Stability Analysis And Control Of Grid-connected Converters Based On DQ Impedance Model

Three-phase voltage-source converters are the basic building block for grid integration of renewable energy.The stability of system will be affected by the coupling between the control system of grid-connected converters and grid impedance under the weak grid conditions.Hence,this paper focuses on the three-phase grid-connected converter under the weak grid conditions and studies the stability of grid-connected system through the impedance-based method,aiming to propose improved control strategies to enhance the stability of converters in a weak grid.The impedance model of the main circuit of three-phase grid-tied converters is firstly developed in the rotating dq-frame.Since the phase-locked loop(PLL)dynamics influence the result of coordinate transformation,the PLL behavior should be taken into account in the impedance model.On this basis,the impedance model is deduced considering current loop,PLL,DC bus voltage loop and AC voltage loop.The accuracy of the impedance model is validated by the frequency scanning method.The influence of reactive current on the stability of grid-connected system under weak grid conditions is analyzed based on the d-q impedance model.The effect of PLL dynamics and grid impedance on the system stability is studies under weak grid conditions by the d-q impedance model.The analysis results show that the error of PLL in the perturbation path increases and the stability is reduced duo to the increase of PLL bandwidth.The variable transfer relationship is used to analyze the influence of PLL perturbations on output current under weak grid conditions.And a novel feedforward method,which considers the PLL dynamics,is proposed in this paper to suppress low order harmonics of the network current.The proposed method is improved for consideration of reactive current effect and the application scope of the improved method is extended in weak grid conditions.The influence of DC bus voltage loop and grid impedance on the stability of grid-connected systems is analyzed based on the d-q impedance model.The research shows that there exists interaction between the DC bus voltage loop and grid impedance due to the grid impedance.The interaction effect results in the increase of the d-axis voltage error of the point of common coupling(PCC)and causes DC bus voltage oscillation.The perturbation path of grid-connected system is found through the transfer relationship between the voltage at the PCC and the DC bus voltage,which is exhibited in the d-q impedance model.The d-axis voltage perturbation of the PCC is added into the output of current loop,and an improved control method based on d-axis voltage perturbation feedforward is proposed to enhance the stability of grid-connected system under weak grid conditions.On the basis of the proposed method,another control method combining PLL perturbation feedforward and d-axis voltage perturbation feedforward is obtained,which takes the PLL dynamics into account.The stability of grid-connected converters is further improved with the comprehensive control method under a weak grid.The simulation model is built in MATLAB/Simulation and experiments are carried out using the three-phase grid-connected converter experiment platform in the lab.The simulation and experimental results verify the validity of theoretical analysis and the effectiveness of the proposed methods.