Load-frequency control

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 (p_tie) 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 (B_f) term. The ACE for each area is a linear combination of the tie-line error and frequency error (Δ_f). The change in reference power setting (Δp_refi) 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 (K_i) 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.