load shedding

Load shedding (LS) 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.

The 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'' (typical situation, especially for UFLS: multiple areas of the grid shed the load in cooperative manner) or ''targeted'' (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.

Under-frequency load shedding (UFLS)

UFLS scheme shed predetermined amounts of load when the system frequency drops below specific thresholds. UFLS typically has short time delays, around 0.2 seconds. The UFLS schemes are most common and often represent a shared shedding paradigm.

Under-voltage load shedding (UVLS)

UVLS scheme is used to protect against excessive voltage decline by shedding load when voltage falls below specified thresholds. Time delays in UVLS are generally longer, typically above 3 seconds to prevent false tripping. Some research suggests using a slope permissive UVLS design to shed load earlier based on predicted voltage drops. UVLS can be employed in targeted load shedding approaches.

By the end of the 20th century, the main cause of blackouts in North America switched from underfrequency to voltage collapse.

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Electric power distribution

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