Switching impulse

Switching impulse is important for high voltage transmission lines. The design withstand level is selected by specifying the risk of flashover, e.g. for 550 kV towers a 0.13 percent probability has been used.


At 345 kV, design is carried out by accepting a switching impulse level of 2.7 p.u, Which corresponds to the lightning level, at 500 kV, however, a 2.7 p.u.
Switching impulse would require 40 per cent more tower insulation than that governed by lightning.
The tendency is therefore for the design switching impulse level to be forced lower with increasing system operating voltage and for control of the surges to be made by the more widespread use of resistance switching in the circuit breakers or use of surge diverters. For example, for the 500 kV networks the level is 2 p.u. and at 765 kV it is reduced to 1.7 p.u. if further increases in system voltage occur, it is hoped to decrease the level to 1.5 p.u.


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