primary is carrying exactly 5A.

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  • If the secondary terminals S1 and S2 are short- circuited, there is no voltage between them.
  • If now the short-circuit be replaced by a resistance of, say, 0.5 ohm the same 5A will flow through, causing a volt-drop of 2.5V and a burden of 5 x 2.5 = 12.5V A. If the resistance were increased to 5 ohms the terminal voltage with 5A flowing would rise to 25V and the burden to 125V A.
  • The greater the resistance, the greater would be the voltage and burden until, as it approached infinity (the open-circuit condition), so also in theory would the voltage (and burden) become infinite. This cannot of course happen in practice because the CT would saturate or the terminals flash over due to the very high secondary voltage between them. But it does show the danger of open-circuiting the secondary of running CT. lethal voltages can be produced at the point of opening. This is why CT secondaries are never fused.
  • The danger from an open-circuited CT is twofold. It can produce lethal voltages and so is a very real danger to personnel. The high voltage across the secondary winding could also cause insulation failure in that winding, leading at best to inaccuracy and at worst to burn- out or fire.
  • Before ever an instrument or relay is removed from the secondary loop of a running CT (if such a thing had to be done), the wires feeding that instrument must first be securely short- circuited at a suitable terminal box or, better, at the CT itself. Similarly, if a running CT is ever to be taken out of circuit, it must first be firmly shorted. CTs with 1 A secondary’s are more dangerous than those with 5A, as the induced voltages are higher.
  • Ammeter resistance is very low ,the current transformer normally works short circuited.
  • If for any reason the ammeter is taken out of secondary winding then the secondary winding must be short circuited with the help of short circuit switch .
  • If this is not done, then due to high m.m.f. will set up high flux in the core and it will produces excessive core loss which produce heat and high voltage across the secondary terminals
  • Hence the secondary of current transformer is never left open

 Sizing of CT for Building:

  • New construction: size the CT to handle about 80% of the circuit breaker capacity. If the building is served by a 2000 amp breaker, use 1600 amp (2000 x 0.8) CT’s.
  • Older buildings: the peak demand can generally be determined from the power company or from past billings. In this case add 20 to 30% to the peak demand and size the CT’s for this load. If the peak demand was 500 kW, the peak current on a 480/3/60 system would be 500,000 / (480 x 1.73 x 0.9 pf) = 669 amps. This assumes a 0.9 power factor. (Peak current would be higher with a lower power Factor.) Use CT’s about 20% larger. 800:5 CT’s would be a good selection.
  • For older buildings with no demand history, size the CT’s the same as for new construction. Where possible, use multi-tap CT’s so that the ratio can be reduced if the maximum load is much less than 80% of the breaker size.
  • CT’s that are used to monitor motor loads can be sized from the nameplate full load motor amps.

 

 

 

 

 

 

Difference between Power T.C & Distribution T.C


Difference between Power Transformer & Distribution Transformer:

  • Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.
  • Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and are generally rated less than 200 MVA.

Transformer Size / Insulation Level:

  • Power transformer is used for the transmission purpose at heavy load, high voltage greater than 33 KV & 100% efficiency. It also having a big in size as compare to distribution transformer, it used in generating station and Transmission substation .high insulation level.
  • The distribution transformer is used for the distribution of electrical energy at low voltage as less than 33KV in industrial purpose and 440v-220v in domestic purpose. It work at low efficiency at 50-70%, small size, easy in installation, having low magnetic losses & it is not always fully loaded.

Iron Loss & Copper Loss:

  • Power Transformers are used in Transmission network so they do not directly connect to the consumers, so load fluctuations are very less. These are loaded fully during 24 hr’s a day, so cu losses & iron losses takes place throughout day the specific weight i.e. (iron weight)/(cu weight) is very less .the average loads are nearer to full loaded or full load and these are designed in such a way that maximum efficiency at full load condition. These are independent of time so in calculating the efficiency only power basis is enough.
  • Power Transformers are used in Distribution Network so directly connected to the consumer so load fluctuations are very high. these are not loaded fully at all time so iron losses takes place 24hr a day and cu losses takes place based on load cycle. the specific weight is more i.e. (iron weight)/(cu weight).average loads are about only 75% of full load and these are designed in such a way that max efficiency occurs at 75% of full load. As these are time dependent the all day efficiency is defined in order to calculate the efficiency.
  • Power transformers are used for transmission as a step up devices so that the I2r loss can be minimized for a given power flow. These transformers are designed to utilize the core to maximum and will operate very much near to the knee point of B-H curve (slightly above the knee point value).This brings down the mass of the core enormously. Naturally these transformers have the matched iron losses and copper losses at peak load (i.e. the maximum efficiency point where both the losses match).
  • Distribution transformers obviously cannot be designed like this. Hence the all-day-efficiency comes into picture while designing it. It depends on the typical load cycle for which it has to supply. Definitely Core design will be done to take care of peak load and as well as all-day-efficiency. It is a bargain between these two points.
  • Power transformer generally operated at full load. Hence, it is designed such that copper losses are minimal. However, a distribution transformer is always online and operated at loads less than full load for most of time. Hence, it is designed such that core losses are minimal.
  • In Power Transformer the flux density is higher than the distribution transformer.

Maximum Efficiency:

  • The main difference between power and distribution transformer is distribution transformer is designed for maximum efficiency at 60% to 70% load as normally doesn’t operate at full load all the time. Its load depends on distribution demand. Whereas power transformer is designed for maximum efficiency at 100% load as it always runs at 100% load being near to generating station.
  • Distribution Transformer is used at the distribution level where voltages tend to be lower .The secondary voltage is almost always the voltage delivered to the end consumer. Because of voltage drop limitations, it is usually not possible to deliver that secondary voltage over great distances. As a result, most distribution systems tend to involve many ‘clusters’ of loads fed from distribution transformers, and this in turn means that the thermal rating of distribution transformers doesn’t have to be very high to support the loads that they have to serve.
  • All day efficiency = (Output in KWhr) / (Input in KWhr) in 24 hrs which is always less than power efficiency.

—————————————————————————————————————————–

Transformer


Standard Transformer Accessories & Fittings:

Standard Transformer Fittings:

1)    Standard Fittings

  • Rating and terminal marking plate.
  • Tap Changing arrangement
  • Off – circuit tap changing switch
  • Off – circuit tap changing link
  • On Load tap changer
  • Two earthing terminals
  •  Lifting Lugs
  • Drain – cum filter valve
  • Pressure Relief Device
  • Silica gel dehydrating breather.
  • Oil Level Indicator.
  • Thermometer Pocket.
  • Conservator with drain plug and filling hole.
  • Air Release plug.
  • Jacking lugs (above 1600 KVA)
  • Filter valve (top tank)
  •  Under base unidirectional flat rollers.

2)    Terminal Arrangement:

  • Bare Bushings  or Cable box.
  • Compound filled for PVC cables (up to 33000 Volts)  or Air filled for PVC cable s (Up to 11000 Volts) or
  • Bus Duct  (Bare bushing enclosed in housing up to 600 Volts)
  • Disconnection chamber between cable box and transformer tank.
  •  Additional bare neutral terminal.

3)    Optional Fittings:

  •  These are optional fittings provided at an extra cost, if customer specifically orders them.
  • Winding temperature  indicator
  • Oil temperature indicator
  • Gas and oil actuated (Buchholz) relay
  • Conservator drain valve
  • Shut off valve between conservator and tank.
  • Magnetic oil level gauge
  • Explosion vent
  • Filter valve (Bottom of tank)
  • Skid under base with haulage holes
  • Junction box.

Standard Transformer Accessories:

1)    Thermometer Pockets:

  • This pocket is provided to measure temperature of the top oil in tank with a mercury in glass type thermometer. It is essential to fill the pocket with transformer oil before inserting the thermometer,  to have uniform and correct reading. One additional pocket is provided for dial type thermometer (OTI) with contacts 

2)    Air release plug:

  • Air release plug is normally provided on the tank cover for transformer with conservator. Space is provided in the plug which allows air to be escaped without removing the plug fully from the  seat. Plug should be unscrewed till air comes out from cross hole and as soon as oil flows out it should be closed. Air release plugs are also provided on radiator headers and outdoor bushings.

3)     Winding temperature Indicator

  • The windings temperature indicator indicates ‘’ Hot spot’’ temperature of the winding. This is a ‘’Thermal Image type’’ indicator. This is basically an oil temperature indicator with a heater responsible to raise the temperature equal to the ‘’Hot spot’’ gradient between winding and oil over the oil temperature. Thus, this instrument indicates the ‘’Hot Spot’’ temperature of the windings. Heater coil is fed with a current proportional to  the windings current through a current transformer mounted on the winding under measurement. Heater coil is either placed on the heater bulb enveloping the sensing element of the winding temperature indicator immersed  in oil or in the instrument. The value of the current fed to the heater is such that it raises the  temperature by an amount equal to the hot spot gradient of the winding, as described above.  Thus temperature of winding  is simulated on the dial of the instrument. Pointer is connected thought  a mechanism to indicate the hot spot temperature on dial. WTI is provided with a temperature recording dial main pointer. Maximum pointer and re setting device and two sets of contacts for alarm and trip.

4)     Oil Temperature Indicator

  • Oil temperature indicator provides local temperature of top oil. Instruments are provided with temperature sensing bulb, temperature recording dial with  the pointer and maximum reading pointer and resetting device. Electrical contacts are provided to give alarm or trip at  a required setting (on capillary tube  type thermometer).

5)    Conservator Tank:

  • It is an Expansion Vessel
  • It maintains oil in the Transformer above a Minimum Level
  •  It has a Magnetic Oil Level Gage.
  •  It can give an alarm if the oil level falls below the limit
  • A portion of the Tank is separated for use with OLTC.
  • This usually has oil level indicators
  • Main Conservator Tank can have a Bellow
  • It has an oil filling provision
  • It has an oil drain valve
  • Provision is there for connecting a Breather

6)    Silica Gel Breather:

  •  Prevents Moisture Ingress.
  • Connected to Conservator Tank
  • Silica Gel is Blue when Dry; Pink when moist
  • Oil Seal provides a Trap for Moisture before passing thro Silica Gel

7)     Cooling:

  • ONAN .. Oil Natural Air Natural
  • ONAF .. Oil Natural Air Forced
  • OFWF .. Oil Forced Water Forced
  • ODWF .. Oil directed Water Forced.
  • By Forced Cooling, the Transformer capacity can be increased by more than 50%

8)    Bushing:

  •  Insulators and Bushings are built with the best quality Porcelain shells manufactured by wet process.
  • For manufacture of electro porcelain,  high quality indigenous raw  materials viz, China Clay,  Ball  Clay,  Quartz  and  Feldspar  is  used Quartz and feldspar are ground to required finesses and then intimately mixed with ball and china clay in high speed blungers. They are then passed through electromagnetic separators, which remove  iron  and  other  magnetic  impurities.  The  slip  produced  is passed to a filter press where extra water is removed under pressure and the resulting clay cakes are aged over a period. The aged cakes are extruded to required form viz., cylinders,  on  high  vacuum  de-airing  pug  mill.  The  extruded  blanks  or  cylinders  are given  shapes  of  Insulators  /  Bushings  which  are  conditioned  and  are  shaped  on copying lathes as the case may be.
  • Testing, Assembly & packing:
  • All insulators & bushings undergo routine electrical and mechanical  tests.  The  tests before  and  after  assembly  are  carried  out  according  to  IS  Specifications, to  ensure their suitability for actual conditions of use. Porosity tests are also carried out regularly on  samples  from  every  batch,  to  ensure  that  the  insulators  are  completely  vitrified. These insulators are then visually checked  and sorted, before they are packed in sea worthy packing, to withstand transit conditions.
  • Types of Insulators & Bushings:
  • Bushing Insulators:  Hollow Porcelain Bushings up to 33 KV
  • Application : Transformers, Capacitors, Circuit Breakers
  • Solid Core Insulators:
  •  Line Post
  • Long Rod
  • Support
  • Special Type Insulators
  • C.T. up to 66 KV
  •  P.T. up to 33 KV
  • Weather Casing
  • L.T. Insulators
  • Shackel Type
  • Spool Type
  • Pin Type
  • Guy strain
  • H.V. Bushings (IS:3347)    
  •  Pin Insulators:  Up to 33 KV
  • Post type Insulators: Post  type  insulators,  complete  with  metal  fittings,  generally  IS Specifications and other  International Standards up to 33 KV
12 to17.5 KV / 250 amps 24 KV / 1000 amps            
12 to 17.5 KV / 630 amps 24 KV / 2000 to 3150 amps
12 to 17.5 KV / 1000 amps     36 KV / 250 amps              
12 to 17.5 KV / 2000 to 3150 amps 36 KV / 630 amps
24 KV / 250 amps               36 KV / 1000 amps     
24 KV / 630 amps 36 KV / 2000 to 3150 amps
  • L.V. Bushings (IS:3347) 
11 KV / 250 amps 1 KV / 2000 amps
1 KV / 630 amps 1 KV / 3150 amps
1 KV / 1000 amps  
  • H.V. Bushings (IS:8603)    
12 KV / 250 amps 36 KV / 250 amps              
12 / 630 amps 36 KV / 630 amps
12 KV / 1000 amps     6 KV / 1000 amps            
12 KV / 2000 to 3150 amps 36 KV 3150 amps
  • C.T. Bushings (IS:5612)   
11 KV 1 KV / 2000 amps
1 KV / 630 amps 1 KV / 3150 amps
1 KV / 1000 amps  
  • Epoxy Bushing:
  • All  Epoxy  Resin  Cast  Components  are  made  from  hot  setting  reins  cured  with anhydrides;  hence  these  provide  class-F  Insulation  to  the  system.  In  an  oxidizing atmosphere, certain amine cured Epoxy Resins can start to degrade at 150ºC whereas the anhydride cured systems are stable at 200ºC therefore our epoxy components are cured with anhydrides which gives them a longer life.

9)    Buchholz Relay:

  • The purpose of such devices is  to  disconnect faulty  apparatus before large scale  damage  caused  by  a  fault  to  the  apparatus  or  to  other  connected  apparatus. Such devices generally respond to a change in the current or pressure arising from the faults and are used for either signaling or tripping the circuits.
  • Considering  liquid  immersed transformer,  a  near  ideal  protective  device  is  available  in  the  form  of  gas  and  oil operated relay described  here. The relay operates on the well known fact that almost every type of electric fault in a liquid immersed transformer gives rise to a gas. This gas is collected in the body of the relay and is used in some way or the other to cause the alarm or the tripping circuit to operate.
  • In the event of fault in an oil filled transformer gas is generated, due to which buchholz relay gives warning of developing fault. Buchholz relay is provided with two elements one for minor faults (gives alarm) and other for major faults (tripping). The alarm elements operate after a specific volume gets accumulated in the relay. Examples of incipient faults which will generate gas in oil are:- Buchholz Relay
  •  i) Failure of core bolt insulation.
  • ii) Shorting of lamination and core clamp.
  • iii) Bad Electrical contact or connections.
  • iv) Excessive hot spots in winding.
  • The alarm element will also operated in the event of oil leakage. The trip element operates due to sudden oil surge in the event of more serious fault such as: –
  • i) Earth fault due to insulation failure from winding to earth.
  • ii) Winding short circuit inter turn, interlayer, inter coil etc.
  • iii) Short circuit between phases.
  • iv) Puncture of bushing.
  • The trip element will also operate if rapid loss of oil occurs. During the  operation of transformer, if there is an alarm transformer should be isolated from lines and possible reasons, listed above for the operation of relay should be checked starting with simple reason such as loss of oil due to  leaks, air accumulation in relay chamber which  may be the absorbed  air released by oil  due to change in temperature etc. Rating of contacts: – 0.5 Amps. At 230 Volts AC or 220 Volts. DC.

Pre commissioning Inspection of Transformer:

  • Sample of oil taken from the transformer and subjected to  electric test (break down value) of 50KV (RMS) as specified in IS : 335.
  • Release trapped air through air release plugs and valve fitted for the purpose on various fittings like radiators, bushing caps, tank cover, Bushing turrets etc.
  • The float lever of the magnetic oil level indicator (if provided) should be moved up and down  between the end position to check that the mechanism does not stick at any point. If the  indicator has signaling contact they should be checked  at the same time for correct operation.  Checking the gauge by draining oil is a more positive test.
  • Check whether  gas operated really (if provided) is mounted at angle by placing a spirit  level on the top of the relay. See that the conservator is filled upto the filling oil level marked on plain oil gauge side and corresponding to the pointer reading in MOG side. Check the operation of the alarm and trip contacts of the relay independently by injecting air through the  top cocks  using a dry air bottle. The air should  be released after the tests. Make sure that transformer oil runs through pert cock of Buchholz relay.
  • Check alarm and trip contacts of WTIs, Dial type thermometer, magnetic oil gauge etc. (if  provided).
  • Ensure that off circuit switch  handle is locked at the desired tap position with padlock.
  • Make sure that all valves except drain, filter and sampling valves are opened (such as radiator valves, valves on the buchholz relay pipe line if Provided).
  • Check  the condition of silicagel in the breather to ensure that silicagel in the breather is active and colour is blue. Also check that the transformer oil is filled in the silicagel breather upto the level indicated.
  • Check tightness of external electrical connections  to bushings.
  • Give a physical  check on all bushing  for any crack or any breakage of porcelain. Bushing  with cracks or any other defects should be immediately replaced.
  • Check the neutral earthing  if specified.
  • Make sure that neutrals of HV / LV are effectively earthed.
  • Tank should be  effectively earthed at two points.
  • Check that the thermometer pockets on tank cover are filled with oil.
  • If  the oil temperature indicator  is not working satisfactorily, loosen and remove the  thermometer bulb from the pocket on the cover and place it with a standard thermometer in a suitable vessel filled with transformer oil. Warm the oil slowly while  string it and take reading of the thermometers if an adjustment of the transformer  thermometer is necessary  the  same many be done. Also check signaling contacts and set for the desired temperature.
  • CT secondary terminals must be shorted and earthed if not in use.
  • Check relief vent diaphragm for breakage. See that the Bakelite diaphragm at bottom and glass diaphragm at top are not ruptured.
  • Check all the gasket joints to ensure that there is no leakage of transformer oil at any point.
  • Clear off extraneous material like tools earthling rods, pieces of clothes, waste etc.
  • Lock the rollers for accidental movement on rails.
  • Touching of paint may be done after erection.

Parts of Transformer:

1)    Transformer Oil

  • Oil is used as coolant and dielectric in the transformer and keeping it in good condition will assist in preventing deterioration of the insulation, which is immersed in oil. Transformer oil is always exposed to the air to some extent therefore in the course of time it may oxidize and form sludge if the breather is defective, oil may also absorb moisture from air thus reducing dielectric strength.

2)    Transformer Winding:

  • The primary and secondary windings in a core type transformer are of the concentric  type only, while in case of shell type transformer these could be of sand-witched type as well. The concentric windings are normally constructed in any of the following types depending on the size and application of the transformer.
  • (1)Cross over Type.
  • (2) Helical Type.
  • (3) Continuous Disc Type.
  •  Distributed.
  • Spiral.
  •  Interleaved Disc.
  •  Shielded Layer

a)    Distributed Winding :

  •  Used   for   HV windings   of   small   Distribution   Transformers where   the   current   does  not   exceed   20   amps  using   circular   cross  section conductor .

b)   Spiral:

  • Used  up   to  33  kv for  low  currents using  strip  conductor. Wound closely  on  Bakelite or press board cylinders generally without cooling ducts. However, multi layer windings are provided with cooling ducts between layers. No Transposition is necessary.

c)    Interleaved Disc:

  • Used for voltages above 145 kv . Interleaving enables the winding withstand higher impulse voltages.

d)   Shielded Layer :

  • Used up to 132 kv in star connected windings with graded insulation. Comprises of a number of concentric spiral coils arranged in layers grading   the   layers.
  • The  longest  at the  Neutral  and  the  shortest  at  the  Line Terminal. The layers are separated by cooling ducts. This type of construction ensures uniform distributed voltages.

e)    Cross-over type winding:

  • It is normally employed where rated currents are up-to about 20 Amperes or so.
  •  In this type of winding, each coil consists of number of layers having number of turns per layer. The conductor being a round wire or strip insulated with a paper covering.
  • It is normal practice to provide one or two extra lavers of paper insulation between lavers. Further, the insulation between lavers is wrapped round the end turns of the lavers there by assisting to keep the whole coil compact.
  • The inside end of a coil is connected to the outside end of adjacent coil. Insulation blocks are provided between adjacent coils to ensure free circulation of oil.

f)     Helical winding:

  •  Used for Low Voltage and high currents .The turns comprising of a number   of   conductors   are   wound axially. Could be   single, double or   multi layer   winding.   Since   each   conductor   is   not   of   the   same   length,   does not embrace the   same   flux and   of  different  impedances,   and  hence  circulating currents, the winding is Transposed.
  • The coil consists of a number of rectangular strips wound in parallel racially such that each separate turn occupies the total radial depth of the winding.
  • Each turn is wound on a number of key spacers which form the vertical oil duct and each turn or group of turns is spaced by radial keys sectors.
  • This ensures free circulation of oil in horizontal and vertical direction.
  • This type of coil construction is normally adopted for low voltage windings where the magnitude of current is comparatively large.
  • Helical Disc winding:
  • This type of winding is also termed “interleaved disk winding.”
  • Since conductors 1 – 4 and conductors 9 – 12 assume a shape similar to a wound capacitor, it is known that these conductors have very large capacitance. This capacitance acts as series capacitance of the winding to highly improve the voltage distribution for surge.
  • Unlike cylindrical windings, Helical disk winding requires no shield on the winding outermost side, resulting in smaller coil outside diameter and thus reducing Transformer dimension. Comparatively small in winding width and large in space between windings, the construction of this type of winding is appropriate for the winding, which faces to an inner winding of relatively high voltage.
  • Thus, general EHV or UHV substation Transformers employ Helical disk winding to utilize its features mentioned above.

g)   Continuous disc type of windings:

  • Used for 33kv and 132 kv for  medium currents. The coil comprises   of   a   number   of   sections   axially.   Cooling   ducts   are   provided between each section.
  • IT is consists of number of Discs wound from a single wire or number of strips in parallel. Each disc consists of number of turns, wound radically, over one another.
  • Arrangement of layers
  • The conductor passing uninterruptedly from one disc to another. With ultiple-strip conductor. Transpositions are made at regular intervals to ensure uniform resistance and length of conductor. The discs are wound on an insulating cylinder spaced from it by strips running the whole length of the cylinder and separated from one another by hard pressboard sectors keyed to the vertical strips.
  • This ensures free circulation of oil in horizontal and vertical direction and provides efficient heat dissipation from windings to the oil.
  •  The whole coil structure is mechanically sound and capable of resisting the most enormous short circuit forces.
  • This is the most general type applicable to windings of a wide range of voltage and current
  • Rectangular wire is used where current is relatively small, while transposed cable Fig. (12) is applied to large current. When voltage is relatively low, a Transformer of 100MVA or more capacity handles a large current exceeding 1000A. In this case, the advantage of transposed cable may be fully utilized
  • Since the number of turns is reduced, even conventional continuous disk construction is satisfactory in voltage distribution, thereby ensuring adequate dielectric characteristics. Also, whenever necessary, potential distribution is improved by inserting a shield between turns.
  •  According to the number of layers used the paper is applied as follows.
  •  Two layers: =Where there are two layers both of them are wound in opposite directions.
  •  More than two layers: =Where there are more than two layers all the layers are applied in the same direction, all,  except  the  outermost  layer  is  butt  wound,  and  the  outermost  layer  is  overlap wound. Within each group of papers the position of the butt joints of any layer relative to the layer below is progressively displaced by approximately 30 percent of the paper width.
  •  Note: Overlapping can also be done as per customer requirements.
  • Grade of paper
  •  The paper, before  application, is ensured to be free  from  metallic  and  other  injurious inclusions  and    have  no  deleterious  effect  on  insulating  oil.
  • The thickness  of  paper used is between 0.025 mm to 0.075 mm.
  • Enameled Conductor
  •  Apart from paper covered conductors, we have all the facilities of producing enameled conductors as per customer specified requirements.
  • Copper –  Usually in 8 – 16mm rods is drawn to the  required sizes and then insulated with paper etc..
  •  Annealing is done for softening and stress relieving in electrically heated annealing plant under vacuum upto 400-500ºC. After 48hrs when the temperature reaches ambient, the vacuum is slowly released and the material is transferred to Insulation section.
  •  Conductors are one  of  the principal materials used in  manufacturing  of  transformers. Best quality of  copper  rods are procured from indigenous as  well as foreign sources. Normally 8 mm & 11 mm rods are procured. For each supply  of input, test  certificate from suppliers is obtained and at times.
  •  After  the  wires  &  strips are drawn  as per clients  requirements they are moved  on  to paper  covering  process.
  • To  prevent  the inclusion  of  copper  dust  or other extraneous matter under paper covering the conductor is fully cleaned by felt pads or other suitable means  before  entering  the  paper  covering  machine.  As  per  the  customers requirements DPC, TPC & MPC conductors are produced. It is ensured that each layer of paper is continuous, firmly applied and substantially free from creases.
  • No bonding or adhesive material  is used except  to  anchor the ends of paper.   Any  such  bonding materials  used  to  anchor  the  ends  do  not have  deleterious  effect  on transformer  oil, insulating  paper  or  the  electric  strength  of  the  covering.    It is ensured  that  the overlapping percentage is not less than 25% of the paper width.
  • The rectangular paper-covered copper conductor is the most commonly used conductor for the windings of medium and large power transformers.
  • These conductors can be individual strip conductors, bunched conductors or continuously transposed cable (CTC) conductors.
  •  In low voltage side of a distribution transformer, where much fewer turns are involved, the use of copper or aluminum foils may find preference.
  • To enhance the short circuit withstand capability, the work hardened copper is commonly used instead of soft annealed copper, particularly for higher rating transformers
  • In the case of a generator transformer having high current rating, the CTC conductor is mostly used which gives better space factor and reduced eddy losses in windings. When the CTC conductor is used in transformers, it is usually of epoxy bonded type to enhance its short circuit strength.

3)    Transformer Core:

  • Purpose of the core: 
  • To reduce the magnetizing current. (For topologies such as Forward, Bridge etc we need the magnetizing current to be as small as possible. For fly-back topology, though the magnetizing current is used to transfer energy, the size of the transformer will be very large to get the required inductance if a core is not used.)
  • To improve the linkage of the flux within windings if  the windings are separated spatially.
  • To contain the magnetic flux within a given volume
  •  In magnetic amplifier applications a saturable core is used as a switch.
  • Core Material:
  •  Different types of material used for cores
  •  Iron-Silicon Steel- Nickel-Iron-Iron-Cobalt-Ferrite-Molybdenum-Met-glass
  •  Salient characteristics of a core material:  
  •  Permeability, Saturation flux density, Coercive force, Remnant flux, Losses due to           Hysteresis & Eddy Current.
  • The power loss is a function of frequency and the ac flux swing and is given by the equation P = K1 * (frequency)K2 * (Flux Density)K3
  •  Every transformer has a core, which is surrounded by windings. The core is made out of special cold rolled grain oriented silicon sheet steel laminations. The special silicon steel ensures low hysteretisis losses. The silicon steel laminations also ensure high resistively of core material which result in low eddy currents. In order to reduce eddy current losses, the laminations are kept as thin as possible. The thickness of the laminations is usually around 0.27 to 0.35 mm.
  • Transformer cores construction is of two types, viz, core type and shell type. In core type transformers, the windings are wound around the core, while in shell type transformers, the core is constructed around the windings. The shell type transformers provide a low reactance path for the magnetic flux, while the core type transformer has a high leakage flux and hence higher reactance.
  • The limb laminations in small transformers are held together by stout webbing tape or by suitably spaced glass fiber bends. The use of insulated bolts passing through the limb laminations has been discontinued due to number of instances of core bolt failures. The top and bottom mitered yokes are interleaved with the limbs and are clamped by steel sections held together by insulated yoke bolts. The steel frames clamping the top and bottom yokes are held together by vertical tie bolts.
  • Grain Oriented steel sheets namely ORIENTCORE, ORIENTCORE H1-B & ORIENTCORE HI-B.LS are some of the finest quality of core.
  • ORENTCORE.HI-B  is  a  breakthrough  in  that  it  offers  higher  magnetic  flux  density, lower  core loss  and  lower  magnetostriction  than  any  conventional  grain-oriented electrical steel sheet.
  • ORIENT.HI-B.LS  is  a  novel  type  with  marked  lower  core  losses,  produced  by  laser irradiation of the surface of ORIENTCORE.HI-B sheets.
  • Annealing of stacked electrical sheets
  • Annealing is to be done at 760 to 845ºC to
  • Reduce mechanical stress
  •  Prevent contamination
  • Enhance insulation of lamination coating
  • Though  ORIENTCORE  and  ORIENTCORE.HI-B  are  grain  orient  steel  sheets  with excellent  magnetic  properties,  mechanical  stress  during  such  operations  as  cutting, punching  and bending  affect their  magnetic  properties adversely.  When these stress are excessive, stress relief annealing is necessary. 
  • Following method is observed for stress relief annealing
  • Available Grades:
  1.  Stacked  electrical  steel  sheets  are  heated  thoroughly  in  the  edge-to-edge direction  rather  than  in the  face-to-face  direction,  because  heat  transfer  is  far faster in side heating.
  2.  A cover is put over sheets stacked on a flat plate. Because ORIENTCORE and ORIENTCORE.HI-B  have  extremely  low  carbon  content  and  very  easily decarburized at annealing temperatures, the base, cover and other accessories used are of very low carbon content .
  3. To prevent oxidation so as to protect the coating on the sheets, a no oxidizing atmosphere free from carbon sources is used having less than 2%hydrogen or high-purity  nitrogen  gas.  Due point of  the  atmosphere  is  maintained  at  0ºC  or less.
  4. Care  is  taken  to  the  flatness  of  annealing  base,  because  an  uneven  base distorts cores, leading to possible  distortion during assembly.
  5.  Annealing  temperature  ranging  from  780ºC  to  820ºC  is  maintained  for  more than 2 hours or more. Cooling is done upto 350ºC in about 15 hours or more.
  •  ORIENTCORE           :M1, M2, M3, M4, M5 & M6
  •  ORIENTCORE.HI-B    :23ZH90, 23ZH95, 27ZH95, 27ZH100, 30ZH100,M-0H, M-1H, M-2H, M-3H
  • ORIENTCORE.HI-B.LS: 23ZDKH90, 27ZDKH95
  •  Non-oriented silicon steel, hot rolled grain oriented silicon steel,cold rolled grain oriented (CRGO) silicon steel, Hi-B, laser scribed and mechanically scribed. The last three materials are improved versions of CRGO.
  •  Saturation flux density has remained more or less constant around 2.0 Tesla for CRGO; but there is a continuous improvement in watts/kg and volt-amperes/kg characteristics in the rolling direction.
  •  The core building technology has improved from the non-mitred to mitred and then to the step-lap construction
  • The better grades of core steel not only reduce the core loss but they also help in reducing the noise level by few decibels
  •  Use of amorphous steel for transformer cores results in substantial core loss reduction (loss is about one-third that of CRGO silicon steel). Since the manufacturing technology of handling this brittle material is difficult, its use in transformers is not widespread
  •  In the early days of transformer manufacturing, inferior grades of laminated steel (as per today’s standards) were used with inherent high losses and magnetizing volt-amperes. Later on it was found that the addition of silicon content of about 4 to 5% improves the performance characteristics significantly, due to a marked reduction in eddy losses (on account of the increase in material resistivity) and increase in permeability. Hysteresis loss is also lower due to a narrower hysteresis loop. The addition of silicon also helps to reduce the aging effects.
  •  Although silicon makes the material brittle, it is well within limits and does not pose problems during the process of core building.
  •  The cold rolled manufacturing technology in which the grains are oriented in the direc tion of rolling gave a new direction to material development for many decades, and even today newer materials are centered around the basic grain orientation process.
  •  Important stages of core material development are: non-oriented, hot rolled grain oriented (HRGO), cold rolled grain oriented (CRGO), high permeability cold rolled grain oriented (Hi-B), laser scribed and mechanically scribed.
  •  Laminations with lower thickness are manufactured and used to take advantage of lower eddy losses. Currently the lowest thickness available is 0.23 mm, and the popular thickness range is 0.23 mm to 0.35 mm for power transformers.
  •  Maximum thickness of lamination used in small transformers can be as high as 0.50 mm.
  •  Inorganic coating (generally glass film and phosphate layer) having thickness of 0.002 to 0.003 mm is provided on both the surfaces of laminations, which is sufficient to withstand eddy voltages (of the order of a few volts).
  •  Since the core is in the vicinity of high voltage windings, it is grounded to drain out the statically induced voltages. While designing the grounding system, due care must be taken to avoid multiple grounding, which otherwise results into circulating currents and subsequent failure of transformers.

4)    Transformer Core:

a)    Core Type Construction: (Mostly Used):

  • Generally in  India, Core  type  of construction  with Two/Three/Five limbed cores are used. Generally five limbed cores are used where the dimensions of the Transformer is to be limited due to Transportation difficulties. In three limbed core the cross section of the Limb and the Yoke are the same where as in five Limbed core, the cross section of the Yoke and the Flux return  path  Limbs are  ver y  less (58%  and  45%  of  the  principal  Limb). 
  • Limb:which is surrounded by windings, is called a limb or leg? 
  • York: Remaining part of the core, which is not surrounded by windings, but is essential for completing the path of flux, is called as yoke.
  • Advantage:
  • Construction is simpler, cooling is better and repair is easy.
  •  The yoke and end limb area should be only 50% of the main limb area for the same operating flux density.
  • Zero-sequence impedance is equal to positive-sequence impedance for this construction (in a bank of single-phase transformers).
  • Sometimes in a single-phase transformer windings are split into two parts and placed around two limbs as shown in figure (b). This construction is sometimes adopted for very large ratings. Magnitude of short-circuit forces are lower because of the fact that ampere-turns/height are reduced. The area of limbs and yokes is the same. Similar to the single-phase three-limb transformer.
  •  The most commonly used construction, for small and medium rating transformers, is three-phase three-limb construction as shown in figure (d).For each phase, the limb flux returns through yokes and other two limbs (the same amount of peak flux flows in limbs and yokes).
  •  limbs and yokes usually have the same area. Sometimes the yokes are provided with a 5% additional area as compared to the limbs for reducing no-load losses.
  •  It is to be noted that the increase in yoke area of 5% reduces flux density in the yoke by 5%, reduces watts/kg by more than 5% (due to non-linear characteristics) but the yoke weight increases by 5%. Also, there may be additional loss due to cross-fluxing since there may not be perfect matching between lamination steps of limb and yoke at the joint. Hence, the reduction in losses may not be very significant.
  • In large power transformers, in order to reduce the height for transportability, three-phase five-limb construction depicted in figure (e) is used. The magnetic length represented by the end yoke and end limb has a higher reluctance as compared to that represented by the main yoke. Hence, as the flux starts rising, it first takes the path of low reluctance of the main yoke. Since the main yoke is not large enough to carry all the flux from the limb, it saturates and forces the remaining flux into the end limb. Since the spilling over of flux to the end limb occurs near the flux peak and also due to the fact that the ratio of reluctances of these two paths varies due to non-linear properties of the core.
  • Fluxes in both main yoke and end yoke/end limb paths are non-sinusoidal even though the main limb flux is varying sinusoidal [2,4]. Extra losses occur in the yokes and end limbs due to the flux harmonics. In order to compensate these extra losses, it is a normal practice to keep the main yoke area 60% and end yoke/end limb area 50% of the main limb area.
  • The zero-sequence impedance is much higher for the three-phase five-limb core than the three-limb core due to low reluctance path (of yokes and end limbs) available to the in-phase zero-sequence fluxes, and its value is close to but less than the positive-sequence impedance value.

b)   Shell-type construction:

  • Cross section of windings in the plane of core is surrounded by limbs and yokes, is also used.
  • Shell   type   of   construction   of   the   core   is   widely   used   in   USA.
  • Advantage:
  • One can use sandwich construction of LV and HV windings to get very low impedance, if desired, which is not easily possible in the core-type construction.
  • Analysis of overlapping joints and building factor:
  • While building a core, the laminations are placed in such a way that the gaps between the laminations at the joint of
  • limb and yoke are overlapped by the laminations in the next layer.
  • This is done so that there is no continuous gap at the joint when the laminations are stacked one above the other (figure). The overlap distance is kept around 15 to 20 mm.
  • There are two types of joints most widely used in transformers: non-mitred and mitred joints.
  • Non-mitered joints:
  • In which the overlap angle is 90°, are quite simple from the manufacturing point of view, but the loss in the corner joints is more since the flux in the joint region is not along the direction of grain orientation. Hence, the on-mitred joints are used for smaller rating transformers. These joints were commonly adopted in earlier days when non-oriented material was used
  • Non-mitered joints:
  • In which the overlap angle is 90°, are quite simple from the manufacturing point of view, but the loss in the corner joints is more since the flux in the joint region is not along the direction of grain orientation. Hence, the on-mitred joints are used for smaller rating transformers. These joints were commonly adopted in earlier days when non-oriented material was used
  • Mitered joints:
  •  The joint where these laminations meet could be Butt or Mitred. In CRGO, the Mitred  Joint is preferred  as it reduces the  Reluctance  of  the  Flux  path  and reduces the No Load Losses and the No Load current (by about 12% & 25% respectively).
  •  The Limb and  the Yoke are made of a number  of Laminations in Steps. Each step  comprises of  some  number  of  laminations  of  equal  width. The  width   of  the  central  strip  is Maximum   and  that at  the  circumference  is Minimum. The   cross  section   of  the  Yoke  and  the   Limb  are  nearly Circular. Mitred  joint  could  be at 35/45/55  degrees but the  45  one  reduces wastage.
  • The angle of overlap (a) is of the order of 30° to 60°, the most commonly used angle is 45°. The flux crosses from limb to yoke along the grain orientation in mitred joints minimizing losses in them. For airgaps of equal length, the excitation requirement of cores with mitred joints is sin a times that with non-mitred joints.
  • Better grades of core material (Hi-B, scribed, etc.) having specific loss (watts/kg) 15 to 20% lower than conventional CRGO material (termed hereafter as CGO grade, e.g., M4) are regularly used. However, it has been observed that the use of these better materials may not give the expected loss reduction if a proper value of building factor is not used in loss calculations
  • The building factor generally increases as grade of the material improves from CGO to Hi-B to scribed (domain refined). This is a logical fact because at the corner joints the flux is not along the grain orientation, and the increase in watts/kg due to deviation from direction of grain orientation is higher for a better grade material.
  • The factor is also a function of operating flux density; it deteriorates more for better grade materials with the increase in operating flux density. Hence, cores built with better grade material may not give the expected benefit in line with Epstein measurements done on individual lamination. Therefore, appropriate building factors should be taken for better grade materials using experimental/test data.
  • Also the loss contribution due to the corner weight is higher in case of 90° joints as compared to 45° joints since there is over-crowding of flux at the inner edge and flux is not along the grain orientation while passing from limb to yoke in the former case. Smaller the overlapping length better is the core performance; but the improvement may not be noticeable.
  •  The gap at the core joint has significant impact on the no-load loss and current. As compared to 0 mm gap, the increase in loss is 1 to 2% for 1.5 mm gap, 3 to 4% for 2.0 mm gap and 8 to 12% for 3 mm gap. These figures highlight the need for maintaining minimum gap at the core joints.
  •  Lesser the laminations per lay, lower is the core loss. The experience shows that from 4 laminations per lay to 2 laminations per lay, there is an advantage in loss of about 3 to 4%. There is further advantage of 2 to 3% in 1 lamination per lay. As the number of laminations per lay reduces, the manufacturing time for core building increases and hence most of the manufacturers have standardized the core building with 2 laminations per lay.
  • Joints of limbs and yokes contribute significantly to the core loss due to cross-fluxing and crowding of flux lines in them. Hence, the higher the corner area and weight, the higher is the core loss.
  • The corner area in single-phase three-limb cores, single-phase four-limb cores and three-phase five-limb cores is less due to smaller core diameter at the corners, reducing the loss contribution due to the corners. However, this reduction is more than compensated by increase in loss because of higher overall weight (due to additional end limbs and yokes).
  • Building factor is usually in the range of 1.1 to 1.25 for three-phase three-limb cores with mitred joints. Higher the ratio of window height to window width, lower is the contribution of corners to the loss and hence the building factor is lower.
  • Step-lap joint :
  •  It is used by many manufacturers due to its excellent performance figures. It consists of a group of laminations (commonly 5 to 7) stacked with a staggered joint as shown in figure.
  •  Its superior performance as compared to the conventional mitred construction.
  •  It is shown that, for a operating flux density of 1.7 T, the flux density in the mitred joint in the core sheet area shunting the air gap rises to 2.7 T (heavy saturation), while in the gap the flux density is about 0.7 T. Contrary to this, in the step-lap joint of 6 steps, the flux totally avoids the gap with flux density of just 0.04 T, and gets redistributed almost equally in laminations of other five steps with a flux density close to 2.0 T. This explains why the no-load performance figures (current, loss and noise) show a marked improvement for the step-lap joints.
  • The   assembled   core   has  to   be   clamped  tightly not  only  to   provide   a  rigid mechanical   structure   but   also   required   magnetic   characteristic.   Top   and Bottom Yokes are clamped by   steel sections using Yoke Studs. These studs do not pass through the core  but held  between steel sections. Of late Fiber Glass Band tapes are wound round the Limbs tightly upto the desired tension and  heat treated. These laminations , due to elongation and contraction  lead to magnetostriction, generally called Humming which can be reduced by using higher  silicon  content  in   steel   but  this  makes  the  laminations become   very brittle.
  •  The choice of operating flux density of a core has a very significant impact on the overall size, material cost and performance of a transformer.
  •  For the currently available various grades of CRGO material, although losses and magnetizing volt-amperes are lower for better grades, viz. Hi-B material (M0H, M1H, M2H), laser scribed, mechanical scribed, etc., as compared to CGO material (M2, M3, M4,M5, M6, etc.), the saturation flux density has remained same (about 2.0 T).
  • The peak operating flux density (Bmp ) gets limited by the over-excitation conditions specified by users.
  • The slope of B-H curve of CRGO material significantly worsens after about 1.9 T (for a small increase in flux density, relatively much higher magnetizing current is drawn). Hence, the point corresponding to 1.9 T can be termed as knee-point of the B-H curve.
  • It has been seen in example 1.1 that the simultaneous over-voltage and under-frequency conditions increase the flux density in the core. Hence, for an over-excitation condition (over-voltage and under-frequency).
  • When a transformer is subjected to an over-excitation, core contains an amount of flux sufficient to saturate it. The remaining flux spills out of the core. The over-excitation must be extreme and of a long duration to produce damaging effect in the core laminations
  • The laminations can easily withstand temperatures in the region of 800°C (they are annealed at this temperature during their manufacture), but insulation in the vicinity of core laminations, viz. press-board insulation (class A: 105°C) and core bolt insulation (class B: 130°C) may get damaged. Since the flux flows in air (outside core) only during the part of a cycle when core gets saturated, the air flux and exciting current are in the form of pulses having high harmonic content which increases the eddy losses and temperature rise in windings and structural parts.

Winding Insulation in Transformer:

  •  Requirement of Insulating Oil:
  •  1.0 lit / kva for Trs from 400 – 1600 Kva
  • 0.6 lit / kva for Trs from 1600 – 80,000 kva
  • 0.5 lit / Kva for Trs above 80,000 Kva.
  • In Transformers, the insulating oil provides an insulation medium as well as a heat transferring medium that carries away heat produced in the windings and iron core. Since the electric strength and the life of a Transformer depend chiefly upon the quality of the insulating oil, it is very important to use a high quality insulating oil
  • Provide a high electric strength.
  • Permit good transfer of heat.
  •  Have low specific gravity-In oil of low specific gravity particles which have become suspended  in the oil will settle down on the bottom of the tank more readily and at a faster rate, a property aiding the oil in retaining its homogeneity.
  •  Have a low viscosity- Oil with low viscosity, i.e., having greater fluidity, will cool Transformers at a much better rate.
  • Have low pour point- Oil with low pour point will cease to flow only at low temperatures.
  • Have a high flash point. The flash point characterizes its tendency to evaporate. The lower the flash point the greater the oil will tend to vaporize When oil vaporizes, it loses in volume, its viscosity rises, and an explosive mixture may be formed with the air above the oil
  • The Core Insulation is:
  •  SRBP- Synthetic Resin Bonded Paper
  •  OIP   – Oil Impregnated Paper
  • RIP   – Resin Impregnated Pape
  • Resin Coated Paper/ Kraft Paper/ Crepe Kraft Paper are used for making core for the above It is Hermetically Sealed.
  •  Pre-compressed pressboard is used in windings as opposed to the softer materials used in earlier days. The major insulation (between windings, between winding and yoke, etc.)
  •  Mineral oil has traditionally been the most commonly used electrical insulating medium and coolant in transformers. Studies have proved that oil-barrier insulation system can be used at the rated voltages greater than 1000 Kv.
  • A high dielectric strength of oil-impregnated paper and pressboard is the main reason for using oil as the most important constituent of the transformer insulation system.
  •  Manufacturers have used silicon-based liquid for insulation and cooling. Due to non-toxic dielectric and self-extinguishing properties, it is selected as a replacement of Askarel. High cost of silicon is an inhibiting factor for its widespread use.
  • Super-biodegradable vegetable seed based oils are also available for use in environmentally sensitive locations.
  • SF6 gas has excellent dielectric strength and is non-flammable. Hence, SF6 transformers find their application in the areas where fire-hazard prevention is of paramount importance.
  • Due to lower specific gravity of SF6 gas, the gas insulated transformer is usually lighter than the oil insulated transformer. The dielectric strength of SF6 gas is a function of the operating pressure; the higher the pressure, the higher the dielectric strength.
  • However, the heat capacity and thermal time constant of SF6 gas are smaller than that of oil, resulting in reduced overload capacity of SF6 transformers as compared to oil-immersed transformers. Environmental concerns, sealing problems, lower cooling capability and present high cost of manufacture are the challenges.
  • Dry-type resin cast and resin impregnated transformers use class F or C insulation. High cost of resins and lower heat dissipation capability limit the use of these transformers to small ratings.
  • The dry-type transformers are primarily used for the indoor application in order to minimize fire hazards. Nomex paper insulation, which has temperature withstand capacity of 220°C, is widely used for dry-type transformers. The initial cost of a dry-type transformer may be 60 to 70% higher than that of an oil-cooled transformer at current prices, but its overall cost at the present level of energy rate can be very much comparable to that of the oil-cooled transformer.

Transformer Noise:

  •  Transformers located near a residential area should have sound level as low as possible.
  • Levels specified are 10 to 15 dB lower than the prevailing levels mentioned in the international standards.
  • Core, windings and cooling equipment are the three main sources of noise.
  • The core is the most important and significant source of the transformer noise.
  • The core vibrates due to magnetic and magnetostrictive forces. Magnetic forces appear due to non-magnetic gaps at the corner joints of limbs and yokes
  •  These magnetic forces depend upon the kind of interlacing between the limb and yoke; these are highest when there is no overlapping (continuous air gap).
  • The magnetic forces are smaller for 90° overlapping, which further reduce for 45°overlapping. These are the least for the step-lap joint due to reduction in the value of flux density in the overlapping region at the joint.
  • The forces produced by the magnetostriction phenomenon are much higher than the magnetic forces in transformers.
  • Magnetostriction is a change in configuration of magnetizable material in a magnetic field, which leads to periodic changes in the length of material. An alternating field sets the core in vibration.
  • This vibration is transmitted, after some attenuation, through the oil and tank structure to the surrounding air. This finally results in a characteristic hum.
  • The magnetostriction force varies with time and contains even harmonics of the power frequency (120, 240, 360, —Hz for 60 Hz power frequency). Therefore, the noise also contains all harmonics of 120 Hz.
  •  The amplitude of core vibration and noise increase manifold if the fundamental mechanical natural frequency of the core is close to 120 Hz.
  • The value of the magnetostriction can be positive or negative, depending on the type of the magnetic material, and the mechanical and thermal treatments.
  • Magnetostriction is generally positive (increase in length by a few microns with increase in flux density) for CRGO material at annealing temperatures below 800°C, and as the annealing temperature is increased (=800°C), it can be displaced to negative values.The mechanical stressing may change it to positive values
  • Magnetostriction is minimum along the rolling direction and maximum along the 90° direction.
  • Most of the noise transmitted from a core comes principally from the yoke region because the noise from the limb is effectively damped by windings (copper and insulation material) around the limb.
  • The quality of yoke clamping has a significant influence on the noise level.
  •  Apart from the yoke flux density, other factors which decide the noise level are: limb flux density, type of core material, leg center (distance between the centers of two adjacent phases), core weight, frequency, etc.
  • The higher the flux density, leg centers, core weight and frequency of operation, the higher is the noise level.
  • The noise level is closely related to the operating peak flux density and core weight.
  •  If core weight is assumed to change with flux density approximately in inverse proportion, for a flux density change from 1.6 T to 1.7 T, the increase in noise level is 1.7 dB
  • Hence, one of the ways of reducing noise is by designing transformer at lower operating flux density. For a flux density reduction of 0.1 T, the noise level reduction of about 2 dB is obtained. This method results into an increase of material content and it may be justified economically if the user has specified a lower no-load loss, in which case the natural choice is to use a lower flux density.
  • Use of step-lap joint gives much better noise reduction (4 to 5 dB).
  • Some manufacturers also use yoke reinforcement (leading to reduction in yoke flux density); the method has the advantage that copper content does not go up since the winding mean diameters do not increase. Bonding of laminations by adhesives and placing of anti-vibration/damping elements between the core and tank can give further reduction in the noise level.
  • The use of Hi-B/scribed material can also give a reduction of 2 to 3 dB. When a noise level reduction of the order of 15 to 20 dB is required, some of these methods are necessary but not sufficient.

Transformer Protection:

Internal Protection:

(1) Bucholtz Relay:  

  • This Gas operated relay is a protection for minor  and major  faults that  may develop inside  a Transformer  and  produce  Gases.
  •  This   relay   is   located   in   between   the   conservator   tank   and   the   Main Transformer tank in the pie line which is mounted at an inclination of 3 to 7 degrees.
  •  A shut off valve is located in between the Bucholtz relay and the Conservator.
  • The relay comprises of  a cast housing    which contains two pivoted   Buckets  counter   balanced  weights.
  • The   relay  also  contains  two mercury y  switches   which   will   send   alarm   or   trip   signal   to   the   breakers controlling the Transformer. In healthy condition, this relay will be full of oil and the   buckets will   also  be   full  of  oil   and   is  counter   balanced  by  the weights.
  • In the event of a fault inside the transformer, the gases flow up to the conservator via the relay and pushes the  oil in the relay down. Once the oil level falls below the bottom level of the  buckets, the bucket due to the weight of oil inside tilts and closes the mercury switch and causes the Alarm or trip to be actuated and isolate the transformer from the system.

(2) Oil Surge /  Bucholtz   Relay for OLTC: 

  • This   relay   operating   on   gas produced  slowly or  in a  surge  due  to  faults inside  the  Diverter  Switch of OLTC protects the Transformer and isolates it from the system.

(3) Pressure Relief Valve for Large Transformers:

  •  In   case of  a   serious fault   inside   the   Transformer,   Gas   is   rapidly   produced.
  • This   gaseous pressure must be relieved immediately otherwise it will damage the Tank and  cause damage to neighboring equipment.
  • This relay is mounted on the  top  cover  or  on  the  side   walls  of  the  Transformer.  The  valve  has a corresponding  port which  will be  sealed by a  stain less steel  diaphragm .
  • The   diaphragm   rests on   a   O   ring   and   is   kept   pressed   by two   heavy springs. If a high pressure is developed inside, this diaphragm lifts up and releases   the   excessive   gas.
  •  The   movement   of   the   diaphragm   lifts the spring  and  causes  a  micro   switch  to  close  its contacts to  give  a  trip signal  to   the  HV  and  LV  circuit  breakers  and  isolate   the  transformer.
  •  A visual   indication can  also  be   seen   on   the  top   of  the  relay.   For   smaller capacity   transformer,   an   Explosion   vent   is   used   to   relieve   the   excess pressure but it cannot isolate the Transformer.

(4) Explosion Vent Low & Medium Transformers  : 

  •  For smaller capacity Transformers, the excessive pressures inside a Transformer due to  major faults inside  the  transformer  can  be  relieved by Explosion vents. But this cannot isolate the Transformer.

(5) Winding /Oil Temperature   Protection   :

  • These   precision   instruments operate on the principle of liquid expansion.
  •  These record the hour to hour temperatures and also record the Maximum temperature over  a  period of time  by a  resettable pointer.
  • These in conjunction with mercury switches provide   signals for   excessive   temperature   alarm   annunciation   and   also isolate   the   Transformer   for   very   excessive   temperatures.
  • These   also switch on the cooler fans and cooler pumps if the temperature exceeds the set values. Normally two separate instruments are used for oil and winding temperatures.
  •  In   some   cases   additional   instruments     are   provided separately for  HV,LV  and  Tertiary winding  temperatures.
  • The  indicator   is provided  with  a  sensing  bulb  placed  in  an  oil  pocket  located  on  the  top cover  of the  Transformer  tank.  The  Bulb  is connected  to  the  instrument housing  by means of flexible connecting  tubes consisting of two capillary tubes.
  •  One   capillary   tube   is   connected   to   an   operating   Bellow   in   the instrument. The  other  is  connected  to  a compensating  Bellow  .
  • The  tube follows the same path as the one with the Bulb but the other  end, it does not   end   in   a   Bulb   and   left   sealed.   This   compensates   for   variations in Ambient Temperatures.
  • As the temperature varies, the volume of the liquid in the  operating  system  also  varies   and  operates the  operating  Bellows transmitting its movements to the pointer and also the switching disc. This disc is  mounted  with  mercury  float  switches which  when  made  provides signals to alarm/trip/cooler controls.
  • Oil and winding temperature indicators work   on   the   same   principles except   that   the   WTI is   provided   with   an additional   bellows     heating   element.   This  heating   element   is  fed   by  a current transformer  with  a current  proportional  to  the load  in the  winding whose   temperature   is   to   be   measured/monitored.   The   tem premature increase of the heating  element is proportional to  the temperature rise of winding over top oil  temperature.
  • The operating bellow gets an additional movement   simulating   the   increase   of   winding   temperature   over   top   oil temperature and represents the Winding Hot Spot. This is called Thermal Imaging process.

(6) Conservator   Magnetic Oil Level Protection   : 

  • Inside   the   conservator tank, a float is used to sense the levels of oil and move. This is transmitted to a switch mechanism by means of magnetic coupling. The Float and the Magnetic   mechanism   are   totally   sealed.   The   pointer   connected   to   the magnetic   mechanism   moves   indicating   the   correct   oil   level   and   also provision is m ade for Low oil level alarm by switch.

(7) Silica gel Breather: 

  • This is a means to preserve the dielectric strength of insulating oil and prevent  absorption of moisture, dust etc. The breather is connected to the Main conservator tank. It is provided with an Oil seal. The breathed in air is passed through the oil seal to retain moisture before the air   passes through  the silica  gel cr ystals which  absorbs moisture  before breathing  into  the  conservator   tank.  In  latest  large  transformers,  Rubber Diaphragm or Air cells are used to reduce contamination of oil.

 (8) Transformer Earthing :

  •  For  Distribution Transformers, normally Dy11 vector Group, the LT Neutral  is Earthed  by a separate   Conductor  section of at least half  the section of the conductor used for phase wire and connected to a Separate Earth whose Earth Resistance must be less than 1 ohm.
  •  The Body of the Tank has two different earth connections, which should be connected to two distinct earth electrodes by GI flat of suitable section.
  • For   Large   Power   Transformers,   Neutral   and   Body  Connections  are   made separately but all the Earth Pits are connected in parallel so that the combined Earth   Resistance   is  always  maintained  below  0.1   ohm.
  • The   individual  and combined   earth   resistance   is   measured   periodically   and   the   Earth   Pits maintained regularly and electrodes replaced if required.

External Protection:

  • Lightning Arrestors on HV & LV for Surge Protection
  • HV / LV Over Current  Protection(Instantaneous /IDMT- Back up)
  • Earth Fault Protection ( Y connected side)
  • REF (HV & LV) ( For internal fault protection)
  • Differential  Protection (for internal fault protection)
  • Over Fluxing Protection (against system Kv & HZ variations)
  • HG Fuse Protection for Small Capacity Transformers.
  • Normally Each Power   Transformers   will   have   a   LV   Circuit Breaker.  For   a Group  of  Transformers  up to  5  MVA  in  a  substation, a  Group  control  Circuit Breaker   is   provided.   Each   Transformer   of   8   MVA   and   above  will   have   a Circuit Breaker on the HV side.

Transformer Cooling:

  •  The Heat in a transformer is produced due to I square R in the  windings and in the core due to Eddy Current and Hysteresis Loss.
  •  In Dry type Transformer the Heat is directly dissipated into the atmosphere but in Oil filled Transformer, the   Heat   is   dissipated   by   Thermosyphon   and   transmitted   to   the   top   and dissipated   into   the   atmosphere   through   Radiators   naturally   or   by   forced cooling   fans   or   by   Oil   pumps   or   through   Water   Coolers.
  •  The   following Standard symbols are adopted to denote the Type of Cooling:
  •   A =Air Cooling
  •  N =Natural Cooling by Convection
  •   B= Cooling by Air Blast Fans
  •  O=Oil (mineral) immersed cooling
  •  W= Water Cooled
  •  F =Forced Oil Circulation by Oil Pumps
  •  S=Synthetic Liquid used  instead of Oil
  •  G =Gas Cooled (SF6 or N2)
  •  D=Forced (Oil directed)
  •  ONAF=Oil immersed Transformer with natural oil circulation and forced air external cooling is designated.
  •  ONAN= Oil Immersed Natural cooled
  •  ONAF= Oil Immersed Air Blast
  •  ONWN=Oil Immersed Water Cooled
  •  OFAF=Forced Oil Air Blast Cooled
  • OFAN=Forced Oil Natural Air Cooled
  •  OFWF=Forced Oil Water Cooled
  • ODAF=Forced Directed Oil and Forced Air Cooling.
  • Cooling e.g., ONAN/ONAF or ONAN/OFAF or sometimes three systems e.g., ONAN/ONAF/ OFAF

Electrical Clearance in Substation


Minimum Clearance in Substation:

Voltage Highest Voltage Lighting Impulse Level (Kvp) Switching Impulse Level (Kvp) Minimum Clearance Safety Clearance(Mt) Ground Clearance(Mt)
Phase-Earth Phase-Phase
11KV 12KV 70   0.178 0.229 2.600 3.700
33KV 36KV 170   0.320 0.320 2.800 3.700
132KV 145KV 550   1.100 1.100 3.700 4.600
650   1.100 1.100 2.700 4.600
220KV 245KV 950   1.900 1.900 4.300 5.500
1050   1.900 1.900 4.300 5.500
400KV 420KV 1425 1050(P-E) 3.400 4.200 6.400 8.000 

 Electrical Clearance in Substation:

Voltage Height of I Bay From Ground (Mt) Height of II Bay From Ground (Mt) Bay Width (Mt) Phase-Phase (Mt) BetweenEquipment Earth Wire From Ground
132KV (Single) 8.0 11.0 3.0 3.0 10.5
220KV (Single) 12.5 18 4.5 4.5 15.5
220KV (Double) 18.5 25 25 4.5 4.5 28.5
400KV 15.6 22 22.0 7.0 >6.0 30.0

Standard Bay Widths in Meters:

Voltage Bay Width (Meter)
11KV 4.7 Meter
33KV 4.7 Meter
66KV 7.6 Meter
132KV 12.2 Meter
220KV 17 Meter
400KV 27 Meter

 Standard Bus and Equipment Elevation

Voltage Equipment live Terminal Elevation (Meter) Main Bus Take of Elevation (Meter)
Low High
11 KV/33KV 2.8To 4 5.5  To6.5 9 6.5To8.5
66KV 2.8To 4 6To8 9To 10.5 9.5
132KV 3.7To5 8To9.5 13.5To14.5 12To12.5
220KV 4.9To5.5 9To13 18.5 15To18.5
400KV 8.0 15.5 23

 Phase spacing for strung Bus:

Voltage Clearance
11KV 1300 mm
33KV 1300 mm
66KV 2200 mm
132KV 3000 mm
220KV 4500 mm
400KV 7000 mm

 Minimum Clearance of Live Parts from Ground:

Voltage Minimum Clearance to Ground (Mt) Section Clearance (Mt)
11KV 3.700 2.600
33KV 3.700 2.800
66KV 4.600 3.000
132KV 4.600 3.500
220KV 5.500 4.300
400KV 8.000 7.000

 Insulator String:

 Voltage No of Suspension String Length (mm) No of Disc for Tension String Length in (mm)
66KV 5 965 6 1070
132KV 9 1255 10 1820
220KV 14 1915 15 2915
400KV 23 3850 2 X 23 5450

 Nominal Span:

Voltage Normal Span (Meter)
66KV 240-250-275
132KV 315-325-335
220KV 315-325-335
400KV 315-325-335

 Minimum Ground Clearance:

Voltage Ground (Meter)
66KV 5.5
132KV 6.1
220KV 7.0
400KV 8.0
800KV 12.4

 Indoor Substation Minimum Clearances

 

Distance Descriptions
0.9 Meter Horizontally between any item of equipment and thesubstation wall
0.6 Meter Horizontally between any Two items of equipment
1.2 Meter Horizontally in front of any HV switchgear

 Clearance of Conductor on Tower

Voltage Tower Type Vertical Space (Mt) Horizontal Space(Mt) Total Height From Ground(Mt)
66KV A 1.03 4.0 15.91
  B 1.03 4.27 15.42
  C 1.22 4.88 16.24
132KV A 7.140 2.17 23.14
  B 4.2 6.29 22.06
  C 4.2 7.15 22.68
  D 4.2 8.8 24.06
220KV A 5.2 8.5 28.55
  B 5.25 10.5 29.08
  C 6.7 12.6 31.68
  D      

 NORMS OF PROTECTION FOR EHV CLASS POWER TRANSFORMERS

Voltage ratio & capacity HV Side LV Side Common relays
 132/33/11KV up to 8 MVA   3 O/L relays + 1 E/L relay 2 O/L relays + 1 E/L relay Buchholz, OLTC Buchholz, OT, WT
 132/33/11KV above 8 MVA and below 31.5 MVA   3 O/L relays + 1 dir. E/L relay 3 O/L relays + 1 E/L relay Differential, Buchholz, OLTC Buchholz, OT, WT
 132/33KV, 31.5 MVA & above   3 O/L relays + 1 dir. E/L relay 3 O/L relays + 1 E/L relay Differential, Over flux,Buchholz, OLTC PRV, OT, WT
 220/33 KV, 31.5MVA & 50MVA 220/132KV, 100 MVA   3 O/L relays + 1 dir. E/L relay 3 O/L relays + 1 dir. relay Differential, Over flux,Buchholz, OLTC PRV, OT, WT
 400/220KV 315MVA   3 directional O/L relays (with dir. High set) +1 directional E/L relays. Restricted E/F relay + 3 Directional O/L relays for action 3 directional O/L relays (with dir. High set)+1 directional E/L relays. Restricted E/F relay Differential, Over flux,Buchholz, OLTC PRV, OT, WT and overload (alarm) relay

 The bottom most portion of any insulator or bushing in service should be at a minimum height of 2500 mm above ground level.

 Location of L.A (From T.C Bushing):

Voltage BIL KV Peak Distance (Mt)
11KV 75 12
33KV 200 15
66KV 325 24
132KV 550 35
220KV 900 To 1050 Close To T.C
400KV 1425 To 1550

Minimum Acceptable Specification of C.T & P.T for Metering


Min. Acceptable Specification of Current Transformer for Metering:

Sr. No Particulars 11 kV 33 kV 132 kV 220 kV 
1 Highest System Voltage (kV rms) 12 36 145 245
2 CT ratio. 2000-1000/1-1 800-400/1-1 400/1-1 800/1-1
1600-800/1-1 600-300/1-1    
1200-600/1-1 400-200/1-1    
800-400/1-1 300-150/1-1    
600-300/1-1 100-50/1-1    
400-200/1-1      
300-150/1-1      
150-75/1-1      
3 Number of metering cores Two Nos Two Nos Two Nos Two Nos
4 Rated continuous thermal current. 120% of rated primary  current 120% of rated primary  current 120% of rated primary  current 120% of rated primary  current
5 Rated short time thermal current of primary for 1 sec. (kA) 25 25 31.5 40
6 CT characteristics :a) Rated primary current (Amps.)  2000-1000  800-400  400  800
1600-800 600-300    
1200-600 400-200    
800-400 300-150    
600-300 100-50    
400-200      
300-150      
150-75      
(b) Rated Secondary current (Amps.) 1 1 1 1
(c) Class of accuracy. 0.2 0.2 0.2 0.2
(d) Max. instrument security factor 5 5 5 5
(e) Rated burden (VA). 30 30 30 40
7 IS to which CT conforms.        
8 IS to which insulating oil conforms.        

 Min. Acceptable Specification of Voltage Transformer for Metering:

Sr. No Particulars     245 kV CVTs      145 kV CVTs 
1 Highest SystemVoltage (kV)          245 kV              145 kV
2 Rated Capacitance (pF)  4400 pf with tolerance + 10% and – 5%
3 For low voltage terminal over entire carrier frequency range.    
  (a) Stray capacitance  Shall not exceed 200 pf
  (b) Stray conductance Shall not exceed 20 us
4 (a) High frequency            capacitance for entirecarrier frequency range within 80% to 150% of rated capacitance
  (b) Equivalent series resistance over the entire frequency range. less than 40 Ohms
5 No. of secondary windings for potential device.             Two              Two
6 Transformation ratio:  
  (i) Winding –I   20 kV- \/3/110 -\/3V
  (ii) Winding –II   20 kV- \/3/110 -\/3V
7 Rated secondary burden    
  (i)Winding –I   (VA) 50 VA 50 VA
  (ii) Winding –II  (VA) 50 VA 50 VA
8 Accuracy Class :    
  (i)Winding –I   (VA)                        0.2 for metering
  (ii) Winding –II  (VA)                        0.2 for metering
9 Voltage factor for winding – IVoltage factor for winding – II 1.2 Cont. & 1.5 for 30 secs.1.2 Cont. & 1.5 for 30 secs.
10 IS to which CVTs conform. IS 3156 with latest amendment
11 IS to which Insulating Oil conform. IS 335 with latest amendment
     

Minimum Acceptable Specification of Single Phase PT for Metering:

Sr.No Particulars        33 kV         11 kV 
1 Highest System Voltage (kV rms)           36             12
2 Transformation ratio.   33kV/ V3/     110/ V3   11 kV/110 V
3 Number of windings.          Two           Two
4 Rated output/ burden (VA) per winding /phase.           50             50
5 Accuracy class. (At 10 to 100% of VA burden)          0.2             0.2
6 Rated voltage factor and duration. 1.2 continuous & 1.5 for 30 secs.
7 IS to which PT conforms.   3156 with latest amendment
       

 

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