Research Of HVDC Participating In Frequency Regulation Of Wind-Thermal-Bundled Islanded System Which Is Connected By HVDC

China is rich in wind power resources,but mainly distributed in the "three north"areas,far from the eastern and central load centers.With the continuous improvement of wind power Grid-connected capacity,there is a need for large-scale and long-distance transmission of wind energy.In recent years,the UHVDC transmission develop rapidly,based on this,China creatively proposed wind-thermal power bundled system which is connected by HVDC,in order to achieve efficient and optimal allocation of renewable energy.However,due to the lack of hydropower in the Northwest of China and the limited frequency regulation capability of conventional units,the frequency stability of isolated island system is particularly prominent under the random fluctuation of wind power.HVDC has flexible control and fast modulation,which can rapidly change the output power to participate in frequency regulation of wind-thermal bundled(WTB)system.Therefore,it is necessary to study the way of HVDC frequency regulation mode under this background,as well as the coordination mode between HVDC modulation and primary or secondary frequency regulation of thermal power units.Firstly,based on the actual engineering data of Northwest Power Grid and Hazheng UHVDC,a simplified model of wind-thermal bundled islanded system which is connected by HVDC is built on PSCAD/EMTDC simulation platform.The model includes thermal power units,DFIG and UHVDC transmission system.Secondly,this paper explores the frequency modulation means of wind-thermal power bundled system which is connected by HVDC,and analyses the primary and secondary frequency modulation characteristics of conventional units,and tunes the key parameters;Combining with the background of wind-thermal bundled island,this paper studies the impact of local load proportion and wind power permeability as key parameters on the frequency stability of the system,and determines the necessity of HVDC participation in frequency regulation.The measures of HVDC participating in the frequency stability of the system are analyzed and compared,and the HVDC frequency limited control(FLC)is selected as a reasonable control measure,then optimizes parameters of the controller and simulates to verify.Finally,aiming at the problem that too small dead-zone setting of FLC will lead to HVDC to follow the large-scale action of low-frequency wind power,resulting in frequent switching of HVDC reactive power regulating equipment,but too large dead-zone setting will weaken the primary frequency modulation and system damping supporting ability of HVDC,this paper studies the feasibility of using AGC to eliminate low frequency wind fluctuations based on the analysis of the traditional FLC control mode and the control characteristics of automatic generation control(AGC).Aiming at building the coordinated control strategy between FLC and AGC of WTB islanded system,this paper uses the relationship between the frequency change rate and the maximum frequency deviation to judge the disturbance degree of the system based on the system frequency response model,then the fluctuation of low frequency wind power is adjusted by AGC,while FLC responds under high frequency wind fluctuation,does not respond under low frequency wind fluctuation and supports the frequency of the system in emergencies.A WTB islanded system is built in PSCAD/EMTDC,and the simulation results show that the proposed strategy can effectively reduce the HVDC action range and the switching times of reactive power equipment while ensuring the frequency stability of the system.