Research On Control Strategy To Enhance The Fast-frequency Response Capability Of DFIG-ES System

Wind energy,as a reliable renewable energy source,has been exploited rapidly in recent years.However,wind power generation is challenging the safety and stability of the power system,as it results in the increasing penetration of wind power generation into the power grid.As the rotor speed of doubly-fed induction generator(DFIG)is decoupled with the system frequency,the frequency regulation contribution from wind farms is nearly none.The energy storage(ES)system has been installed in wind farms to respond to frequency deviation with doubly-fed induction generators(DFIGs).In view of the high allocation and maintenance cost of the ES system,the capacity allocation scheme of the ES system—especially for fast-frequency response—was proposed in this paper.After the capacity of the ES system was defined,the coordinated control strategy of the DFIG-ES system with consideration of wind speed was proposed in order to improve the frequency nadir during fast-frequency response.In particular,a novel virtual inertia control method of DFIG was put forward to release rotational kinetic energy and produce power surge by means of continuously modifying the proportional coefficient of maximum power point tracking(MPPT)control.During the deceleration period,the power reference smoothly decreased with the rotor speed until it reached the MPPT curve,wherein the rotor speed could rapidly recover by virtue of wind power so that the secondary frequency drop could be avoided.Afterwards,a nonlinear programming model,whose objectives were maximum frequency nadir and maximum frequency response of frequency nadir,was built to distribute output power between the DFIG and ES system and was solved by improved particle swarm optimization.What’s more,a fuzzy logic controller(FLC)was designed to decide the maximum regulation range of DFIG rotor speed according to the rotor speed of DFIG and wind power penetrating rate.Thus the scheme enabled the DFIG-ES system to respond to frequency deviation in most cases while preventing the secondary frequency drop.The test results,which were based on the simulation system on MATLAB/Simulink software,verified the effectiveness of the proposed control strategy by comparison with other control methods and verified the rationality of the designed fuzzy logic controller and proposed capacity allocation scheme of the ES system.