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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Power Transformer Protection R. P. Gupta, M. B. Lonkar Global R & D Centre Crompton Greaves Limited, Mumbai India Ramprakash.Gupta@cgglobal.com,Manoj.Lonkar@cgglobal.com

Abstract—A power transformer is a very valuable and an important link in power transmission system. It provides voltage transformation and power transfer. Faults occur in transformer due to insulation breakdown, aging of insulation, overheating due to over-excitation, oil contamination and reduced cooling. It is an important aspect to implement reliable, secure and fast protection scheme which is essential to minimize damage. In this paper, various faults taking place in transformer and suitable protections are described. This paper will be useful to enhance knowledge of the students and practicing engineers in the area of substation engineering.

consisting of bus fault logic, feeder back-up logic, and two-bank substation load shedding are described [1]. Multifunctional microprocessor-based relays are used for protection. According to additional features and applications, the complexities in these relays are increasing. A sophisticated computer software is required for the analysis, setting coordination, data acquisition and testing of intelligent electronic devices (IEDs). The paper describes automatic test system used for advancement in transformer protection [2]. The inrush current in power transformers is causing unreliable operation of numerical relay. Artificial Neural Network (ANN) is applied to inrush detection. As the saturation of protective Current Transformers (CT) cannot be totally eliminated, ANN is used for the reconstruction of distorted secondary CT currents. ANN is applied in both cases. It is experimentally proved the enhancement in reliability due to reconstruction of distorted CT secondary. The results confirm faster and more reliable recognition of transformer inrush, as well as satisfactory reconstruction of the distorted secondary CT currents. Thus, the possibility of improving digital power transformer protection is described [3]. Protection of transformer is described using principle of excitation impedance. On occurrence of external faults, fault components of terminal voltages and currents are obtained. The large magnitude and positive values are used for calculating excitation impedance. On occurrence of internal fault, the impedance is calculated with fault component. It is the impedances of the transmission lines and equivalent sources of system. In this situation, the magnitude is smaller than that calculated when external fault occurs, and the sign is negative. In such situation of inrush current, the sign of fault component impedance is positive and the magnitude is larger than that in internal fault situation. The method described can act as the primary protection of transformers with wye-delta connection. The simulation is also given which shows that the implemented technique can immunize the maloperation when transformer is energized, and has high sensitivity and reliability [4]. A fault detection technique using fault generated high frequency current transients is described. The transients are to be generated through a designed relay unit. The relay is to be tuned to a band of high frequencies. It is connected to the transformer through CTs on both the high and low voltage sides of the transformer. When a fault is detected, the transient currents from both ends of CTs are extracted by the detector. The working of detector is based on differential and average currents

Keywords—Differential protection, back-up protection, over fluxing, Buchholz relay

INTRODUCTION

Power Transformer plays a vital role in power system. It is everywhere–in all parts of power system, between all voltage levels, and existing in many different sizes, types, and connections. The increasing demand of power in transmission and distribution system has resulted in design of very large capacity transformers. The choice of suitable protection is decided on the basis of economical considerations. Circuit breaker or other disconnecting device is available at or near the winding terminal of transformer bank. The transformer bank thus can be connected directly to a bus, line, or a power source. For both phase and ground faults, differential protection provides, the best overall protection. In ungrounded system, differential protection provides only phase fault protection. Differential protection is usually provided to transformer of 10 MVA and above. IDMT relays are to be used to provide back–up protection. In this paper, the authors have given various transformer faults conditions and accordingly protective schemes.

II.

POWER TRANSFORMER PROTECTION

In this section, specific areas of transformer protection are given. In case of power plant transformer protection, the protective schemes are described for over-excitation, differential restraint, generator step-up unit (GSU) transformer ground fault protection, and auxiliary/start I up and industrial transformer protection. For transmission substation transformer protection, over-excitation (V/Hz) protection and sudden pressure relay (SPR) blocking is given. For distribution substation transformer protection, under-frequency / under-voltage load shedding is described. The logic schemes for distribution substation

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

saturate the energized transformer. It thus results in an apparent inrush current

between the two ends. The spectral energies of these current signals are then extracted to produce the operate and restraint signals. On the basis of comparison between the levels of the two signals, it comes to know whether the fault is internal or external to the protected zone [5]. The current ratio differential relaying (CDCR) with harmonic restraint is used for transformer protection. The second harmonic current components depend on the capacitance of the high voltage status and underground distribution. It can be reduced with necessary changes in material of iron core or its design technology. Modification in the CDCR relay is thus required. A numerical algorithm for enhanced power transformer protection is described. The algorithm enables a clear distinction of internal faults, magnetizing inrush and steady state. It uses the RMS fluctuation of terminal voltage, instantaneous value of differential current, RMS changes, harmonic component analysis of differential current, and analysis of flux-differential slope characteristics. The result shows more rapid and reliable performance of protection of transformer [6].

Transformer deenergized at this point

Transformer reenergized at this point

(a)

III. POWER TRANSFORMER PROTECTION SCHEMES Transformer deenergized at this point

Various faults that can occur in power transformer: HV and LV bushing flashover (external to tank) HV winding earth fault LV winding earth fault Inter turn fault Core fault and Tank fault Phase to phase faults are relatively rare within the tank. These are more likely to take place external to tank on HV and LV bushing. The required protections for power transformers are given in this section. Terms related with energizing current of transformer are given below. i) Energizing Inrush-It is caused by remanance (residual flux) in the core and the point in the voltage waveform when the transformer breaker closes. Under the normal steady state conditions, the magnetizing current required to produce the necessary flux is relatively small, usually less than 1% of full load current. In case of transformer is energized at voltage zero, the magnitude of flux during first half of voltage cycle becomes as high as twice the normal current flux. The energization with and without inrush is represented in fig. 1(a) and fig.1 (b) respectively. The relay may be given a setting higher than the maximum inrush current. Also, the time setting may be such that before the relay operates, magnetizing current should fall to a value below the primary current. ii) Recovery inrush- In case of fault or momentary dip in voltage, an inrush may occur when the voltage return to normal. It occurs at the clearing of an external fault. The current level is less than it is during the transformer energization. iii) Sympathetic inrush- It is due to energization of adjacent transformer. A common case is paralleling a second transformer bank with a bank already in operation. The dc component of inrush current can also

(b) Fig.1. Energization (a) without inrush (b) with inrush

A. Fuse Protection: Fuses are economical, require less maintenance, and do not need external power supply. Fuses can protect some power transformer against damage from primary and secondary faults. Fuses are insensitive and relatively slow except at higher current levels. It does not sense low level faults. In case of fault current magnitude is extremely high; a fuse can be faster than a circuit breaker and can clear the fault within 0.5 to 2 cycles. For fused transformer protection, it is recommended to implement phase and ground over current relays with a low side circuit breaker. This is to provide back-up protection of secondary faults. B. Differential Protection: It compares currents entering and leaving the protected zone and operates when the differential current exceeds the predetermined value. The type of differential scheme normally applied to a transformer is called the current balance or circulating current scheme. The CTs are connected in series and the secondary current circulates between them as in fig. 2 (a). In case of internal fault, the relay operates since both the CT secondary currents add up and passes through the relay as represented in fig. 2(b).

Transformer reenergized at this point

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

CT 1

differential scheme is represented for a delta–star transformer in fig.3.

CT 2

Fault

(a) CT 1

CT 2 C

Fault

BW-Biased Winding, A/B/C-Operating Coil/Winding Fig.3. Differential protection scheme for a delta–star transformer

(b) Fig.2. (a) Differential connection for single phase transformer (b) Differential relay operation for internal fault

Differential protection provides faster detection of faults. The determination of fault location depends on size of zone. Relays of three general classes are used in current differential scheme Time over current relay Percentage differential with restraint actuated by currents going into and out of protection zone Percentage differential with restraint actuated by one or more harmonics in addition to the restraint actuated by currents going into and out of protection zone The CTs are connected in series and the secondary current circulates between them. In case of internal fault, the relay operates since both the CT secondary currents add up and pass through the relay. There are some factors which need consideration in differential scheme.

In case of transformer differential scheme, most relays are provided with bias with slope of 20%, 30% and 40% as required. The requirements of differential protections: i) Triple pole with individual phase indication ii).Unrestrained instantaneous high set over-units which should not operate during inrush. iii) One bias winding per phase and per CT input iv) An adjustable operating current v) An adjustable multi-bias setting vi) Second harmonic or in-rush proof vii) Fast operating time C. Restricted Earth fault Protection (REF): The biased differential relay is not sensitized for certain earth faults within winding. This situation occurs in case of transformer is resistance or impedance earthed and current in the internal fault is disproportionately low. The earth fault protection gets improved by the application of unit differential or restricted earth fault systems. The residual current of three line CTs is balanced against the output current if CT in the neutral conductor. It makes the protective scheme stable for all types of faults outside the zone. For low voltage side, the value of earth fault current may less than full load current in case of fault occurs in midwinding. In such cases, the value of voltage available is half the line voltage. High voltage over current will not provide required protection. The delta winding cannot supply zero sequence current to the system. A relay, an instantaneous high impedance type, is to be connected to monitor the residual current will provide restricted earth fault protection. The protective scheme is represented in fig.4.

i) Transformer vector group ii) Mismatch of HV and LV CTs iii) Varying currents due to on-load tap changer (OLTC) iv) Magnetizing in-rush currents (from one side only) v) The possibility of zero sequence current destabilizing the differential for an external earth fault. Factor (i) can be overcome by connecting the HV and LV CTs in star/delta respectively (or vice versa). The opposite vector group connections counter balance the effect of phase shift occurred in transformer. The delta connection of CTs provides a path for circulating zero sequence current. Thus is stabilizes the protection for an external earth fault as required by factor (v). To overcome the current unbalance caused by factors (ii) mismatch of CTs and (iii) OLTC, it is necessary to bias the differential relay. Harmonic filters are to be used to stabilize the protection in case of magnetizing current in-rush i.e., for factor (iv). The operating range of CT causes the biggest current unbalance under healthy conditions. Hence, desired setting is dictated by the operating range of the OLTC. A

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

i) It should be of three pole type ii) IDMT characteristics, directional on LV side iii) Variable setting range 50-200% of rated current iv) Variable setting 50-200% v) Characteristics angle 45-90 degree lead vi) Hand reset indicators per phase

R R

Fig. 4. Restricted earth fault relay

The requirement of REF protection: i) It should be of single type ii) Operating current sensitivity of at least 10% iii) Be tuned with system frequency iv) High or low impedance principle v) Suitable value on non-linear resistor to limit peak voltage during in-zone faults

Ground faults in case of delta winding connected to solidly grounded power system are relatively easy to sense. Time delay is must be used for high sensitivity setting. The requirement of ground over current protection: i) It should be of single pole type ii) IDMT characteristics iii) Variable setting 20-80% of rated current iv) Characteristics angle 45-60 degrees lag v) Hand reset indicators per phase

D. Overfluxing Protection: The increase in input voltage increases the level of working flux. This results in increasing magnetizing and iron current. The core and core belt get heated leads to weaken the interlamination insulation and core belt insulation. Also, the same effect is observed in case of reduction in supply frequency. In such cases V/f relays are used to detect over fluxing in transformer. This phenomenon is more in generator transformers. For 50 Hz AC supply, setting of relay should be 1.0 to 1.3 on 110 V. The relay is provided adjustable time delay so as to prevent transient operation due to momentary disturbances. The transformer over fluxing protection is implemented for both sides of interconnecting transformer. This is to cover all possible operating conditions. For other transformers, over fluxing relay should be provided on the untapped winding of transformer. The requirement of over fluxing protection: i) Phase to phase connection ii) Operation on voltage to frequency ratio iii) High resetting ration of 98 % or more iv) Inverse time characteristics compatible with transformer over fluxing v) Independent alarm with definite time delay at value voltage to frequency between 100-130%

I OL

RCL

I OL I OL

Phase Relays

3 IOL / R

R 87 3 IOL / R

Fig. 5. Ground fault protection for delta-star transformer with residual over current and differentially ground relay

(b) Negative sequence over current It is particularly applicable to delta-star grounded transformer where only 58% of secondary per unit phase to ground fault current appears in any one phase. It can be connected in primary supply of transformer. The sensitivity of setting may be according to protection against secondary phase-ground or phase-phase-faults. The relay setting should be higher than negative sequence current due to unbalanced load. F. Sudden Pressure protection: This protection scheme is used for 5 MVA and above transformer typically provided with gas cushion. The gas pressure is generated by an arc under the coil. It thus leads to oil decomposition and gas product formation. The relay is sensitive to both low- and high energy arcs within the transformer and have inverse time characteristics. It is fast for heavy faults and slow for light faults. It is usually connected to the contact parallel to differential relay and other trip relay contacts. As represented in fig.6, the change in pressure actuates bellows 5 closing micro-switch contact 7. Equalizer port 8 is much smaller than port 4. It prevents bellows movement for slow changes in gas pressure. This type of protective scheme is only designed to respond only to arcs within the coil. It detects an increase in gas pressure.

E. Over current Protection: (a) Phase-or and ground- fault over current The external faults are the short circuits or earth faults outside the transformer. At about 10 MVA and below, primary fuse may be used. The inverse time over current relays and in case of high voltage line, distance relays provide protection for transformer. Instantaneous over current relays may be applied as supplemental differential or over current relays. The relays must not operate on magnetizing inrush unless harmonic restraint is used. The high fault current passing through transformer can cause thermal as well as mechanical damage. The relays or fuses should protect the transformers against through faults. The ground fault protection for delta-star transformer with residual over current and differentially ground relay is represented in fig. 5. The requirement of phase over current protection:

228 ÂŠ 2009 ACEEE

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

the setting of restraint is adequate to prevent the relay operation. In fig.8, various protection schemes are represented for power transformer. Also, differential protection scheme for 13.8 kV, breaker failure for 115 kV may exist.

115 kV 50/51 P,N,Q

400:5

Fig. 6. Sudden Pressure relay 63 49

G. Turn- to- turn fault protection: Due to electromechanical /electromagnetic forces on the winding, chafing or cracking of insulation between turns takes place. Ingress of moisture content in to the oil can also be a contributing factor. When the transformer is subjected to lightning surge, the steep–fronted surges may hit the end windings causing the puncture in the insulation. It thus leads to short circuit. Phase differential relay may detect a turn-to-turn fault protection as this fault changes the transformer turns ratio. The fault case is given in fig.7.

18/24/30 MVA 400:5

RG 400A 20 Ω 10 Sec

RG 51G

87N

Trip Dir 51 P.Q,N Partial Differential

2000:5 Bus BU

67/51 P,Q,N 32

E Short Turn

27 P, 59P/N, 47

L O A D

2000:5 51 P.Q,N

Fig.7. Turn-Turn fault

Bus Tie 52

13.8 kV

2000:5

H. Excessive high voltage protection: The relay offers high impedance to CTs. Hence, CTs are required to develop an extremely high voltage. To limit this voltage, a voltage dependent resistor (VDR) is normally mounted across the relay to prevent the external flashover. It is usually preferred in polluted environments. For maximum efficiency, transformers are operated near the knee of their saturation curve. So, at voltage above about 110% of rated, the exciting current becomes very high. These large currents can destroy the units. The protection against over voltage is included in the regulating and control devices. I.

Harmonic restraint protection:

The magnetizing inrush current has a high harmonic content, particularly second harmonic. The second harmonic can be used to restrain and thus desensitize the relay during energization. While setting the relay in such case, it should be made sensitive to internal fault. In harmonic restraint relay unit, a second harmonic blocking filter is connected in operating coil circuit, whereas a second harmonic pass filter in restraint coil circuit. Thus, the predominant second harmonic characteristics of an inrush current produce ample restraint. It requires minimum operating energy. The circuit is designed to hold open its contact when secondary harmonic component is higher than 15% of fundamental. For all inrush currents,

a) Relay units: 50-Instantaneous, 51-Time over current, 49-Thermal, 63-Sudden Pressure, 67-Directional over current, 87-Differential b) 52AC circuit breaker c) RG – Ground resistance Fig.8. Various protection for power transformer.

J. Thermal Protection The conventional thermal relays measure the oil temperature and transformer current to calculate hot-spot temperature. It is usually supplied as a part of transformer. It is used for monitoring and alarm, but may be used for tripping. K . Gas Detection: Gas detection devices can be applied only to the transformer units built with conservator tanks. A gas accumulator device commonly known as Buchholz relay is connected between the main and conservator tanks. Failure of winding insulation will result in some form of arcing. It collects any gas rising through oil. It decomposes the oil into hydrogen, acetylene, methane. It can detect the core fault. The intense localized heat damages the winding insulation. Localized heating can also precipitate a breakdown of oil into gas. One part of this device accumulates gas over a time period and provides sensitive indication of low energy arcs. It is used for alarm. The other part of this device

Auto Tap Changer

87R 87U

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010 [2] Apostolov, A.; Vandiver, B., “Automatic test system advances transformer protection”, Computer Applications in Power, IEEE, Volume 13, Issue 2, Apr 2000 Page(s):31 – 36 [3] Pihler J. et al. “Improved operation of power transformer protection using artificial neural network” IEEE transactions on power delivery , 1997, vol. 12, no3, pp. 1128-1136 (19 ref.) [4] Suonan J., et al., “A Novel Transformer Protection Principle Based on the Excitation Impedance” Developments in Power System Protection, 2008. DPSP 2008. IET 9th International Conference on Volume , Issue , 17-20 March 2008 Page(s):308 – 314 [5] Aggarwal, R.K et al. “A new relaying principle for power transformer protection using transient comparison technique” , Developments in Power System Protection, Sixth International Conference on (Conf. Publ. No. 434) Volume , Issue , 25-27 Mar 1997 Page(s):139 - 142 [6] Chul-Won Park et al. “Numerical Algorithm for Power Transformer Protection” 14b6 KIEE International Transactions on PE, Vol. 4-A No. 3, pp. 146-151, 2004 [7] Rockfeller G. Basler Electric,” Transformer Protection Application Guide” revised 2006/07 John Horak. [8] Hewiston L.G., Mark Brown ,Ben Ramesh “Practical Power System Protection” Newnes Publications 2004, Burlington MA 01803, ISBN 0 7506 6397 9 [9] J.L.Blackburn, “Protective Relaying Principles and Applications Second Edition” Marcel Dekker ,Inc.USA.1998, ISBN 0-8247-9918-6 [10] W.A.Elmore, ABB Power T&D Co.Inc. ,Florida “Protective Relaying Theory and Applications” Marcel Dekker ,Inc.USA.1994, ISBN 0-8247-9152-5

responds to heavy faults operating the relay with high velocity to give trip signal. It can detect the core fault. L . Tank fault: Loss of oil through leak in the tank can cause a reduction in insulation and possibly overheating on normal load takes place. Oil sludge can also block the cooling duct and pipes, which may cause overheating. The MV fuses are to be used for protection in such cases. CONCLUSION Various faults taking place in transformer are described. The required protective schemes are given accordingly. The utility may prefer the required protection necessary for power transformers. ACKNOWLEDGEMENT The authors duly acknowledge the support provided by Global R & D Center, Crompton Greaves Limited, Mumbai, India. REFERENCES [1]

Mozina, C.J., Beckwith Electr. Co. Inc., Largo, FL; “Protection of power plant transformers using digital technology”, Transmission and Distribution Conference, 1999 IEEE, Publication 11-16 Apr 1999, Volume: 1, page(s): 421-432