Control of Switching Overvoltage:
The Control of Switching Overvoltage and power frequency may be controlled by energization of transmission lines in one or more steps by inserting resistances and withdrawing them afterwards,
Phase controlled closing of circuit breakers,
Drainage trapped charges before reclosing,
Use of shunt reactors, and
Limiting switching surges by suitable surge diverters.
Insertion of Resistors:
It is normal and a common practice to insert resistances R in series with circuit breaker contacts when switching on but short circuiting them after a few cycles. This will reduce the transients occurring due to Control of Switching Overvoltage.
The voltage step applied is first reduced to Z0/(R + Z0) per unit where Z0 is the surge impedance of the line. It is reflected from the far end unchanged and again reflected back from the near end with reflection factor (R – Z0)/(R + Z0) per unit. If R = Z0, there is no reflection from the far end. The applied step at the first instance is only 0.5 per unit.
When the resistor is short circuited, a voltage step equal to the instantaneous voltage drop enters the line. If the resistor is kept for a duration larger than 5 m s (for 50 Hz sine wave = 1/4 cycle duration), it can be shown from successive reflections and transmissions, that the overvoltage may reach as high as 1.2 p.u. for a line length of 500 km.
But for conventional opening of the breaker, the resistors have too high an ohmic value to be effective for resistance closing. Therefore, pre-insertion of suitable value resistors in practice is done to limit the overvoltage to less than 2.0 to 2:5 p.u. Normal time of insertion is 6 to 10 ms.
Phase Controlled Switching:
Overvoltages can be avoided by controlling the exact instances of the closing of the three phases separately. But this necessitates the use of complicated controlling equipment and therefore is not adopted.
Drainage of Trapped Charge:
When lines are suddenly switching off, “electric charge” may be left on capacitors and line conductors. This charge will normally leak through the leakage path of the insulators, etc. Conventional potential transformers (magnetic) may also help the drainage of the charge.
An effective way to reduce the trapped charges during the lead time before re closing is by temporary insertion of resistors to ground or in series with shunt reactors and removing before the closure of the Control of Switching Overvoltage.
Normally all HIV lines will have shunt reactors to limit the voltage rise due to the Ferranti effect. They also help in reducing surges caused due to sudden energizing. However, shunt reactors cannot drain the trapped charge but will give rise to oscillations with the capacitance of the system.
Since the compensation given by the reactors will be less than 100%, the frequency of oscillation will be less than the power frequency and overvoltages produced may be as high as 1.2 p.u. Resistors in series with these reactors will suppress the oscillations and limit the overvoltages.