Insulation Coordination in a Generating Station-2

Part 2 - Slow Front (Switching) Overvoltage Simulation

In a typical generating station, a medium voltage generator is connected to a short high voltage transmission line via a generator transformer (T1). The transmission line terminates in a high voltage transmission substation. The generator is connected to the medium voltage terminal of T1 via generator circuit breaker and isolated phase bus.

A model is developed based on the single line diagram of the generating station, using Specialised Power System of Simscape physical modelling section. A suitable 500 kV surge arrester is added to the model. A combination of 13.8 kV surge arrester and capacitors are added to the generator output system. A simulation is carried out.

In the simulation, a switching overvoltage of standard waveform (250 x 2500 microsecond) having a peak value of 823 kV is applied to the remote end of the transmission line. The incoming switching overvoltage, 500 kV surge arrester discharge voltage and the overvoltage at the 500 kV terminal of T1 are measured in a multichannel scope.It was found that the overvoltage at the high voltage terminal of T1 is less than the switching impulse withstand level of T1.

During simulation, the voltage at the 13.8 kV terminal of T1 and the generator terminal are also measured in a separate multichannel scope. It was found that the 13.8 kV equipment and the generator low frequency short duration withstand levels were higher than the overvoltage at the terminals.

Simulation results are indicated in the model.