Bus Backup Distance Relay Operation For Remote Fault Due To Load Current

An electromechanical distance relay intended to provide backup protection for 161 kV line terminals at a large nuclear generating station operated for a slow-cleared 46 kV system fault (nearly seven seconds) at a remote station near the generating plant before the backup protection at that station could operate. This resulted in erroneously splitting the 161kV buses at the generating station and interrupted three 161 kV transmission lines. Neither the forward zone 2 at the generating station on the line to the remote station nor the reverse zone at the remote station was set to detect the 46 kV fault which occurred. The 161 kV switchyard at the generating station is configured in what has been called a zig-zag bus (Z-bus for short). There are two 161 kV main buses, each terminating several 161 kV lines on single breakers, with five double-breaker bus tie bays. The backup distance relay is fed from voltage transformers on main bus 1 and receives summed current from the five bus tie breakers connected to bus 1. The backup distance relay is part of a bus backup scheme that provides for relay failure, current transformer failure, and breaker failure for several 161 kV line terminals connected to one of the 161 kV buses at the generating station. The reach of the distance relay was correctly set to detect faults at the end of the longest line connected to the bus section by apparent impedance with infeed. By fault study without load this element would not have detected the 46 kV fault. But with load across the bus ties at the generating station, the apparent impedance was within backup distance relay reach. It was determined that the load across the bus ties had to exceed 600 MVA in order for the distance relay to operate. It is extremely difficult for a settings engineer to have known or predicted the expected maximum load across the bus ties. Mitigating actions included extending the reach of the forward zone 2 on the line to the remote station, extending the reach of the reverse zone at the remote station, and installing individual backup relays on two additional 161 kV lines. The paper will discuss the configuration of the switchyard, the settings of the distance relays involved, the actual event, the estimation of load across the bus ties, and the chosen mitigating solutions.