Generator in a power station requires different type of

Transcription

1 Laboratory Simulation of Generator Protection Rashesh P. Mehta, Member, IEEE, Bhuvanesh Oza, Member, IEEE Abstract Generator is the most important and costly equipment in the power system. For the reliability of power system, protection of the generator is very important. There are different types of generator protection which are existing in actual field like reverse power protection, stator and rotor earth fault protection, negative phase sequence protection, over current protection, over voltage protection etc. To give a feel and exposure of these protections, hardware simulation is implemented. Protections against six faults and abnormalities have been simulated. This simulation panel has been practically built at the Power System Laboratory of BVM Engineering College, Vallabh Vidyanagar, Gujarat. In this project, we are using contactor instead of circuit breaker. For the protection, we have used electromagnetic and static relays. It is an academic effort to demonstrate the concepts and complicacies of generator protection in the laboratory environment. I.INTRODUCTION Generator in a power station requires different type of protections. These protections are provided by various types of relays which may be electromagnetic or static. In case of numerical relays, most of the electrical protections can be taken care by a single relay. To understand the phenomena, consequences and detection of faults, it is essential to simulate the faults practically in the laboratory. Many efforts have been done in the academic environment to demonstrate the concepts of protection for various equipments of power system. In the past two decades, different laboratories focusing on teaching and researching the area of power system protection have been reported [1] [8]. Sidhu and Sachdev [1], [2] describe a laboratory at the University of Saskatchewan that focuses on designing relay strategies, modeling them and testing them using high speed digital signal processing (DSP) boards and an array of design softwares. Redfern et al. [3] describe testing relays using actual voltage and current data converted from the data files generated by power system simulation software. Lee et al. [4] report a relay performance testing facility using simulated transmission line modules. The paper describes both the Manuscript received July 10, This work was supported by MODROBS grant from the All India Council for Technical Education (AICTE). R. P. Mehta is with the Electrical Engineering Department, B.V.M. Engineering College, Vallabh Vidyanagar, Gujarat, India ( rpmehta1968@hotmail.com). B. A. Oza is with the Electrical Engineering Department, B.V.M. Engineering College, Vallabh Vidyanagar, Gujarat, India hardware and the software strategy and documents the performance results of an instantaneous overcurrent relay and a reverse power relay. Carullo and Nwankpa [5] describe a laboratory that focuses on the data acquisition, energy management and supervisory control aspects of a power system that form the basis of a modern protection system. Kabir [6] documents the performance of a laboratory experiment on a scaled down power system protected by a single computer implementing an over-current protection strategy. Chen et al. [7] report the laboratory implementation of an intelligent embedded microprocessor based overcurrent protection scheme. McLaren et al. [8] report a relay testing facility based on Real Time Digital Simulator (RTDS). Bhuvanesh Oza [9] has reported a novel power system protection laboratory based on senior design projects. As reported in [9], in the same laboratory we have implemented a simulation of generator protection. The purpose is to give professional experience and relay setting calculations exercise to students in the laboratory sessions. Moreover, best utilization of the available relays and equipments was also a driving force in designing this panel. In this project, we have simulated six protections which are (1) Reverse power protection (2) Stator earth fault protection (3) Negative phase sequence protection (4) Overload alarm and over current protection (5) Over voltage protection (6) Zero sequence voltage protection. Reverse power relay, I.D.M.T over-current relays are electromagnetic relay and remaining relays are static relay. 10/5 A C.T.s are used and 230/110 V P.T.s are used. As it is the case with field practices, we are also controlling the generator and its prime mover (D.C machine) from remote control. The motor-generator assembly lying in the machine lab is controlled from power system lab about 40 to 50 meters away from the machine lab. Speed, voltage, load current and frequency are the parameters being controlled. CLASS A and CLASS B protections are annunciated on the panel. II. SPECIFICATIONS OF THE EQUIPMENTS The specifications of various equipments used in the simulation are enlisted below. 1) Generator: 3-Phase, 415 V, 50 Hz, 4.33 A, 3 KVA, 1500 R.P.M. 2) Prime Mover (D.C Shunt Motor): 230 V (D.C.), 10 H.P., 1440 R.P.M., 41.5 A. 3) Earth Fault Relay [11]: Rated Current of Relay: 1 amp Auxiliary Voltage: 240 V (A.C.) Plug setting range: % of 1 A. T.M.S: 0.1 to 1.0 Instantaneous Over Current Setting: 1-4 A. 131

2 4) Reverse Power Relay [12]: Rated current of Relay: 1 amp. Rated voltage: 110 V (A.C) Auxiliary voltage: 220 V (A.C) Maximum Torque Phase Angle: 30 degree (lead) Time Range: 1 to 10 sec. 5) Overload and Overcurrent Relays [13]: Similar relays are used for these two protections. Extremely inverse I.D.M.T over current relay Rated current of relay: 1 A. Auxilary voltage: 240 V (A.C) Plug setting range: 50 to 200% of 1 amp. T.M.S: 0.1 to 1.0 Inst. Over current setting : 2 to 18 amp. 6) Over-voltage Relay [14]: Rated current: 1 amp. Auxilary voltage: 110 V (D.C) V1 = 80 to 140 V V2 = 0 to 16 V S = 0.28 to 2.8 sec. 7) Negative Phase-sequence Relay [15]: Rated current: 1 A. Auxilary voltage: 110 V (D.C) In = 1 A. %In = 3 to 10 X %Ir = 0.45 to 0.9 S for alarm (t) = 1.3 to 13 sec. S for trip (K) = 2 to 10 sec. 8) Zero Sequence Voltage Relay [16]: Rated current: 1 amp. Auxilary voltage: 110 V (D.C) % Un// 3: 5.0 to 40 S: 0.28 to 2.8 sec. III. DESIGN OF CONTROL CIRCUIT A. A.C. Power and Control Circuit-I Fig. 1. A.C. Power Circuit I The power and control circuit connections for reverse power relay, stator earth fault relay and the synchronizing arrangement are shown in the figures 1 and 2 respectively. The START/STOP pushbuttons are for energizing/deenergizing the circuit breakers (here simulated by a contactor) for the Generator and the BUS. Both the START pushbuttons are paralleled with NO contacts of the respective contactors to provide self hold-on feature. The contact A3-1 is for tripping the Generator circuit breaker due to operation of any one out of the Earth Fault Relay or the Reverse Power Relay, when that fault or abnormal condition is simulated on the panel. The ACCEPT push-button is for the purpose of simulating similar facility used actual practice for the acceptance of the fault condition by the operator on the control panel. Thus, the professional practices followed in control panel wiring for protective relays are sufficiently exposed in this panel. Fig. 2. Control Circuit I B. D.C. Shunt Motor Power and Control Circuit For starting the generator, first start the prime mover (D.C shunt motor) by pressing the start push button and bell switch (S2) simultaneously until the motor field gets its rated current (1.4 amp) provided on the controlling desk with maximum position of armature rheostat (40ohm, 8 amp) of motor, minimum position of field rheostat (345ohm, 1.5 amp) of motor and maximum position of field rheostat (185ohm, 2.3 amp) of generator as shown in the circuit diagram. Now, for increasing the speed of the prime mover (D.C shunt motor) increase the motor armature current and decrease the motor field current by decreasing the value of motor armature rheostat and increasing the value of motor field rheostat respectively. Generated voltage can be increased by increasing the generator field current by decreasing the value of generator field rheostat. The generator voltage is 200 volts (phase to neutral) at its speed of 1250 to 1300 rpm. Frequency meter (its terminals 270 and 0 volts) is connected across phase and neutral terminals of generator stator, coming to panel from the machine lab. Measured frequency is proportional to the speed of the generator, so we can also continuously measure the speed of the generator. If any fault 132

3 occurs in D.C shunt motor, like opening of field during the performance, speed of the motor will increase dangerously to a high value. Hence, we have implemented field failure protection as shown in the figure 3 and figure 4, which gives a protection against opening of field and disconnect D.C shunt motor (prime mover) from the supply. Fig. 5. Generator Field Control Circuit D. A.C. Power and Control Circuit - II Fig. 3. D.C Shunt Motor Power Circuit Fig. 6. A.C. Power Circuit II Fig. 4. D.C Shunt Motor Control Circuit C. Generator Field Control Circuit The generator field control is shown in figure

4 A. Stator Earth Fault Protection TABLE I SETTING OF EARTH FAULT RELAY Plug setting T.M.S 10 % of 1 amp. 0.1 Fig. 7. A.C. Control Circuit II The power and control circuit arrangements for overload relay, overcurrent relay, negative phase sequence relay and zero sequence relay are shown in figures 6 and 7 respectively. The description of the control circuit II shown in figure 7 is similar to that for control circuit I shown in figure 2. Only difference is that here negative phase sequence, zero sequence, over-current and over-voltage conditions are being simulated. So these relays contacts are connected in series with auxiliary relay which is used to automatically trip the generator circuit breaker (load contactor). IV. SIMULATION OF FAULTS The panel with all relays mounted and the power and control circuit wired looks as shown in figure 8. The method of simulation for various faults is as follows. Fig. 8. Front View of the Simulation Panel In the simulation, between one of the three phases and ground we have connected an E/F switch as shown in the A.C Power Circuit-I. When we close the switch, earth fault occurs in the generator and according to the ground relay setting, relay will give the signal to the contactor (in actual field, circuit breaker). As stator earth fault protection is CLASS A protection, it will trip the load contactor (generator breaker), generator field contactor (generator field breaker), and also trip the prime mover (D.C shunt motor) along with CLASS A TRIP indication on panel. On the tripping of field contactor, field suppression will come into the picture and energy stored in the field winding will be consumed through the field suppression resistance, as shown in the figure 5. Release the field suppression by the push button provided on the panel and reset the accept button to de-energize the relay coil, after a few minutes to allow restarting of the generator. B. Reverse Power Protection TABLE II SETTING OF REVERSE POWER RELAY Max. torque phase angle Time setting 30 (lead) 1.0 sec. In this simulation, first we are synchronizing generator with system bus (GEB bus). For that we are comparing four quantities of the generator and the system bus (1) voltage (2) frequency (3) phase sequence and (4) instantaneous value of voltage of one of the phases (phase difference). First we start the generator by pushing ON button and start the D.C shunt motor (prime mover) as shown in figure 4. Now, generator starts generating the voltage. To increase the generated voltage we have to increase the speed of generator by increasing armature current of shunt motor and decreasing field current of the motor. After doing this, we are comparing generator voltage, generator frequency, phase sequence of the generator and phase difference of the generator with the system bus. When all the four quantities are equal and the synchroscope pointer is in 12 O clock position, synchronize the machine with the system bus by making the generator breaker ON. Now the machine has synchronized with system bus. So, for reverse power, switch off the D.C shunt motor (prime mover) and the machine now works as a synchronous motor drawing active power from the system. Now, flow of power is in reverse direction which will cause 134

5 reverse power relay to trip, as per the relay setting. Reverse power protection is also a CLASS A protection. It will trip load contactor (generator breaker), Field contactor (field breaker), and prime mover (D.C shunt motor) along with an indication of CLASS A TRIP on the annunciation panel. On the tripping of field contactor, field suppression will come into the picture and energy stored in the field winding will be consumed through the field suppression resistance, as shown in the figure of field suppression circuit. Release the field suppression by the push button provided on the panel and reset the accept button to de-energize the relay coil, after a few minutes to allow restarting of the machine. C. Overload Protection TABLE III SETTING OF OVERLOAD RELAY Plug setting T.M.S 50 % of 1 amp. 0.1 Over load relay is wired only in R phase. For the simulation of overload protection, we have connected three phase load bank to the generator. First start the generator at the rated voltage and frequency (230 V Ph-N, 50Hz) as explained above. During this protection, close the switch S which is shown in the control circuit-ii. Now set the overload relay at 50% of the generator rated current. Plug setting is 50% and time setting is 0.1. Here, we set the over load relay at half of the rated current because, we are doing simulation. If we pass the fault current repeatedly in the laboratory sessions then it can damage the insulation of generator winding and it can deteriorate the performance of the generator. For the simulation we increase the load gradually with the help of the load bank. When current exceeds plug setting, the relay will sense over load phenomenon and it will give a signal to the buzzer for alarm. when current exceeds plug setting, over current is sensed by the relay and it will give a trip signal to the load contactor and generator contactor, because it is a CLASS A PROTECTION. All the sequence of events will occur as per control circuit-ii. E. Overvoltage Protection TABLE V SETTING OF OVERVOLTAGE RELAY V1(volts) V2(volts) S 100 V 0 V 0.28 sec. Start the generator as the procedure explained above. Over voltage relay is connected across secondary of a P.T(220/110 v) as shown in power circuit(2). For the setting of over voltage relay V1 and V2 is given. We set V1=100 V and V2=0 V, so setting of the relay is 100 v (V1+V2). Usual practice is to set the over voltage relay at 110% of rated voltage but as we can not stress the insulation of the generator winding, we are using lower setting. By increasing the excitation of the generator field (110 V D.C.) we can increase generator voltage. We set the over voltage relay setting less than rated voltage of generator. This is because we are doing simulation of over voltage frequently in the laboratory sessions. Such operation for many times can damage the insulation of the generator windings. As per the pre set value when voltage exceeds the set value (V1+V2), relay will sense the over voltage phenomenon and it gives a trip signal to the load contactor and the generator contactor, because it is a CLASS A PROTECTION. So all the sequence of events will occur for CLASS A PROTECTION. We can do over voltage simulation by another method also. Connect the 3 ph star connected load bank to the generator. Set the over voltage relay as pre defined setting at constant load. Now by suddenly disconnecting some load on the generator symetrically which will effect as an over voltage on the generator, and CLASS A PROTECTION will occur. D. Overcurrent Protection TABLE IV SETTING OF OVERCURRENT RELAY Plug setting T.M.S 75 % of 1 amp. 0.1 In the simulation, we have used I.D.M.T extermely inverse over current relay in all the phases. 3-phase star connected load bank is connected to the generator. First, start the generator as the procedure explained above. Now open the switch S shown in figure 7, because at the time of over current protection over load alarm should not sound. Here also we set the plug setting of the relay less than rated current of generator because of damage of generator winding insulation problem. Plug setting of the relay is 0.75 and T.M.S is 0.1. So F. Negative Phase Sequence Protection TABLE VI SETTING OF NEGATIVE PHASE SEQUENCE RELAY %In X % Ir t(alarm) K (trip) sec. 2 sec. In the simulation, negative phase sequence current can be generated either by unbalancing the load with the help of a load bank or with the help of a rheostatic method (connect between phase to neutral). And it can be done also by opening one of the phase of the generator terminals. Now, start the generator as the procedure explained above. Here, negative phase sequence relay measures percentage of unbalance current in the stator winding. So, we set In = 1 amp, K = 4, 135

6 %In = 5, %Ir = 0.9 and S = 1.3 sec, where In is a negative phase sequence current passing through relay, K decides characteristic of relay, %In is a percentage of negatve phase seq. current, S is the time of operation and %Ir is a percentage of %In. Now when %Ir exceeds 0.9 value, NPS relay will give an alarm on the annunciation pannel, and when %In exceeds beyond 5 then it starts for tripping signal and after pre defined value of S decided by setting K, relay will trip and send the signal to load contactor and disconnect the load because it is a CLASS B PROTECTION. G. Zero Sequence Voltage Protection TABLE VII SETTING OF ZERO SEQUENCE VOLTAGE RELAY % Un/ 3 S sec. Setting of the zero sequnce voltage relay is in terms of %Un/ 3, where Un = 110 V and %Un/ 3= 20 and S (time of operation) = 0.28 sec. We can not set the relay below 20, otherwise relay will operate in normal running condition due to P.T error. Here three single phase P.Ts are used with primary winding and secondary windings are connected in star. Secondary goes to the zero sequence voltage relay in which star connection is converted to open delta connection to sense the ground faults. In normal condition no voltage induced in open delta connection (V1+V2+V3 = 0). Now for the simulation of this we are connecting rheostat between phase anb neutral on the generator terminals and by changing the value of rheostat we can induce the zero sequence voltage in the relay. So when ground fault occurs, zero sequence voltage is induced in open delta connection and according to the relay setting, zero sequence voltage phenomenon is sensed by the relay and it will give trip signal to load contactor and field contactor because it s a CLASS A PROTECTION. All the consequences will occur as per explained in control circuit-ii. It can not respond to three phase to ground faults (L-L-L-G) because in this condition no zero sequence voltage induced in open delta connection. This relay is not supposed to operate in phase faults. V. CONCLUSION Simulation of all the protections discussed above are giving satisfactory performance. The purpose of providing professional touch in academic environment of the laboratory is served by this endeavour. The level of interest and confidence of students in the subject of power system protection is enhanced greatly. All the protections which are simulated can be similarly implemented by using a single numerical relay. As frequency meter is connected for measuring the speed of generator (frequency is proportional to the speed). This frequency meter is an analog instrument, so accurracy of this meter is not good and there is an absence of controlling torque too. To eleminate this problem we can use digital tachogenerator which can measure the speed directly propotional to the generated voltage. REFERENCES [1] T. S. Sidhu and M. S. Sachdev, Laboratory setup for teaching and research in computer-based power system protection, in Proc. Int. Conf. Energy Manage. Power Delivery, vol. 2, 1995, pp [2] M. S. Sachdev and T. S. Sidhu, Laboratory for research and teaching of microprocessor-based power system protection, IEEE Transactions on Power System., vol. 11, no. 2, pp , May [3] M. A. Redfern, R. K. Aggarwal, and G. C. Massey, Interactive power system simulation for the laboratory evaluation of power system protection relays, in Proc. Int. Conf. Develop. Power System Protection, vol. 302, 1989, pp [4] L.Wei-Jen, G. Jyh-Cherng, L. Ren-Jun, and D. Ponpranod, A physical laboratory for protective relay education, IEEE Transactions on Education, vol. 45, no. 2, pp , May [5] S. P. Carullo and C. O. Nwankpa, Interconnected power system laboratory: a computer automated instructional facility for power system experiments, IEEE Transactions on Power System., vol. 17, no. 2, pp , May [6] S. M. Lutful Kabir, Computer operated coordinated over-current protection scheme, in Proc. Univ. Power Engg. Conf., 2000, pp [7] Z. Chen, A. Kalam, and A. Zayegh, Advanced microprocessor based power protection system using artificial neural network techniques, in Proc. Int. Conf. Energy Manage. Power Del., vol. 1, 1995, pp [8] P. G. McLaren, R. Kuffel, R. Wierckx, R. J. Giesbrecht, and L. Arendt, A real time digital simulator for testing relays, IEEE Transactions on Power Delivery., vol. 7, no. 1, pp , Jan [9] Bhuvanesh A. Oza and Sukumar M. Brahma, Development of Power System Protection Laboratory through Senior Design Projects, IEEE Transactions on Power System., vol. 20, no. 2, pp , May [10] Dr. M.A. Date, Prof. Bhvanesh Oza, Dr. N.C.Nair, Power System Protection, Bharti Prakashan. [11] Technical Literature of Earth Fault Relay Type, Universal Electric Co. [12] Technical Literature of Reverse Power Relay Type CCUM, English Electric Co. [13] Technical Literature of I.D.M.T Overcurrent Relay Type EM, Universal Electric Co. [14] Technical Literature of Static Overvoltage Relay Type TMV111, Aluminum Industries Co. (ALIND) [15] Technical Literature of Static Negative Phase Sequence Relay Type TMAIS, Aluminum Industries Co. (ALIND) [16] Technical Literature of Static Zero Sequence Voltage Relay Type TMVH, Aluminum Industries Co. (ALIND) Rashesh P. Mehta (M 08) was born in 1968 at Anand, India. He received his B.E. (Electrical) Degree in 1989 from Sardar Patel University, Gujarat, India and M.Tech (PEPS) Degree in 2007 from Department of Electrical Engineering, Indian Institute of Technology, Bombay, India. He is currently Senior Lecturer at Electrical Engineering Department, Birla Vishwakarma Mahavidhyalaya (B.V.M. Engineering College), Vallabh Vidyanagar, Gujarat, India. His areas of interest include real-time digital simulation and power system protection. Bhuvanesh A. Oza (M 08) was born in Rajkot, India, in He received the B.E. and M.E. degrees, both in electrical engineering, from Sardar Patel University, Gujarat, India, in 1972 and 1982, respectively. His industrial experience from 1974 to 1986 includes working with Engineering Cell in GEB for design of relaying system for Ukai Thermal Power Plant. Since 1986, he has been with Birla Vishwakarma Mahavidhyalaya Engineering College, Vallabh Vidyanagar, India, first as a Lecturer and from 1990 onwards as an Assistant Professor. His areas of interest are power system protection and operation. 136