Study On Grid-connected Control Strategy Of Renewable Energy Generation And Oscillation Mechanism Analysis

To achieve the grand goal of "carbon neutral" and "carbon peak",China is transforming its energy structure.The traditional synchronous generators are being gradually replaced by renewable energy sources led by wind power and photovoltaic.The penetration rate of power electronic equipment is increasing significantly.The grid-connected control strategy is one of the pivotal parts of renewable energy sources.The structure and parameter sets of the control strategies mainly determine the grid-connection characteristics of the renewable sources.Moreover,it is also recognized that the broadband oscillation brought by renewable sources is closely related to the controllers of the power electronic equipment.Consequently,it is urgent to research the advanced grid-connected control strategies and the oscillation affected by control characteristics.The paper focus on wind power and photovoltaic,which are two major new energy grid-connected fields.Two different scenarios:grid-connected voltage source inverter and doubly-fed induction generator(DFIG),are studied.The fundamental response characteristics and harmonic response characteristics of the inverter based on different control strategies are compared.A novel voltage modulated direct power control(VM-DPC)strategy was studied,which overcomes the drawbacks of conventional VOC,such as high precision requirements of phase-locking and asymmetrical control structure.The paper also presents the application of VM-DPC in DFIG,which improves the speed of dynamic response without using PLL.The admittance modeling method of DFIG using VM-DPC is also proposed.Based on the admittance model,the factors related to the grid-connected stability of DFIG using VM-DPC are revealed.(1)The normalization analysis framework of control strategies,including VOC,PR,and VM-DPC,is constructed.The power tracking dynamics of three different types of controllers are unified into the same second-order transfer function.On this basis,this paper compares the characteristics of the three control structures under non-ideal grid conditions.The results show that VM-DPC has better anti-interference characteristics than VOC and PR control.(2)The sequence admittance model of grid-connected inverter based on VM-DPC is first established.The model is built based on the normalized analysis framework by using the harmonic linearization method.The admittance characteristics and grid-connected stability of the inverter using VM-DPC are compared with the inverter using VOC and PR control.By using the symmetrical PLL technique and second-order bandpass filter,the frequency-coupling effects caused by three controllers are largely reduced.Therefore,the harmonic responses of the three control strategies after lin-earization can be approximately modeled as single-input single-output sequence admittance models.The accuracy of admittance modeling was verified by the frequency scanning method.The grid-connected stability of the inverter is analyzed by using proposed admittance models and the grid equivalent admittance.The accuracy of the stability criterion based on proposed admittance modeling was verified by using realtime simulation built on RT-Lab platform.(3)An improved VM-DPC is designed for DFIG.The control characteristics of DFIG using VM-DPC are compared with the DFIG using classical VOC in detail.The advantages of VM-DPC are illustrated from the aspects of dynamic response,steady-state characteristic,and control complexity.In addition,a parallel compensator is designed for negative sequence current suppression based on VM-DPC is proposed to deal with the voltage imbalance condition,which can effectively reduce the negative sequence current output and power fluctuation of the system under the condition of the unbalanced power grid.The proposed strategies are verified by simulation and hardware experiments.(4)A sequence admittance modeling method for the DFIG based on VM-DPC is proposed.The accuracy of admittance modeling is verified by the frequency scanning method.The influence of the control parameters,grid parameters,and operating point on admittance characteristics and grid-connection stability is analyzed.The Simulation results verify the accuracy of stability criteria based on the proposed admittance modeling method.