Load-frequency control (LFC) is vital for maintaining power system stability, ensuring that frequency and power flows remain within acceptable limits during load changes. Turbine-governor control eliminates rotor accelerations and decelerations following load changes. However, a steady-state frequency error persists when the change in the turbine-governor reference setting is zero. In an interconnected power system, each area agrees to export or import a scheduled amount of power through transmission-line interconnections or tie-lines. Each area absorbs its own load changes to maintain scheduled net tie-line power (ptie) flow. The main roles of LFC are to return the steady-state frequency error to zero after a load change and maintain the net tie-line power flow at its scheduled value.
The Area Control Error (ACE) is defined as the net tie-line power flow deviation from its scheduled value plus a frequency bias (Bf) term. The ACE for each area is a linear combination of the tie-line error and frequency error (Δf). The change in reference power setting (Δprefi) of each turbine-governor under LFC is proportional to the integral of the ACE. This integral action ensures that both the frequency error and tie-line power error are driven to zero.
The frequency bias constant and integrator gain (Ki) affect the transient response to load changes. The frequency bias should be high enough for adequate frequency control. Cohn's method suggests setting frequency bias equal to the area frequency response characteristic for satisfactory performance. The integrator gain should not be too high to avoid instability. Automatic generation control strategies coordinate LFC with economic dispatch objectives by adjusting turbine-governor reference power settings. The desired outputs of each generator, as determined by an economic dispatch program, are updated at intervals, ensuring efficient and stable power system operation.
In sudden power surges, load-frequency control stabilizes the grid. LFC's goals are to nullify steady-state frequency error after load changes and maintain each area's scheduled tie-line power flow.
Area Control Error, combining tie-line power flow deviations and frequency error, guides adjustments to each turbine governor's reference power setting.
Commands to raise or lower these settings occur every few seconds, allowing new steady-state operations when all settings and ACE of all areas are zero.
Frequency bias should be high enough for area contribution to frequency control without causing instability through high integrator gain.
Additional LFC objectives include controlling 60-Hz motor-driven clocks' timing and energy transfers by nullifying frequency and net tie-line errors.
During normal load and frequency changes, LFC maintains control.
Adjusting turbine governors' reference power settings accomplishes LFC and economic dispatch objectives.
In a coordinated automatic generation control strategy, ACE is computed first, with a share allocated to each unit.
Then, deviations of total generation from the desired are computed and allocated, dictating each unit's raise or lower signals.
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