Accurate Performance of Residual Voltage Transfer Schemes

Transferring motor loads from one bus to another bus, due to a source disconnection from a main-tie-main arrangement, is not a trivial action. The transfer is dependent on detection of breakers' status, switch positions, and accurate measurements of line and bus voltages. The mechanism of transferring loads from one bus to the other imposes a number of requirements for the relay controlling the closing of the tie-breaker. The main requirement in this process is to accurately measure both the bus voltage from the “healthy” source and the decaying voltage from the bus disconnected from its supplying source. This is done to assure accurate and smooth transition of the motors to the healthy source. To perform accurate ac measurements, many protective devices are built to track the power system frequency from one voltage source. However, when the source for the frequency tracking is lost, the accuracy of the measurements associated with the voltages and currents from the healthy source is jeopardized. In such conditions, the relay may declare an absence of healthy bus voltage and could prematurely issue a close command to the tie-breaker, while the magnitudes of both bus voltages are still high and out of phase. Closing of the tie-breaker under such conditions can stress the motors and damage them due to a high torque. This paper reveals a method for accurate voltage measurement and shows a number of tests proving that inaccurate voltage measurement, due to decaying frequency in existing relay designs, could cause incorrect tie-breaker closure during transfer operations. Both scenarios of decaying voltage used for frequency tracking and decaying voltage from the other bus are explored. This paper discusses alternative methods for accurate bus voltage measurements during frequency decay of the reference voltage and makes recommendations for optimal relay performance.