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Automatic Under-Frequency Load Shedding

The maintenance of maximum service reliability has always been the primary concern of the electric utility industry. To ensure this, power systems are always designed and operated such that working is not affected in any system conditions and load requirements are always met. Usually the designing is such that it can hold up service continuity even under emergency situations, but sometimes, unpredictable conditions of faults, forced outages, etc. may occur. When this happens, it is important to ensure that steps are taken to ensure that a major system outage doesn’t occur.

Any part of a power system will begin to deteriorate if there is an excess of load over available generation. If there is an excess of load over generation ratio, the frequency decreases. It is generally recognised that a sudden drop in generating capacity results in a drop in frequency. This drop is not immediate, but rather, happens gradually.

One way to attain the balance between generation and load, before the decaying frequency affects performance, is to increase generation. However this isn’t always possible practically due to system limitations or due to time constraints. So, a more common method is to employ Automatic Under-Frequency Load Shedding (AUFLS). What this does is that it employs a quick and effective means of attaining a balance of generation and load. The application of AUFLS relays throughout the load area, preset to drop increments of load at specific values of low frequency, provides a simple and direct method of minimizing service interruption and alleviating system overloads.

The Load Shedding function provides under-frequency protection at the main distribution substation. As system frequency decreases, load is disconnected in discrete steps according to frequency thresholds. Protective relays are used for automatic gradually under-frequency load shedding. Under and over-frequency relays are specified by frequency settings and delays. And all this can be incorporated by using IoT Gateway which requires minimum system integration and is fully compatible with most of the applications.

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