Slope-Permissive Under-Voltage Load Shed Relay for Delayed Voltage Recovery Mitigation

In this paper, a new relay algorithm is proposed for under-voltage load shedding (UVLS) based on rate of voltage recovery and predicted time to recover above a preset voltage threshold. Simulation experience in the Southern Electric System has shown that conventional UVLS relay logic is ill-suited to the task of mitigating delayed voltage recovery and fast voltage collapse situations while maintaining compliance with existing NERC Reliability Standards. The proposed scheme continuously monitors both voltage magnitude and rate-of-change of voltage magnitude and uses these values to determine a projected voltage recovery time following a contingency event. If the projected recovery time is such that resultant system conditions are unacceptable (e.g., major load loss, loss of generation, or voltage collapse), the relay will operate to shed load at the distribution feeder level. This unique scheme has been implemented in a commercially-available programmable relay and is presently installed and active in a number of substations in Georgia Power Company.