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Ferro-resonance due to interruption of magnetizing currents of a cable fed transformer

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Ferro-resonance due to interruption of magnetizing currents of a cable fed transformer

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Ferro-resonance due to interruption of magnetizing currents of a cable fed transformer

  1. 1. PROF. S. RENGARAJAN PESENTED BY K.RATHAN Aurora’s engineering college, Bhongir (TECH. SEMINAR GUIDE) 24 January 2014 1
  2. 2. CONTENTS  ABSTRACT  INTRODUCTION 1. TRANSFORMER • FERRO-RESONANCE IN TRANSFORMERS • ANALYSIS IN ATP-EMTP SOFTWARE • EFFECTS OF FERRO-RESONANCE • MITIGATING METHODS • CONCLUSION 24 January 2014 2
  3. 3. ABSTRACT Transformers are used in generating stations to step-up the voltage to an extra high voltage. Transformer is a static device which transforms the energy from one circuit to other circuit without change in frequency. Transformers are widely used in power system networks due to their advantages like high efficiency, low losses. But the problem is over voltages are generating due to disconnection of transformer under no load. Under no-load, transformer takes only magnetizing currents order of few amperes and their chopping factor can be very high. When disconnecting transformers or reactors, the current is low but highly inductive. Circuit breakers such as Air blast, SF6, Vacuum are high speed in operation and designed normally to interrupt very high short-circuit currents. If these circuit breakers are made to interrupt magnetizing currents of transformer, they force the current to be interrupted before its natural current zero and this phenomenon is known as current chopping. Because of this current chopping over voltages are generated in transformer and the magnitude of these over voltages depends on the capacitance and inductance values of transformer. This over voltages causes the severe problems to transformer insulation such as fast ageing of winding insulation, if generated voltages are above the BIL of transformer it leads to breakdown of winding insulation. By using a software ATP-EMTP package we can analyze this phenomenon easily. 24 January 2014 3
  4. 4. INTRODUCTION 1.TRANSFORMER  It is a static electric device which transfers the energy from one electrical circuit to other electrical circuit through the medium of magnetic field and without change in frequency.  Transformers are most widely used in power system networks because of their advantages 1. High efficiency 2. Low maintenance However transformer core tends to saturate at higher currents, there by exhibits non-linear flux-current characteristics. 24 January 2014 4
  5. 5. FERRO-RESONANCE IN TRANSFORMERS Ferro resonance is the disturbance that involves high levels of overvoltage and over currents generation. When a no-load cable connected transformer disconnected from the source, over voltages and currents are generated. Capacitor discharged through transformer primary winding, then the iron core is saturated. 24 January 2014 5
  6. 6. Conti…  Due to iron core is saturation the change in flux with current decreases, due to this the inductance of the transformer decreases. The cable and transformer resonance causes a typical voltage and current oscillations called Ferro Resonance.  So the voltage at the primary winding becomes as a rectangular wave due to dc component, and current is very high magnitude with triangular wave shape. 24 January 2014 6
  7. 7. ANALYSIS IN ATP-EMTP SOFTWARE Ferro-resonance in transformer Resonance in RLC circuit Above circuit is simulated in ATP-EMTP software , and the results voltage and current waveforms are shown below. 24 January 2014 7
  8. 8. Voltage at inductor 24 January 2014 8
  9. 9. Current at inductor 24 January 2014 9
  10. 10. Voltage at transformer Fig: voltage at node RD05 24 January 2014 10
  11. 11. Current at transformer Fig: current at node RD05 24 January 2014 11
  12. 12. Effects of Ferro-resonance  high current flow in the primary winding of transformer causes heating and damage of insulation. hence a mitigation Ferro-resonance phenomena is warranted. 24 January 2014 12
  13. 13. MITIGATING METHODS  Secondary winding of transformer should not be at no-load at the time of opening.  RC-suppressor may be connected at secondary side of transformer. 24 January 2014 13
  14. 14. CONNECTING A LOAD AT SECONDARY SIDE Fig: voltage at primary side with load 24 January 2014 14
  15. 15. Conti… Fig: current at primary side with load 24 January 2014 15
  16. 16. CONCLUSION  Ferro-resonant oscillations initiated by transient events such as switching operations.  No-load transformer should not be disconnected from the source. If in case the transformer must be disconnected from the source, secondary side of transformer must be connected with some load or connect RC-suppressor. 24 January 2014 16
  17. 17. THANK YOU 24 January 2014 17

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