Residual transfer

A residual transfer aka residual voltage transfer aka slow transfer is one of three possible reactions to losing the Main Generator. Fast transfer is when the main generator shuts down and breakers readjust so that off-site power is used for plant loads. A Residual transfer is when the EDG are started based on a detected loss of power and non-vital power is cut; however, off-site power is connected prior to aligning on-site power. Lastly, no transfer, which is when the EDG starts and it is used as the power source. (One last event, but its beyond design bases, the station blackout involving a generator trip, no EDG, and no offsite power.)

Consequences of a residual transfer

A residual transfer makes a otherwise normal shutdown into a more difficult shutdown. The primary reason is that non-vital power is cut. Without this then all the Operators can use are ECCS systems. Main condenser can not be used so MSIVs have to be closed. If MSIVs are closed then reactor pressure has to be controlled through SRVs. The feedwater can not be used so reactor level has to be controlled by HPCI or RCIC injection.

Fortunately, a residual transfer doesn't mean that off-site power is actually lost (aka LOOP) and therefore this event is not reportable as a loss of safety function. EDGs are initiated, therefore, reportable as a system actuation.

Other notes

Some plants are designed to have a residual transfer of the non-safety electrical buses from the generator to an off-site power source when the turbine trip is caused by a generator protective feature. The residual transfer automatically trips large electrical loads to prevent damaging plant equipment during re-energization of the switchgear. These large loads include the reactor feedwater pumps, reactor recirculation pumps, and condensate booster pumps. After the residual transfer is completed the operators can manually restart the pumps from the control room. The turbine trip will result in a reactor scram. Should the trip of the reactor feedwater pumps be counted as a scram with a loss of normal heat removal?

Resolution: No. In this instance, the electrical transfer scheme performed as designed following a scram and the residual transfer. In addition the pumps can be started from the control room. Therefore, this would not count as a scram with a loss of normal heat removal.