Adaptive control of load shedding relays under generation loss conditions

Two approaches to emergency load shedding have been evaluated by means of simulations of a representative system over a range of loads. The load shedding schemes examined were a conventional discrete frequency system, and an adaptive rate-of-change of frequency system. It was found that more accurate load shedding was achieved with the adaptive rate-of-change of frequency technique, the mismatch being 21%, compared with 37% for the discrete frequency scheme. The performance of the adaptive scheme was limited by a loss of discrimination in cases where the largest infeeds did not correspond to the most severe potential generation outages from the point of view of required load shedding. Further rationalization of load shedding requires a system-wide communications facility to identify the tripped generating unit, and instruct the appropriate relays to shed the minimum load in sufficient time to ensure frequency recovery