NUMERICAL RELAY BASED TRANSFORMER PROTECTION

1VRUSHALI BORIKAR, 2T.G ARORA

1,2Department of Electrical Engineering, RCOEM, Nagpur E-mail: [email protected], [email protected]

Abstract - In power system the transformer is the most vital and expensive component. So we provide differential protection to the power transformer in order to prevent from abnormal condition. When internal fault occur this system works rapidly. However it should not operate under normal condition or non-fault condition such as a flowing inrush current. MATLAB/SIMULINK platform were used to study differential protection relay and under voltage relay for a power transformer. The basic approach is to give protection to the power transformer against internal faults and prevent interruption due to other operating conditions. In this paper, transformer terminal voltage and current signals are sensed by current transformer and potential transformer and utilized to enhance the reliability of differential protection.The proposed relay was able to distinguished between fault and no-fault conditions.

Keywords - Differential Protection, Power Transformer, Matlab, Fault Conditions.

I. INTRODUCTION The Matching of CT ratio In order to prevent from phase error and ratio error Power transformers are subject to different types of we used same CT ratio that is 1000/1 on primary as transient disturbances including internal fault, and well as on secondary side of transformer. Therefore it external fault and through-fault currents. If a power compensate these two problem of transformer. This is transformer experiences a fault, it is necessary to take the one of major advantage of proposed method. the transformer out of service in order to prevent from damages. The costs associated with repairing a Phase Shift through the Transformer damaged maybe high. Due to imbalance of current Having eliminated the problem of zero sequence cause the relay to malfunction. This is due to inrush currents through faults will still produce positive and magnetizing currents, stationary over excitation of a negative sequence currents that will be seen by the Core, external faults in the presence of current protection CTs. These currents may experience a transformer (CT) saturation, and/or CT and power phase shift as they pass through the transformer transformer ratio mismatch [5]. One of the most depending upon the transformer vector group. CT effective methods of protection to protect Power secondary connections must compensate to avoid transformers is the Differential protection method by imbalance and a possible mal-operation [7]. using differential relay circuits. This scheme is based on the principle of that the power input to the transformer under normal conditions is equal to the power out. [5] By proper connection of the secondary’s of current transformers (CT), under normal conditions, no current will flow into the relay coil. Whenever a fault occurs the current balance will no longer exist and relay contacts will close and Figure 1. CT Secondary Current release a trip signal to cause a certain Circuit Breakers (CB) to operate in order to disconnect the III. MATERIALS &METHODS faulty equipment. The Simulink Power system library browser in the II. DESIGN CONSIDERATION Matlab/ Simulink environment is used to model the power transformer protection system. A number of factors have to be taken into account in designing a scheme to meet these objectives. These The following components make up the fault include: simulation model: A. Three-Phase Breaker  The matching of CT ratios B. Three-Phase Source  Phase shift through the transformer C. Three-Phase Transformer  Magnetizing inrush current D. Three-Phase Fault E. Relay F. Three-Phase Series RLC Load

Proceedings of WRFER International Conference, 16th April 2017, Pune, India 115 Numerical Relay Based Transformer Protection

Figure.2 Block Diagram

One current transformer (CT) is connected in the I1= Primary Current, primary winding and a second one used in the I2= Secondary Current, secondary winding. The outputs of the CTs are B = Biasing connected in such a way that the current of each CT opposes the other, thus, it gives normally a zero sum Under Voltage Relay of the two currents. If the sum is not zero, this may A relay that has contacts that operate when the mean that the ratio of one of the windings has been voltage drops below a set voltage. Under voltage changed because of a short between the windings and relays are used for protection against voltage drops, consequently a trip signal can be sent by the to detect short-circuit faults, etc protection relay to the circuit breaker in order to trip Therefore the principle of both the relay illustrate in and disconnect the transformer from the power line figure 4 are as follows. and load[9]. IV. SIMULATION The major problem to the transformer differential protection is sustain security during CT saturation for The simulation has been done on 600KVA, 50Hz, external faults (e.g. short circuit in the secondary of 11kV/440V, the transformer) while maintaining sensitivity to Three phase ∆/ Transformer. The Three phase detect low magnitude internal faults. CT saturation source is connected to RL circuit and then circuit reduces the secondary Output current from the CT, breaker is connected. and causes a false differential current to appear to the After circuit breaker the current transformer is used to relay [1]-[3]. When the transformer is connected to measure primary and secondary current and lastly power line, it energizes the transformer and produced load is connected. The relay will provide the trip an inrush current which is similar to internal fault. If signal which is shown in Figure 3. no prevention is considered there may be chances of false tripping single given to circuit breaker. Simulation Parameter In this paper two relay principles have been explained are as follow: Table no.1 Simulation Parameters 1. Percentage Differential Relay Parameter Value 2. Under Voltage Relay Three Phase Transformer 600KVA Percentage Differential Relay Frequency 50 Hz This relay is employed for the protection of transformer against internal short circuit.It compares Primary Voltage 11kV the current entering the transformer with the current Secondary Voltage 440 V leaving the element. CT Ratio 1000/1 If they are equal there is no fault inside the zone of protection. No fault Condition: (1 − 2) = (1 + 2)/2 If they are not equal it means that a fault occurs between the two windings Fault: (1 − 2) > (1 + 2)/2

(I1-I2)= Differential Current (I diff) B (I1+I2)/2 = Restraining Current

Where, Figure.3: Matlab/Simulink Model of Transformer Protection

Proceedings of WRFER International Conference, 16th April 2017, Pune, India 116 Numerical Relay Based Transformer Protection 4. Is2 0.576 5. Vph 432.7

Figure 4: Relay Logic Figure 5.Primary current

In this method the primary current and secondary currents are first calculated using current transformer. Then secondary voltage measure by using potential transformer and compared with some constant value. Here we used under voltage relay if voltage is below the constant value the trip single will provided. Now Figure 6.Secondary current primary current and secondary current are calculated. The secondary current is referred to the primary current using turn ratio. At the same time, the primary current goes through a time delay which compensates the phase shifts between the primary and secondary.The small changes has been done to compensate magnetizing current and core loss. Figure 7. Phase Voltage (Vph) : In normal case the differential current will be equal to zero. However in internal fault condition it will be not equal to zero. During inrush condition there may be chance of false tripping so therefore the resulting current is calculated and subtracted the RMS valve of differential current. Figure 8. Trip

IV. RESULTS AND DISCUSSON Figure 9. Shows that there is no tripping signal provided by relay to the circuit breaker. Simulation Results: A. Simulation is done during Inrush without load The external and internal fault cases are simulated. Condition : These currents are generated when the circuit breaker In this condition, the transformer first energize which is closed to connect the transformer and external fault simulate without any internal fault. The following appears. The currents are measured by current differential current RMSvalue is obtained. Now transformers and then introduced to the relay. noticed the current increases during first cycle and The following conditions are presented in this paper, then decays gradually. 1. Healthy condition 2. Magnetizing inrush condition Table No 3. Inrush without load Condition 3. External faults condition SR Peak Value RMS Value 4. Internal fault condition No 1. Ip1 41.37 The study of current, voltage waveform and trip 2. Ip2 0.3257 condition are shown in fig 3. Is1 0 4. Is2 0 A. Simulation is done during Healthy Condition: 5. Vph 419.8

In this condition, the transformer is operating normally and the currents are well below the peak value. Table No 2. Healthy Condition SR Peak Value RMS Value No 1. Ip1=32.2A 23.26 2. Ip2 0.576 Figure9.Primary current 3. Is1=820A 576

Proceedings of WRFER International Conference, 16th April 2017, Pune, India 117 Numerical Relay Based Transformer Protection

Figure10. Secondary current Figure15. Phase Voltage (Vph)

Figure16.Trip

Figure11. Phase Voltage (Vph) The relay does not issue the trip signal which is shown in figure16, as the restraining current are well below the operating current. This is similar to other external fault condition. Thus this relay provides stability during external fault condition by properly adjusting the biasing of percentage differential relay.

Figure 12.Trip B. Simulation is done during Internal Fault Condition :

Figure 12. Shows that there is no tripping signal Now, an internal fault was occurs the transformer generated because there is continuously decaying of energies and relay was able to detect it and trip signal current to zero steady state value. So even though the is given to circuit breaker shown in figure. 16 current value is greater, the controller was able to recognize that is due to inrush current. Table No 5. Internal Fault Condition SR Peak Value RMS Value B. Simulation is done during External Fault No Condition : 1. Ip1 212.13 2. Ip2 0.070 Table No 4. External Fault Condition 3. Is1 11.45 SR Peak Value RMS Value 4. Is2 0.0154 No 5. Vph 7.11 1. Ip1 160.8 2. Ip2 4.057 3. Is1 4006 4. Is2 3.646 5. Vph 8.119

Figure 17.Primary current

Figure 13.Primary current

Figure 18. Secondary current

Figure 14. Secondary current Fig 19 .Phase Voltage (Vph)

Proceedings of WRFER International Conference, 16th April 2017, Pune, India 118 Numerical Relay Based Transformer Protection CONCLUSION

In this paper, an attempt has been made through the use of MATLAB to analyze numerical differential protection. The obtained result illustrate that the proposed differential relay represents the method works properly. The proposed relay was able to

Figure 20.Trip discriminate between internal fault, external fault and inrush current. DISCUSSION ACKNOWLEDGMENT The tests are conducted with four situations, Healthy condition, Inrush without load, external faults and The authors would like to thank the authorities of internal faults. R.C.O.E.M Dr T.G. Arora, Dr M.M Renge and Summary of all tested cases: DrS.B.Bodkhe,Nagpur for providing facilities to carry out the research work. Table No 6. Summary SR CASE TYPE RELAY NO RESPONSE REFERENCES 1. Healthy Condition No trip 2. Inrush without Load No trip [1] Y. G. Paithankar and S. R. Bhide, “Fundamentals of Power System Protection,” Prentice – Hall of India Private Limited, 3. External Fault No trip 2003. 4. Internal Fault Trip [2] Poljak, M. and N. Kolibas, " Computation of Current Transformer Transient Performance," IEEE Trans. On Power Therefore from the above table we summarized that Delivery, Vol.3,No.4, pp.635-645(2010). [3] M. A. Rahman and B. Jeyasurya, “A state-of-the-art review the relay will provide tripping signal during internal of transformer protection algorithms”, IEEE Trans. Power fault condition And dose not operate during other Delivery, vol. 3, pp. 534–544, Apr. 2011. non-fault condition. [4] Adel Aktaibi and M. Azizur Rahman, “Digital differential Differential protection relay for a large power protection of power transformer using MATLAB”, Chapter 10, Intech publications. transformer [5] C. D. Hayward, ―Harmonic-Current Restrained Relays for Transformer Differential Protection, AIEE trans., vol. 60, VII. ADVANTAGES pp 276, 2009. [6] M. S. Sachdev, T. S. Sidhu, H. C. Wood, ―A Digital Relaying Algorithm for Detecting Transformer Winding 1. The CT secondary connections compensate Faults, IEEE Transactions on Power Deliver, vol. 4, No. 3. phase shift between two windings. July 2011. 2. This system compensate phase error and ratio [7] Adel Aktaibi and M. A. Rahman, ―A Software Design error. Technique for Differential Protection of Power Transformers, International Electric Machines & Drives 3. CT Ratio can be kept same on both sides of Conference (IEMDC 2011), IEEE, 15-18 May 2011, Page(s): transformer. 1456–1461 4. This system doesn’t require FFT analysis as well [8] H. S. Bronzeado, P. B. Brogan, and R. Yacamini, “Harmonic as filter. analysis of transient currents during sympathetic interaction” IEEE Trans. Power Syst., vol. 11, no. 4, pp. 2051–2056, Nov. 5. Compatible to the microcontroller 2010. [9] Abdelrahman H. Hamouda, Fadel Q. Al-Anzi, Hussein K. Gad, Adel Gastli,”Numerical Differential Protection th Algorithm for Power Transformers”, 7 IEEE GCC Conference and exhibition, November 17-20,

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