Automatic load shedding is the primary protection against frequency instability due to power deficiency. This paper describes the design of a digital adaptive under-frequency load shedding relay. The relay implements a new adaptive load shedding scheme, in which the set of shedding feeders is selected adaptively based on the online measurement of load power distribution, so as to reduce over or under shedding and speed up the frequency recovery. In addition, an adaptive frequency estimation technique is proposed to provide fast and accurate frequency estimation with high harmonic suppression ability. The test results in laboratory and field environments confirm that the developed relay can offer better protection than a traditional load shedding relay.
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