FERRORESONANCE SIMULATION STUDIES USING EMTP Jaya Bharati, R. S. Gorayan Department of Electrical Engineering Institute of Technology, BHU Varanasi, India jbharatiele@gmail.com, rsgorayan.eee@itbhu.ac.in Abstract Ferroresonance is a special case of resonance which occurs due to interaction of nonlinear inductance of iron core reactors or transformer with the distribution network capacitance. There are situations like open conductor when these two make a series circuit and may produce resonance. Resonant over voltages are dynamic in nature and are severe enough to cause damage to lightning arresters, transformers and loads at receiving end of a three phase distribution network. Analysis of circuits prone to ferroresonance is thus necessary for the system designer. Many papers are available both analytical and TNA simulations for analysis and design of distribution networks considering ferroresonance. In the present work EMTP software has been used for analysis. One conductor and two conductors open cases have been analyzed. Recommended preventive measures like grounding of transformer neutral through resistance, dummy secondary load (resistive) in the transformer secondary and limiting the lateral cable length have analyzed. switching operation and total losses. So its predictability is quite complex and difficult [3]-[7]. Ferroresonance is initiated after some type of switching event such as load rejection, fault clearing, transformer energization, single-phase switching or loss of system grounding [8]. II. SIMULATION MODEL A simple three phase ferroresonant circuit as shown in Fig.1 has considered for study. It consists of a three phase transformer, and a capacitor by an ac source. The magnetising reactance of a transformer is represented by non linear inductor [3], [5]. The transformer rating considered in simulation is 13800/400, 1000 KVA. Shunt capacitance of the lateral between switching device and transformer has considered value of 0.37µF/km. Keywords: ferroresonance, distribution transformer,nonlinear circuit, electromagnetic transient program. F I. INTRODUCTION ERRORESONANCE is a complicated electrical phenomenon which has long been a problem for power engineers. It is a nonlinear, dynamic and complex electrical phenomenon in power system which may lead to overvoltages and overcurrents in an electrical power system thus may cause damage to the transmission system, switchgear, and the operational personnel [1],[2],[4].Nowadays, ferroresonance is a widely studied phenomenon in power systems involving capacitors and saturable inductors. It occurs frequently in electrical distribution systems, due to the transformer nonlinear inductance and the capacitive effect [1], [6]. The capacitance in power system is due to a number of elements, such as: The phase-to-phase capacitance Conductor to earth capacitance Circuit breaker grading capacitance Busbar capacitance Bushing capacitance. Ferroresonance phenomenon depends on many factors and conditions such as initial conditions of the system, transformer iron core saturation characteristic, residual fluxes in the transformer core, type of transformer winding connection, capacitance of the circuit, point-of-wave Fig.1 Single line diagram of three phase switching transformer 1
m3 m2 AC2 m1 Fig.2 Nonlinear characteristics of transformer core One conductor open: 7.97kV /_0 AC3 AC1 7.97kV /_120 Fig.3 7.97kV /_240 SW3-1 1E15 0 SW1 1E15 1ms 0 SW2-1 1E15 0 RL3 RL1 RL2 C3 Simulation model of three phase equivalent circuit C2 C1 xfmr1 xfmr2 xfmr3 VM VM ScopeView VM Fig.3 shows the significant details of a 3-phase equivalent circuit to represent the simple circuit of Fig.1.The entire system up to the single-phase switching device is represented by a 3-phase grounded voltage source. The lateral between the single-phase switching device and the transformer bank is represented by its resistance, inductance and shunt capacitance. The transformer bank as shown represents a wye-wye bank formed by appropriate connections of three equivalent circuits of a single-phase transformer. In Fig.3 1, 2 and 3 denotes phase a, phase b and phase c respectively. The effects of one open conductor (phase a ) with neutral isolated are shown in Fig.4 and Fig.5. Waveforms shown in Fig.4 and Fig.5 are primary current and primary terminal voltage of transformer respectively. Examinations of these figures indicate that very high values of abnormal voltage are obtained under no load isolated neutral condition. Resistance grounded neutral: The ferroresonance can be avoided by the use of a neutral resistor of proper value. Effects of neutral resistance with phase a open is shown in the table1.the result indicated that the neutral resistor value should not exceed 0.05X M. Fig.4 Transformer primary current for isolated neutral TABLE I R N EFFECTS OF VARIATION OF NEUTRAL RESISTANCE WITH ONE CONDUCTOR OPEN V N I N V a (Volts) (ohm) (Volts) (Ampere) (Volts) (Volts) 100 174 1.74 5206 7792 8045 200 383 1.94 6325 7856 8247 300 590 1.96 7087 7899 8470 400 760 1.90 7387 7908 8664 500 907 1.81 7540 7902 8833 600 1050 1.74 7631 7887 9000 700 1346 1.68 7728 7831 9145 800 1348 1.67 7728 7831 9300 900 1504 1.67 7759 7825 9456 1000 1671 1.66 7763 7814 9616 Fig.5 Transformer primary voltage for isolated neutral Fig.6 Transformer primary voltage at neutral resistance=700ω 2
From the result shown in Fig.6 it is indicated that the elimination of abnormal voltage condition produced by open conductor can be accomplished by grounding the neutral of transformer bank through a suitable value of resistance. It is also indicated that the value of resistance will be 0.05 X M. Secondary load required to prevent ferroresonance overvoltage: Ferroresonant overvoltages can be avoided by the use of a secondary load. It is seen that a dummy load on the secondary of the transformer whose rating is 5% of transformer rating permanently connected prevents ferroresonance [3],[5]. The effect of variation secondary load resistance with phase a open is shown in table II. TABLE II EFFECTS OF VARIATION OF TRANSFORMER RESISTIVE LOAD WITH ONE CONDUCTOR OPEN. R (secondary load requirement in ohms) Va 0.1 1.6 197 200 0.5 8 194 202 1 16 1906 2 32 184 213 3 47 177 220 4 64 170 225 5 81 166 230 6 100 163 230 7 115 160 238 8 126 154 243 9 147 143 254 10 177 133 260 Effect of feeder length: The effect of variation of lateral line length between switching device and transformer with phase a open is shown in table III. At lower value of lateral length the transformer primary voltage reaches very high value. TABLE III EFFECTS OF VARIATION OF LINE LENGTH WITH ONE CONDUCTOR OPEN Line length Voltage ( in Km) Phase a Phase b Phase c 0.50 10830 12214 12034 0.75 30823 26478 27588 1.00 26323 22802 22588 2.00 5720 9271 10936 3.00 4633 9036 10512 4.00 4261 9000 10400 5.00 4021 9195 10020 Two conductors open: For two open conductors simulation is done with phase a and phase b open and the result is shown in below figure. Fig.8 Transformer primary current for isolated neutral Fig.7 Transformer secondary voltage for resistive load= 3Ω The effects of secondary resistive load (3Ω) on the transformer with one open conductor (phase a ) are shown in Fig.7. Fig.9 Transformer primary voltage for isolated neutral 3
The effects of two open conductors with neutral isolated are shown in Fig.8 to Fig.9. Waveforms shown in Fig.8 and Fig.9 are primary current and primary voltage of transformer respectively. Examinations of these figures indicate that very high values of abnormal voltage are obtained under no load isolated neutral condition. Resistance grounded neutral: The effects of variation neutral resistance in the case of two phases (phase a phase b ) open is shown in table IV. The result indicated in table.1, it is clear that the neutral resistor value should not exceed 0.05 X M. TABLE IV EFFECTS OF VARIATION OF NEUTRAL RESISTANCE WITH TWO CONDUCTORS OPEN. R N (ohms) V N I N (amperes) V a 100 94 0.94 4042 4042 7935 200 214 1.01 4708 4708 8116 300 314 1.03 4802 4802 8190 400 400 1.00 4875 4875 8235 500 488 0.99 4921 4921 8284 600 591 1.00 5005 5005 8351 700 693 1.00 5092 5092 8423 800 805 1.00 5168 5168 8445 900 915 1.00 5264 5264 8462 1000 1040 1.00 5346 5346 8531 phase b ) with secondary resistive load 3Ω are shown in Fig.11. TABLE V EFFECTS OF VARIATION OF TRANSFORMER RESISTIVE LOAD WITH TWO CONDUCTORS OPEN. R (secondary load requirements in ohms) V a 0.1 3.1 3.1 6.4 0.5 16 16 31.8 1 31 31 62 2 56 56 112 3 76 76 149 4 93 93 175 5 106 106 193 6 116 116 208 7 124 124 220 8 134 134 237 Fig.11 Transformer secondary voltage at load= 3Ω Effect of feeder length: The effect of variation of lateral line length between switching device and transformer with primary voltage of two conductors (Phase a and phase b ) open is shown in table VI. At lower value of lateral length the transformer primary voltage reaches very high value. Fig.10 Transformer primary voltage at neutral resistance R N=700Ω From the result shown in Fig.10 it is indicated that the elimination of abnormal voltage produced by open conductors can be accomplished by grounding the neutral of transformer bank through a suitable value of resistance. It is also indicated that the value of resistance will be 0.05 X M. Secondary load required to prevent ferroresonance overvoltage: The effect of variation secondary load resistance with two conductors open (phase a and phase b ) is shown in table V. The effects of two open conductors (phase a and TABLE VI EFFECTS OF VARIATION OF LINE LENGTH WITH TWO CONDUCTORS OPEN Line length ( in Km) Voltage Phase a Phase b Phase c 0.50 30648 30648 34262 0.75 27335 27335 31208 1 30251 30259630 2 5334 5334 6662 3 5302 5302 6154 4
4 4791 4791 5890 5 4021 4021 5800 III. CONCULUSION The opening of one conductor results high transient of long duration, sustained under voltage or overvoltage and reversal of phase rotation, all causes serious effect on transformer. The opening of two conductor results high transient of long duration, sustained under voltage or overvoltage which also causes dangerous effect to the transformer. For one and two open conductors high abnormal voltages are nonsinusoidal. By comparing the waveform of two cases of one open conductor and two open conductors it is clear that higher abnormal voltage is produced in the case of two open conductors. In this paper three remedial measures have been analyzed for prevention of ferroresonance. 1. By grounding primary neutral through resistance- From the result shown it is found that for mitigation of ferroresonance requirement of neutral resistance is about 0.05 times of magnetising reactance. It is an effective method. 2. By transformer load- The use of dummy resistance load on the transformer whose rating is 5% of the transformer rating permanently connected prevents ferroresonance.. This method is not economical due to cost of losses in secondary load but it is highly effective 3. By limiting the line length- The effect of increased line length results reduction in magnitude of all abnormal overvoltage. REFERENCES [1] Owen C. Parks, Adly A. Girgis, Ph.D., Marion E. Frick, Simulating ferroresonance in a series compensated distribution network. IEEE ninth international conference on harmonics and quality of power, vol.1,pp. 162-167, 2000. [2] R. H. Hopkinson, Ferroresonance During Single-Phase Switching of 3- Phase Distribution Transformer Banks, IEEE Trans. PAS, vol. 84 No.10, pp. 289 293, April, 1965. [3] Ralph H. Hopkinson, Ferroresonant Overvoltage Control Based on TNA Tests on Three-Phase Wye-Delta Transformer Banks, IEEE Trans. on PAS, p352-361, PAS-87, Feb. 1968. [4] P. E. Hendrickson, I. B. Johnson, and N. R. Schultz, "Abnormal voltage conditions produced by open conductors on three phase circuits using shunt capacitors," AIEE Trans. (Power Apparatus and Systems), vol. 72, p. 1183, 1953. [5] R. H. Hopkinson, "Ferroresonant overvoltage control based on TNA tests on three-phase delta-wye transformer banks," IEEE Trans. Power Apparatus and Systems, vol. PAS-86, pp. 1258-1265,October 1967. [6] J. Horak, 2004, "A review of ferroresonance ", Basler Electric s Technical Papers, 57 th annual conference for protective relay engg.,pp.1-29,2004. [7] M. Roy, and C. K. Roy, Experiments on Ferroresonance at Various Line Conditions and Its Damping.IEEE joint International conference on ower system technology and power india conference, POWERCON, pp.1-8, 2008. [8] D. A. N. Jacobson, Examples of Ferroresonance in a High Voltage Power System., Power Engineering Society General Meeting,, IEEE power engineering Society general meeting, Vol. 2, 2003. 5