Probability Indices For Composite Frequency Regulation By PMSG And VSC-HVDC

The application of the voltage source converter based high-voltage direct-current(VSC-HVDC)decouples the frequency of the interconnected power grids,leading to the loss of frequency support at the receiving end and enlarging the challenge of the frequency.Moreover the integration of the large scale offshore wind farms to the traditional power grid through the VSC-HVDC reduces the system inertia and compromises the frequency stability.The composite frequency regulation effect of the wind farm and the VSC-HVDC is influenced by the operation conditions and the fault scenarios.The dynamic models of the traditional AC power grid with the VSC-HVDC interconnection and the offshore permanent-magnet synchronous-generator(PMSG)based wind farm integrated through the VSC-HVDC are established in this paper.The frequency regulation utilizing VSC-HVDC and the coordination of the VSC-HVDC and the PMSG based wind farm is studied.The main research content involves:(1)For the VSC-HVDC interconnected power grids,the fast response of the VSC-HVDC control may be utilized to participate in frequency regulation.The additional frequency control may be provided by the converter station at the receiving end to improve frequency regulation effect.Based on direct power flow model,the response of the generator speed under the load disturbance is derived considering the rotor motion equation and the governor system of synchronous generator.The additional power-frequency droop control of converter station at the receiving end is introduced,based on which the analytical model to quantify the transient frequency of the power system equipped with the VSC-HVDC is proposed.(2)The control of the offshore PMSG based wind farm integrated through the VSC-HVDC to participate in the frequency regulation is studied.The probability statistics indices based on the time domain simulations are proposed to quantify the frequency regulation effect.The control strategies and parameters of the frequency regulation as well as the locations and scales of the load disturbances are enumerated and the frequency responses of the power grid are obtained based on the time domain simulations.The indices concerning the system frequency,e.g.maximum deviation,drop rate,recovery rate,and steady state deviation,are calculated.The expectation values of the aforementioned indices are obtained based on the probability statistics of the load disturbances.