Active Support Control Strategy Of PMSG And Its Adaptability Research Under Weak Grid

The increase in wind power permeability reduces the moment of inertia and damping in the power system,and thus cannot provide effective support and adjustment for the system frequency and voltage fluctuations.Especially when the wind turbine is connected to the weak grid,the situation will become worse.Therefore,in order to improve the active support capability of wind turbines and the stability under weak grid,this paper adopts an active support control strategy suitable for permanent magnet synchronous generator(PMSG).In this strategy,the grid-side converter is equivalent to a voltage source-type virtual synchronous generator,so that the PMSG can use the rotor kinetic energy of the wind turbine to provide inertial support when the system power fluctuates.In addition,the rotor-side converter keeps the DC-side voltage constant through constant DC-voltage control.Unlike traditional phase-locked loop(PLL)-based virtual inertia control,active support control strategies can not only provide the necessary inertia and damping for the system,but also achieve self-synchronization with the power grid,thereby avoiding the problems such as deterioration of PLL dynamic performance under weak grid.The control strategy mentioned in this paper is of great significance to improve the ability of wind turbines to participate in the frequency and voltage regulation of the power grid,as well as the stable operation in weak grid.Firstly,the overall structure of the PMSG grid-connected system and the traditional PLL type control model are analyzed,and the inertia of the system before and after the PMSG is connected to the weak grid is analyzed,thereby the virtual inertia is introduced to improve the system's inertia support capacity.Next,the PMSG grid-connected system simulation model is built in PSCAD/EMTDC,and then the situation of PMSG based on traditional PLL-type control strategy participating in system frequency and voltage regulation under different grid strengths is compared and analyzed.The results show that in the weak grid environment,even if the PLL-type control strategy with virtual inertia is adopted,the system frequency and the voltage at common coupling point will still oscillate,making it difficult to maintain stability.Secondly,for the above-mentioned problems that exist when the PMSG is connected to the weak grid,an active support control strategy is designed.This control strategy compares with the traditional synchronous generator model,the reactive-voltage controller,active-frequency controller and inner loop controller are designed.And utilizes the voltage feedforward compensation link to realize the constant voltage control of the voltage at common coupling point.In addition,on the basis of the proposed control strategy,the output external characteristics of the PMSG grid-connected system are analyzed,so that the virtual impedance is designed to adjust the output impedance of the converter to meet the output power decoupling conditions.Finally,for the adaptability of the proposed control strategy under weak grid,a small signal model of active support control was established,and the Bode diagram was used to analyze the influence of different short circuit ratio(SCR)and main control parameters on the stability of PMSG grid-connected system.In addition,the reason for the decline in active power transmission capacity and voltage instability of the PMSG grid-connected system under weak grid is analyzed,and this problem is improved based on active support control.In order to verify the correctness of the proposed control strategy and the above theoretical analysis,the operation of the PMSG connected to the weak grid under the proposed control and the traditional PLL control is analyzed comparatively through simulation.And under different working conditions and SCR,the simulation verified the active support ability of the proposed control to the system frequency and voltage.