Distribution of Switching Surges (EHV and UHV Systems)
- Switching surges originate when there is sudden release of internal energy stored in the electrostatic form (in the capacitance) or in the electromagnetic form (in the inductance).
- This can happen in following situations :
(i) Current chopping phenomenon : Interruption of magnetizing currents (transformers/reactors re switched off) or low inductive currents by high speed circuit breaker.
(ii) Capacitive current switching : Interruption of small capacitive currents e.g. switching off of unloaded lines, capacitors banks installed for power factor improvement etc.
(iii) Ferro-resonance condition : May occur when poles of a circuit breaker do not close simultaneously.
(iv) Energization of long EHV or UHV lines.
- The transient overvoltage occurring in the order of 2 to 3.3 p.u and will be of high magnitudes of the órder 1200 kV to 2000 kV on 750 kV systems. The time duration of these may vary from 1 to 10 msec. depending on circuit inductance and capacitance parameters.
- The other situations of switching that give rise to switching over voltages of short duration (0.5 to 5 ms) and lower magnitudes (2 to 2.5 pu) are :
(a) Single pole closing of circuit breaker.
(b) Resistance switching used in circuit breaker.
(c) Switching lines with transformers at far end.
(d) Compensated lines (Using series capacitors).
(e) Interruption of fault current when L-G or L-L fault is cleared.
(f) Sparking of surge diverter located at the receiving end of the line to limit lightning over voltages.