Extreme stresses on generator circuit breakers

The operation of generator circuit breakers, located between generator and step-up transformer, is associated with extreme stresses due to the conduction and interruption of very large current, both in normal (up to 50 kA) as well as in fault situations (up to 200 kA). Moreover, after interruption, very severe transient recovery voltages appear across the breaker. This contribution discusses the technical solutions that are involved in the design and testing of generator circuit breakers, by example of a newly developed and certified generator circuit breaker for 28,000 – 50,000 A nominal current and 210 kA short-circuit current, for generating units up to 2000 MW. Key issues here are fault current commutation inside the breaker chamber and reduction of conductive heat losses. In addition, the actual standardisation status is described, along with the fault scenarios that are envisaged. Adequate test circuits, procedures and results, both for short-circuit test duties as well as for conductive heat loss evaluation are described.