FREQUENCY STABILITY ANALYSIS AND CONTROL OF AC/DC SYSTEM

Frequency oscillation between regions occurs when the interconnected system is disturbed by load, which is not conducive to the safe and stable system operation. First, by analyzing the characteristics of fast controllable and short-time overload of DC transmission power, a scheme of coordinated control of load frequency control and voltage source converter based high voltage direct current modulation controller is proposed to alleviate the frequency oscillation between interconnected systems. Secondly, the influences of two frequency modulation parameters, unequal unit rate and frequency modulation dead zone on the frequency stability of the system is analyzed. Finally, taking the minimum system frequency deviation and tie line power deviation as the objective function, the genetic algorithm is used to optimize the controller parameters. And the global optimization is applied to achieve the rapid restoration of the system stability. The simulation results of the AC/DC interconnected system show that the maximum frequency deviation amplitude can be reduced and the frequency oscillation between the systems can be effectively suppressed by the interconnection of DC modulation controller and AC line. The smaller setting values of frequency modulation dead zone and unit speed unequal rate, the smaller the maximum frequency deviation and final stable frequency of the unit, and the better the effect of primary frequency modulation. Compared with the traditional controller, the intelligent optimal control proposed in this paper has better control performance and could adapt to the change of load.