Choice of generator circuit breaker

A generator circuit breaker (GCB) is placed between the generator and the step-up transformer. For a hydropower station, an SF6 generator circuit breaker (or a vacuum GCB for small and medium units), designed and tested to IEEE C37.013 / IEC 62271-37-013, is suitable. A normal distribution breaker built to IEC 62271-100 is not adequate.

Why a special GCB is needed

1. High DC component: the generator circuit has a high X/R ratio, so the fault current DC offset decays slowly; currents can have delayed current zeros. The GCB must interrupt such asymmetric currents.
2. High rate of rise of TRV: the transformer and generator have small capacitance, so the transient recovery voltage rises very fast; SF6 and vacuum interrupters can withstand it.
3. High continuous current: e.g. a 30 MVA unit at 11 kV carries about 1.6 kA; large units carry tens of kA.
4. Out-of-phase switching: the GCB must interrupt during unsuccessful synchronisation.
5. Frequent operation: hydro units start and stop daily (peaking), so a long mechanical and electrical life is needed.

Why SF6 (or vacuum)

• SF6 has excellent arc-quenching and dielectric strength, handles high currents and asymmetry, and is compact; it is the usual choice for medium and large hydro units.
• Vacuum GCBs are compact and maintenance-free for small/medium units; precautions: current chopping may cause overvoltage, so surge arresters and RC surge suppressors are fitted near the generator and transformer.
• Air-blast and oil breakers are obsolete for this duty.

Benefits of using a GCB

• Generator faults are cleared fast and selectively; the unit is synchronised on the LV side.
• Station auxiliaries can be fed from the grid through the GSU when the generator is off (back-feeding), so a separate station transformer may not be needed.
• Better protection of the GSU and generator, higher plant availability.