Main unit electrical protection at Sizewell `B´ power station

For any nuclear power station, reliable electrical protection of the main generating units (generators plus generator transformers) has important commercial implications. Spurious trips cause loss of generation and consequent loss of revenue, while failure to rapidly isolate a fault leads to unnecessary damage and again, loss of generation and revenue. While these conditions apply equally to Sizewell B there are additional factors to be taken into consideration. A spurious trip of a main generating unit may lead to a trip of the reactor with an associated challenge to the shutdown and core cooling plant. The generator transformers, besides exporting power from the generators to the 400 kV National Grid, also import power from the Grid to the 11 kV main electrical system, which in turn is the preferred source of supply to the essential electrical system. The main unit protection of the Sizewell B power station is designed to clear generator faults leaving this offsite power route intact. Hence failure to operate correctly could affect the integrity of the essential electrical supplies