Load shedding (LS) is a protective method of
emergency power control
Power system operations is a term used in electricity generation to describe the process of decision-making on the timescale from one day (day-ahead operation) to minutes prior to the power delivery. The term power system control describes action ...
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
A cascading failure is a failure in a system of interconnected parts in which the failure of one or few parts leads to the failure of other parts, growing progressively as a result of positive feedback. This can occur when a single part fails, in ...
of the
power grid
''Power Grid'' is the English-language version of the second edition of the multiplayer German-style board game ''Funkenschlag'', designed by Friedemann Friese and first released in 2004. ''Power Grid'' was released by Rio Grande Games.
I ...
.
The load shedding is activated when the
line frequency
The utility frequency, (power) line frequency (American English) or mains frequency (British English) is the nominal frequency of the oscillations of alternating current (AC) in a wide area synchronous grid transmitted from a power station to t ...
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
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
.
References
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Electric power distribution
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