Insulation co-ordination aspects for power stations with generator circuit-breakers

The generator circuit-breaker (gen.c.b.) located between the generator and the step-up transformer, is now being applied world-wide. It has become a recognized electrical component of power stations which is largely due to economical advantages and increased power station availability. Technical protection considerations for power stations have always been the reason for discussion and the object of improvement. With the use of a gen.c.b., some points of view need to be considered anew. Not only the protection system in case of fault conditions will be influenced, but also the insulation co-ordination philosophy. The results of some calculations concerning expected overvoltages are presented. These calculations are based on a transformer rated 264/15.5 kV, 220 MVA, but the results are transferable to other power plants. Some measurements carried out on a transformer of the same rating complement the calculations. The findings may contribute to an improvement in insulation co-ordination and protection of the electrical system generator/step-up transformer