Tertiary frequency modulation method for power system

A tertiary frequency modulation method for a power system, characterized in that Automatic Generation Control (AGC) based on power plant input performs active power regulation for tertiary frequency modulation in following modes in power plants in case of secondary frequency modulation failure of the power system and unidirectional deviation of grid frequency from rated frequency for a period:

• tertiary frequency modulation mode based on reserved capacity of secondary frequency modulation:
• if adjustable capacity is reserved for the secondary frequency modulation in the power plant, and an absolute deviation between an active power set value and an active power planned value of the power plant is smaller than the reserved capacity of secondary frequency modulation, correct the active power set value in a cyclic iterative manner based on the reserved capacity of secondary frequency modulation and the tertiary frequency modulation coefficient;
• or tertiary frequency modulation mode based on a regulation amount of primary frequency modulation:
• obtain the regulation amount of primary frequency modulation according to an actual regulation amount of primary frequency modulation, a target regulation amount of primary frequency modulation or a corrected regulation amount of primary frequency modulation, assign the regulation amount of primary frequency modulation to an element in an array, and correct a plant active power set value according to a weighted average after several cycles;
• or tertiary frequency modulation mode based on random parameters:
• equally segment an actual output value of plant active power, generate random numbers by a real-time monitoring system according to a number of segmented units, and assign the random numbers to each element in the array in turn;
• obtain a corrected power according to the grid frequency deviation, capacity of segmented units, a number of specified values in the array and a preset power regulation factor of tertiary frequency modulation in each cycle, and correct the plant active power set value according to the corrected power;
• after the corrected active power set values of the plant are obtained in each mode, AGC modifies the active power set value of each unit and performs closed-loop active power regulation for each unit;
• when the grid frequency deviation falls back or AGC state changes reach a stop trigger condition, stop tertiary frequency modulation.