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REALISTIC SPECIFICATION FOR CURRENT TRANSFORMER

Dr. K Rajamani and Ms. Bina Mitra

Reliance Energy Ltd., Mumbai

1. INTRODUCTION

Current transformers (CT), though may appear quiet insignificant in the huge electrical power network, play a vital role in protection and metering systems. The key elements in a protection system (Refer Fig.1) are:

i. Instrument transformers (Current and voltage transformers) – sensors in the system. ii. Protective relays – locating and initiating

isolation of faults in the system.

iii. Circuit breaker – isolating faults from the system.

iv. AC and DC wiring related to the above elements.

Fig.1. Protection System

Faults in the system can be cleared successfully when all the above elements of protection chain work perfectly. The success of fault clearance, irrespective of use of ‘advanced numerical relays’ and ‘VCBs’ is still critically dependent on faithful reproduction of primary quantities on secondary side by instrument transformers. This paper discusses realistic specification of current transformer in particular to achieve the above objective. Initially few basic concepts which play a vital role in specifying current transformer parameters are explained.

1.1 Equivalent circuit of current transformer Refer Fig. 2 for equivalent circuit of current transformer.

ES = Secondary induced EMF VS = Secondary output voltage IP = Primary current

IS = Secondary current IE =Excitingcurrent Ic = Core loss component

IM = Magnetising component

Primary connected to current source

Fig.2. Equivalent Circuit of Current Transformer 1.2 Phasor diagram of current transformer Refer Fig. 3 for phasor diagram of current transformer.

ϕ : Flux

ISRS : Secondary resistance voltage drop ISXS : Secondary reactance voltage drop IP NP : Total primary ampere turns.

ICNp : Component of primary ampere turns required to supply core losses (usually very small)

IM NP : Component of primary ampere turns required to produce the flux.

ISNS : Secondary Ampere Turns.

IP’ NP : Component of primary Ampere Turns required to neutralize secondary Ampere Turns; opposite to ISNS

For bar primary, NP =1

Fig. 3. Phasor Diagram of Current Transformer As seen from the phasor diagram, the primary current IP is made up of two components:

i. Exciting current IE -magnetizes the core and supplies the core losses.

ii. Reflected secondary current - IP’.

The errors in current transformation are due to the exciting current. The proportionality between primary current and secondary current is not

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strictly maintained and results in magnitude (ratio) and phase angle errors.

1.3 CT saturation

When a CT is saturated, the tight linear relationship between primary and secondary is lost and the CT is unable to replicate faithfully. Under healthy conditions very little current is used for excitation and majority of the primary current is transformed into secondary (Refer Fig.4).

Fig.4. Healthy Current Transformer

However, under saturation conditions, majority of the primary current is used in exciting the core and very little is transformed into secondary current which flows in the burden (Refer Fig 5).

Fig.5. Saturated Current Transformer

The CT excitation characteristic linearity is maintained up to knee point voltage (Vk) (defined later) (Refer .Fig.6). Beyond knee point voltage, current transformer starts saturating.

Fig.6. CT Excitation Characteristic 1.4 Voltage developed across CT secondary Another important function of a current transformer is to develop enough voltage to drive required current through circuit burden in addition to faithfully reproducing the primary current. In case of CT saturation, since major portion of primary current is used in exciting the core, the CT is unable to develop enough voltage across CT secondary to drive the required current through the connected burden. This concept plays an

important role in specifying parameters for both general protection class and special protection class CTs.

2. CURRENT TRANSFORMER CLASSIFICATION

Current transformers may be classified in the following categories based on the application: i. General protection class used for protective

relaying.

ii. Special protection class (Class PS) used in current balance protection schemes. iii. Metering class used in metering circuits. 3. PARAMETERS FOR CURRENT TRANSFOMER SPECIFICATION

The key parameters required for complete current transformer specification:

i. C.T. Ratio ii. Number of cores

3.1 Parameters based on application of current transformer

3.1.1 General protection class i. Accuracy class

ii. Accuracy limit factor (A.L.F) iii. Rated burden

3.1.2 Special protection class i. Knee point voltage (Vk) ii. Exciting current (Iex)

iii. Secondary winding resistance (Rct) 3.1.3 Metering class

i. Accuracy class

ii. Instrument security factor (I.S.F) iii. Rated burden

4. CT RATIO

CT ratio is defined as the ratio of rated primary current to the rated secondary current.

4.1 Rated primary current

Factors influencing rated primary current: i. Rating based on continuous thermal rating ΙA: Maximum load current (mandatory) + 20% overload capacity.

ii. Rating based on short time thermal rating ΙB: Rated short time current for 1 sec / 150 The higher current of the above two values (IA, IB) decides primary current rating. This ensures robust construction of the current transformer.

Short circuit current through the current transformer can be maximum 150 times the rated CT current for 1 sec. Based on Ι2t criteria,in case fault current (ΙF) is larger than 150 times the rated primary current, then short circuit withstand time will be less than ‘t’ seconds,

t = 1502 ΙP2 / ΙF2

The fault shall be cleared within ‘ t ‘ seconds to avoid CT damage.

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Fault Current IF = 40kA

Short Circuit withstand time t = 1502 x 2002 / (40,000)2 = 0.57 sec

The fault shall be cleared within 0.6 sec to avoid damage of current transformer.

A special mention is required for CTs used for equipment of small rating connected to high voltage and high short circuit level networks. In such networks low ratio CTs will be heavily saturated under short circuit conditions causing mal operation of over current protection. For such situations IEEE (C37.20.2) recommends use of two sets of CTs. One set with a low ratio to be used for metering and another set with a high ratio to be used for protection. The combination can thus provide accurate metering and adequate short circuit protection. This may be useful particularly in design of auxiliary system of power plants where the motor rating at 6.6kV can vary from 200kW to 9000kW. The rating of CT for protection application may be standardized as per the criteria given above whereas the ratings for metering CTs may vary as per the individual load ratings.

4.2 Rated secondary current

The standard CT secondary current ratings are 1A and 5A. The selection is based on the lead burden used for connecting the CT to meters/ relays. 5A CT can be used when current transformer and protective devices are located within same switchgear. 1 A CT is preferred if CT lead goes out of the switchgear. For example, if CT is located in switch yard and CT leads have to be taken to relay panels located in control room which can be away, 1A CT is preferred to reduce the lead burden. For CT with very high lead length, CT with secondary current rating of 0.5A 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 ratio becomes very high and CT becomes unwieldy.

5. GENERAL PROTECTION CLASS 5.1 Accuracy class

Standard accuracy classes available are 5P and 10P. The figure ‘5’ in ‘5P’ indicates the accuracy limit in percent expressed in terms of composite error. Generally, 5P Class CTs are employed. 5.2 Accuracy limit factor (A.L.F)

Accuracy limit factor (A.L.F) is the ratio of largest value of current to CT rated current, up to which CT must retain the specified accuracy.

Example: C.T.: 5P20, 5 VA. In this case, ALF = 20 and composite error < 5 % up to 20 times rated current for burden of 5VA. If the actual burden < 5 VA, composite error is less than 5%, even for currents > 20 times rated current.

Specifying ALF > 20 is not useful as relay operating time characteristic flattens out at 20 times rated current (Refer Fig.7).

Fig.7. IDMT Characteristics

A.L.F. is relevant only for protection class CTs since it is required to retain specified accuracy at current values above normal rating to faithfully reflect the fault currents. A.L.F is not relevant for CTs mounted on neutral circuit in medium and high resistance grounded systems and for metering class.

5.3 Rated burden

Burden is the load burden in VA, of all equipment connected to CT secondary circuit, at rated CT secondary current.

Burden and accuracy limit factor (ALF) are two sides of the same coin. The selection of these two parameters depends on the voltage required to be developed by the current transformer during faults. For protection class CTs the actual voltage required on CT secondary (Refer Fig. 8)

VACTUAL = IF (RCT + 2 * RL+ RR) ,where

IF = Reflected fault current, RCT = CT resistance, RL = Lead resistance, RR = Relay resistance

Fig.8

It may be mentioned in passing that, even if very low burden numerical relays are used, only RR in above expression is low but other factors are significant.

The design value of CT secondary voltage is given by

VDESIGN = Burden x Accuracy Limit Factor (A.L.F) ΙRAT (Secondary)

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As the rated CT secondary current is known, any standard value of A.L.F and burden may be selected to satisfy

Design voltage across CT > Actual volts required, VDESIGN > VACTUAL Example: CT : Ratio - 800 /1: 5P20, 10 VA IF =30kA; RCT = 3Ω ; RL = 1Ω; RR = 0Ω VACTUAL = (30000/800) * (3 + 2*1) = 187.5 V VDESIGN = 20 x 10 / 1 = 200 V

The chosen parameters are acceptable since VDESIGN > VACTUAL.

6. SPECIAL PROTECTION CLASS 6.1 Knee point voltage (Vk)

Knee point voltage (VK) at which CT starts saturating is defined as the point where exciting current increases by 50% for 10% increase in voltage (Refer Fig. 6). Knee point voltage is relevant only during external fault conditions and does not have significance during normal operating conditions. The knee point voltage (Vk) for Class PS CTs used in high impedance scheme is calculated for the worst condition that one of the CTs is fully saturated and the other CT has to develop enough voltage to drive current through the other CT circuit to ensure stability during external fault.

A typical current balanced scheme which operates by sensing the difference of two or more currents measured by the CTs located on two sides of the protected object is shown in Fig. 9.

Fig.9. Current Balanced Scheme

During internal fault conditions, CT2 presents an open circuit (Refer Fig. 10).

Fig.10

During external fault conditions CT2 presents short circuit when it is saturated (Refer Fig. 11).

Fig.11

Now, CT1 has to develop enough voltage to drive current through the complete CT circuit.

VREQUIRED during external fault condition with CT2 saturated, VREQUIRED = IF (Rct1 + RL1+RL3+Rct2 + RL4+ RL2) Assuming, Rct1 = Rct2 = Rct and RL1= RL3= RL4= RL2= RL VREQUIRED = IF (2*Rct + 4*RL) VREQUIRED = 2* IF (Rct + 2*RL)

Therefore, knee point voltage, for Class PS CTs is Vk (min) > VREQUIRED = 2 * IF (RCT + 2RL)

where,

VK (min) = Minimum Knee Point Voltage IF = Max. through fault current to which CTs

are subjected to.

RCT = C.T secondary resistance typically varies from 1 to 8 Ω

RL = Lead resistance typically 8 ohms / km for 2.5 mm2 Cu control cable

Modern numerical relays offer low impedance biased schemes as an alternate which achieves stability during through faults by algorithmic calculation after measuring CT secondary currents. In such cases, the CT requirements furnished by relay manufacturer may be followed. 6.1.1 Fault current for CT sizing

Following guidelines are used for choosing appropriate fault current IF for knee point voltage calculations of CTs used in biased differential protection scheme of transformer to avoid CT oversizing:

i. LT side of transformer - LT system fault current or 20 times rated current of LT CT, whichever is lower.

ii. HT side of transformer - HT system fault current or 20 times rated current of HT CT, whichever is lower.

The rational for the above is as follows:

i. In case of LT side fault, fault current will not exceed 20 times rated current assuming minimum transformer impedance as 5%.

ii. In case of HT side fault, only CTs on HT side carry current. Assume relay pickup setting as 10% (0.1 A for 1A CT) and fault current 20 times rated current. Now, even if 19A is consumed in saturation, the available secondary current of 1A is enough to operate the relay.

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6.2 Exciting current (IEX)

Error in transformation is due to exciting current (IEX) because of which the proportionality between primary and secondary current is not maintained. For Class PS CT, this proportionality is retained to a high degree by specifying a low exciting current. Usually IEX <30mA is specified for 1A CT and IEX < 150mA is specified for 5A CT at VK / 4.

6.2.1 Why IEX <30mA or IEX <150mA?

In current balanced scheme to avoid mal operation of protection scheme during normal operating conditions, the spill current through the differential relay should be less than the relay pick up (Refer Fig. 12).

Fig.12

Therefore for such schemes the relay pickup current is set based on the number of CTs in the circuits and the exciting current for each CT. Assuming a relay pickup of 10% i.e 0.1 A for a 1A CT, the exciting current of CTs can be <30mA when used for a three winding transformer (Refer Fig.13). It can be even 45mA for a 1A CT for a two winding transformer. On similar lines 150mA is normally specified for 5A CT.

Fig.13

6.2.2 ΙEX to be specified at VK / 4 or VK / 2?

ΙEX is relevant only during normal operating conditions to ensure stability and prevent false tripping and is not relevant during faults. Under fault conditions,

Knee point voltage (Vk) = 2ΙF (RCT + 2 RL) Under normal conditions,

ΙRAT VCT = ΙRAT (RCT + 2 RL) = Vk --- 2ΙF ΙRAT 3 KA e.g. - --- = --- = 0. 05 ⇒ 5% 2ΙF 2 x30 KA

As seen from above, under healthy conditions, voltage required to be developed by CT is only 5% of the knee point voltage. Therefore, specifying ΙEX @ VK / 4 (25%) is more than adequate whereas specifying ΙEX @ VK / 2 (50%) is a conservative design resulting in bigger size of CTs. The exciting current at VK / 4 is less than that at VK / 2 (Refer Fig. 14). Considering a limiting value of 30mA for exciting current, specifying 30mA @ Vk/4 is adequate.

Fig.14

Generally identical class PS CTs are used in both sides of the protected equipment. It is not necessary to order both CTs from same vendor as long as class PS requirements are met. Point by point matching of saturation characteristics for the CTs is not mandatory and not required. For example, if ΙEX < 30 mA @ VK/4 for both the CTs, they are acceptable. (Refer Fig. 15).

Fig.15

A point may be noted here that it is not mandatory to use Class PS CTs in current balanced schemes. General protection class CTs can be used as long as the CT have low exciting current and is able to develop enough knee point voltage required for the said application. The site test results given in table (Table-1) shows that the exciting current for a protection class CT is less than that of a Class PS CT. Also, it has a higher knee point voltage compared to a Class PS CT.

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Special Protection

Class General Protection Class 1600/5A, Cl. PS, Vk >

130, Ie < 150mA @

Vk/4, Rct < 0.8 ohm 1600/5A, Cl. 5P20, 20VA Volts Current (mA) Volts Current (mA)

10 10 10 5 75 42 40 12 130 71 80 20 143 85 120 33 158 (Vk) 111 171 (Vk) 77 174 181 190 132 Table-1

6.3 Secondary winding resistance (Rct)

Winding resistance is part of the CT burden and is taken into account while determining knee point voltage requirement of CT. For special protection class CTs (Class PS), CT secondary winding resistance is usually specified. However it is preferable to furnish expression for knee point voltage, fault current and lead resistance values and not to specify both knee point voltage and Rct to the vendor. The vendor can then optimally choose Rct to get the desired knee point voltage. This will avoid oversizing of CTs.

7. METERING CLASS 7.1 Accuracy class

Accuracy class is defined as the maximum ratio error at rated current and at rated burden. Class 0.1, 0.2 and 0.5 CTs are used for precision industrial metering / tariff metering. As per IS -2705 accuracy is not guaranteed for current less than 20% of the rated current. If current through the metered line is much less than the rated current of CT, for majority of time, anticipated accuracy is never realised in practice. This is mostly true for tie lines connecting industrial plants with captive power plant and grid.

7.2 Instrument security factor (I.S.F)

Instrument security factor (I.S.F) is defined as the ratio of minimum value of primary current to the rated current at which composite error of CT is greater than 10%. This signifies the current at which the CT starts saturating to protect the apparatus supplied by CT in the event of the system fault. Therefore it may be emphasized that metering CTs should saturate after certain current may be10ΙN to protect meters while protection CTs should not saturate up to 20ΙN toensure accuracy during fault conditions. Therefore knee point voltage and ALF are not relevant for metering

CTs. The site test results given in the table below (Table-2) shows that knee point voltage for a metering CT is much less than that of a protection class CT. This is one way to identify metering core at site.

Metering Class General Protection Class 1600/5A, Cl. 0.5, 15VA 1600/5A, Cl. 5P20, 20VA

Volts Current (mA) Volts Current (mA)

6 5 40 12 10 7 80 20 20 12 120 33 30 (Vk) 22 171 (Vk) 77 33 33 190 132 Table-2

Generally I.S.F. is specified less than 5. However this does not have much practical significance and I.S.F = 10 is acceptable as the ammeters and current coils of meters are designed to withstand 10 times the rated current for 5 seconds.

It may be noted that a current transformer with high accuracy class and low I.S.F cannot be realised in practice. High accuracy class requires low excitation current which in turn results in bigger core. The saturation point of a bigger core is high which contradicts the requirement of low I.S.F.

It may be worth mentioning that meters can also be connected to protection core for feeders with instantaneous protection where fault clearing time is less than 100ms. As mentioned above, meters are designed to withstand 10 times the rated current for 5 seconds and faults are generally cleared within one (1) sec. Therefore for 1 sec the coil can withstand,

Ι2 x 1 = (10 Ι

R)2 x 5 = 500 ΙR2

Ι = 22.4 ΙR , where ΙR is the CT rated current Therefore, if ΙFAULT is less than 22.4 times CT rated primary current, indicating meters can be connected to protection core and no separate metering core is required.

7.3 Rated burden

Burden usually expressed in VA indicates the impedance of the CT secondary circuit at a specified power factor and at the rated secondary current. The accuracy requirements are specified at rated burden. For a current transformer the rated burden should be carefully chosen based on the equipments connected in the secondary circuit as burden has bearing on the price of CT.

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8. CONCLUSION

The article covers salient aspects that the user should consider to realize CTs that are not oversized. A radical rethink when selecting primary rating of CT for protection application is needed. Extreme care shall be exercised when selecting knee point voltage and exciting current for CTs used in current balanced schemes. I.S.F for metering CTs can be 10 without endangering meters. The practicing engineer is encouraged to apply the ideas presented here to realize optimally sized CTs.

9. REFERENCES

i. Protective Relays- Application Guide – GEC Measurements

ii. The design of Electrical Systems for large projects (in India) – N Balasubramanyam iii. Electrical Measurements and Measuring

References

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