Realistic Ct Specs

REALISTIC SPECIFICATION FOR CURRENT TRANSFORMER Dr. K Rajamani and Ms. Bina Mitra Reliance Energy Ltd., Mumbai IM = Magnetising component 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.

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.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.

Fig. 3. Phasor Diagram of Current Transformer

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 = Exciting current Ic = Core loss component

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 1

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

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).

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

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).

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)

Fig.5. Saturated Current Transformer

3.1.3 Metering class i. Accuracy class ii. Instrument security factor (I.S.F) iii. Rated burden

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.

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.

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

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. Eg: CT Ratio = 200 / 1 2

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

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.

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.

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.

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

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.

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) 3

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

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.

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

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.

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.

Fig.9. Current Balanced Scheme

The rational for the above is as follows:

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

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.

Fig.10 4

3 KA ΙRAT 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.

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