Insulation Coordination in a Generating Station

Part 1 - Fast Front (Lightning) 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 the Simscape Electrical physical modelling section. A suitable 500 kV surge arrester is added to the model. A combination of 13.8 kV surge arresters and capacitors were also added to the medium voltage generator output system. A simulation is carried out.

In the simulation, a lightning overvoltage of 2000 kV (peak) per microsecond is applied at the remote end of the 500 kV transmission line. The voltage waveform of the incoming lightning overvoltage, 500 kV surge arrester voltage and overvoltage appearing across the 500 kV terminal of T1 was mesured in a multichannel scope. It was found that the 500 kV surge arrester clamped the overvoltage down, below the lighting impulse withstand level (LIWL) of the 500 kV winding of the transformer.

During simulation, the overvoltage at 13.8 kV terminal of T1 and Generator terminal was also measured in a multichannel scope. The peak value of the overvoltage at 13.8 kV terminal was lower than the lightning impulse withstand level of T1 (13.8 kV) and other 13.8 kV equipment. The overvoltage appearing at generator terminal was less than the calculated lightning impulse withstand level (LIWL) of the generator.

Simulation results are indicated in the model.