Q14Power Quality and FACTS

Causes of Power Frequency Variation

System frequency is directly governed by the balance between total generation and total load (including losses) in an interconnected power system, since generator rotor speed and frequency are proportional. Frequency variation occurs whenever this generation-load balance is disturbed.

• Sudden loss of a large generating unit, causing frequency to drop as remaining generators supply the deficit
• Sudden loss of a large load or line, causing frequency to rise momentarily
• Faults and switching operations that disconnect generation or transmission lines
• Poor governor response or inadequate spinning reserve during load changes
• Islanding of a section of the grid, where local generation-load mismatch is more severe

Effects of Power Frequency Variation

Frequency deviations affect nearly all equipment that is designed to operate at a fixed synchronous speed or timing reference. Induction and synchronous motors experience speed changes proportional to frequency, which affects the output of connected pumps, fans, and production machinery. In generating stations, sustained under-frequency operation increases mechanical stress and vibration on steam turbine blades, and severe under-frequency can trigger automatic load-shedding or even unit trip to protect turbine blades from resonance-induced fatigue damage.

Frequency-sensitive protection relays (such as under-frequency and rate-of-change-of-frequency relays) may operate incorrectly or trigger unwanted load shedding if frequency deviates significantly from nominal. Clock-driven and timing-synchronized equipment that derives time reference from mains frequency can lose accuracy. In extreme cases, sustained frequency excursions can lead to cascading trips and system-wide blackouts if corrective action (governor response, automatic generation control, load shedding) is not fast enough.

Corrective Measures

• Automatic Generation Control (AGC) and governor droop control to restore frequency
• Spinning reserve maintained to cover sudden generation loss
• Under-frequency load shedding schemes to arrest severe frequency decline