Research On Grid Connection Strategy Of Permanent Magnet Direct Drive Wind Power System Based On VSG

Wind resource is the most abundant renewable energy in the world.In recent years,with the improvement of people’s awareness of environmental protection and the increase of energy demand,wind power generation has developed rapidly.However,with the increasing installed capacity of wind turbines connected to the power grid,it has a serious impact on the stability and power quality of the power grid,resulting in the reduction of damping and inertia of the power system.In order to solve this problem,many scholars have proposed control strategies of virtual synchronous generator with different structures.The existing grid connection control strategies of permanent magnet direct drive wind power systems in China are mostly the control strategies under normal power grid,but the actual power grid may have non-ideal working conditions such as three-phase power grid voltage imbalance,DC component,and harmonic component.Taking permanent magnet direct drive generator as the object,this thesis studies the control strategy of wind power generation system.Firstly,the mathematical model of permanent magnet synchronous generator is established,and the traditional permanent magnet direct drive power generation system is built.The machine side zero d-axis vector control strategy and the grid side converter grid voltage oriented vector control strategy are verified under the normal working condition of grid voltage,and the phase-locked principle of traditional synchronous reference frame phase-locked loop(SRF-PLL)is introduced in detail,In view of the failure of the traditional SRF-PLL under single voltage in the non-ideal power grid,a PLL based on third-order generalized integrator is proposed in this paper,and two improvements are made.They are type I improvement scheme based on notch filter and delay signal cancellation(DSC)hybrid filter and type II improvement scheme based on Notch cascade.Finally,the input signal contains a DC bias component The correctness of the proposed PLL is verified under three working conditions: single-phase voltage,two-phase voltage drop,and input voltage containing harmonics.Different from the traditional grid side control strategy of permanent magnet direct drive power generation system,the virtual synchronous generator can provide damping and inertia support for the power grid,and can realize phase-locked loop-free control,simplify the control strategy and save cost.Firstly,this paper designs the body model,virtual governor,and virtual exciter of virtual synchronous generator,and simulates and verifies the designed virtual synchronous generator(VSG)in the island and grid-connected 2 operation mode.Then,the non-ideal grid voltage components and the reasons for the power fluctuation of ordinary VSG when the grid voltage is unbalanced are analyzed,the VSG grid connection control strategy under complex working conditions is designed,and the third-order general integrator(TOGI)is used to eliminate the influence of the DC component of grid voltage,On the basis of VSG control under ordinary unbalanced voltage,the modified delay signal cancellation(MDSC)method is used to eliminate the harmonic of positive sequence command current to obtain the positive sequence current command,and then the control strategy of cascade PI of improved quasi proportional resonant(QPR)is used to further suppress the harmonic current,so as to realize that the harmonic content of grid-connected current meets the requirements of grid connection,The negative sequence current command is calculated with the control objectives of output three-phase current balance and constant active and reactive power.Finally,under the non-ideal grid voltage,the VSG control strategy under complex working conditions and the VSG control strategy under ordinary unbalanced voltage is simulated and compared in MATLAB to verify the correctness of the proposed control strategy.