Prof. S. Rengarajan presented a technical seminar on ferroresonance in transformers. Ferroresonance occurs when a no-load transformer is disconnected from the power source, causing overvoltages and overcurrents. It was analyzed using ATP-EMTP software, showing high voltage and triangular current waveforms. Ferroresonance can damage transformer insulation if voltages exceed ratings. Mitigation methods include connecting a load to the secondary winding or an RC suppressor to dissipate resonant energies when disconnecting no-load transformers.

1. PROF. S. RENGARAJAN
PESENTED BY
K.RATHAN
Aurora’s engineering college, Bhongir
(TECH. SEMINAR GUIDE)
24 January 2014
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2. CONTENTS
 ABSTRACT
 INTRODUCTION
1. TRANSFORMER
• FERRO-RESONANCE IN TRANSFORMERS
•
ANALYSIS IN ATP-EMTP SOFTWARE
•
EFFECTS OF FERRO-RESONANCE
• MITIGATING METHODS
• CONCLUSION
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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.
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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.
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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.
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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.
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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.
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8. Voltage at inductor
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9. Current at inductor
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10. Voltage at transformer
Fig: voltage at node RD05
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11. Current at transformer
Fig: current at node RD05
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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.
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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.
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14. CONNECTING A LOAD AT SECONDARY SIDE
Fig: voltage at primary side with load
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15. Conti…
Fig: current at primary side with load
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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.
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17. THANK YOU
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