load shedding

Load shedding (LS), often spelled loadshedding, is a protective method of emergency power control where, during a large disbalance between supply and demand, the demand (load) is intentionally disconnected ("shed") so that the available electricity supply within a safe timeframe can meet the remaining demand, thereby preventing a cascading collapse of the power grid.
Load shedding is activated when the line frequency becomes too low (under-frequency load shedding, UFLS) or the line voltage is below the specified level (under-voltage load shedding, UVLS). The very act of disconnecting the load introduces yet another disturbance into the grid, so the selection of the bus about to be shed is chosen based on the bus distance from the contingency location as well as economic considerations. The shedding can be shared via "rolling blackouts" or "rolling brownouts" (the typical situation, especially for UFLS: multiple areas of the grid shed the load in cooperative manner) or targeted via a "targeted blackout" (the load shedding is concentrated in a particular area). Typically the shedding is performed automatically in multiple stages, each with a pre-programmed threshold and the load amount to shed.

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