Jun 30, 2013

CONTROL OF OVER VOLTAGES DUE TO SWITCHING



Switching surges cause over voltages  on the line and has to be controlled effectively to protect the line and connected equipment’s   as well.  This can be accomplished   by the following methods:

a)  Insertion of resistors :

Normally a resistor R is in series with circuit breaker contacts is inserted, while switching on, but short circuiting them next few cycles. This will decrease the transients causing over voltages occurring during switching on the breaker.

b)  Phase-controlled closing of breakers:

Over voltages must be   evaded by controlling the particular instances of the terminating of the 3 phase supply independently. But this requires usage of complicated   monitoring equipment’s and hence not considered for adoption.

 c)  Drainage of trapped charge before re-closing the breakers:

At what times of lines are quickly switched off, residual electric charge might be absent on capacitors and transmission line conductors. This electric electric charge has a tendency to leak through the insulators to ground.   Normally it is the practice to insert resistors to ground or ill series with shunt reactors and take away before the termination of the switches. 

d)  Usage of shunt reactors:

Shunt reactors are to EHV lines to restrict the voltage rise caused by Ferranti Effect. But there would be some oscillations in the power network at that time and insertion of resistance in series with reactors will suppress   the oscillation and restrict the over voltages.

e) Restraining switching surges by appropriate surge diverters:

Connecting suitably rated Surge diverters or lightning arresters would help to drain the energy caused by over voltage to ground and suppress the over voltage.


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