Load Shedding

Load shedding is the controlled, intentional disconnection of electrical loads to prevent a system-wide blackout when generation, imports, and reserves are insufficient to meet demand. It is the last line of defense in the power system security hierarchy, activated only after all other remedial actions have been exhausted.

Key Aspects of Load Shedding:

• Last-Resort Measure: Load shedding is deployed only after exhausting all other options, dispatching reserves, activating demand response, increasing imports, and voltage reduction. It is considered an emergency action because it directly interrupts electricity supply to customers.
• Priority-Based Schemes: Load shedding plans assign priority levels to different feeders or substations. Critical loads such as hospitals, emergency services, water treatment, and telecommunications are on the lowest-priority shedding group (shed last), while less essential loads are disconnected first.
• Underfrequency Load Shedding (UFLS): Automatic relay-based schemes that trip pre-assigned load blocks when system frequency drops below defined thresholds (e.g., 49.0 Hz, 48.8 Hz, 48.6 Hz in a 50 Hz system). UFLS provides millisecond-speed response to sudden, large generation losses that outpace operator reaction time.
• Manual vs. Automatic: Manual load shedding is ordered by system operators during foreseeable shortages (e.g., planned maintenance coinciding with extreme weather). Automatic schemes (UFLS, undervoltage load shedding) respond to rapid, unplanned events like generator trips or major transmission faults.
• Rolling Blackouts: When a generation deficit persists over hours or days, operators may implement rolling load shedding, rotating disconnections among different areas in scheduled blocks, to share the impact equitably across customers and limit the duration of interruption for any single area.