Fast bus transfer scheme and its performance in nuclear power generating stations

During a cold start of the nuclear power generating station, Startup Transformers (SUT) energized from the switchyard provide power for recirculation pumps, feedwater pumps, condensate and condensate booster pumps in a Boiling Water Reactor (or reactor coolant pumps, circulating water pumps etc. in a Pressurized Water Reactor). When the reactor reaches rated pressure and temperature and 6-10 percent power, the main generator is synchronized to the grid and these loads are transferred manually to the Unit Auxiliary Transformer (UAT) energized by the main generator. On the trip of the turbine, the power to these loads is switched automatically back to the SUT. Without the provision of a fast transfer scheme (4 to 10 cycles) from UAT to the SUT, recovery from scram may be quite complex. The complexities that could result, including rendering the feedwater system inoperable. In the Sequential Fast Transfer Scheme an early 'bb' auxiliary contact of the UAT (normal source) breaker is used to initiate closing of the SUT breaker (alternate source). Bus dead time of 4 to 10 cycles is obtained. Some licensees use a voltage decay scheme (also called residual bus transfer scheme) as a backup to the fast transfer scheme. ANSI C50.41-1982, "Polyphase Induction Motors for Generating Stations," recommends that the power supply system be designed so that the resultant vectorial volts/hertz between at the instant the transfer or reclosing is completed does not exceed 1.33 per unit volts/hertz on the motor rated voltage and frequency bases. High speed sync-check relays or the use of the state of the art technology like a microprocessor based sync-check devices in a fast transfer scheme can improve the reliability of the auxiliary power distribution system.