Research On Model Predictive Control Algorithm For Three Phase Converter With LCL Filter

With rapid development of renewable energy technologies,such as wind energy,and photovoltaic techniques,the converter has become one of the most important techniques in the field of power electronics.At present the converter is of VSC structure together with PWM control strategy,high order harmonics inevitably occur in output current,which will cause harmonic contamination if the converter is directly connected to the grid.Compared with conventional single L filter,LCL filter is more widely applied in the field of power electronics converter due to its small volume,less weight and strong decay of high order harmonics.However,the resonance problem of LCL filter itself may affect the system stability.Therefore,an additional damping method should be applied,Meanwhile the system is transformed from 1-to 3-order system after usage of LCL filter which requires more advanced control algorithm.In this thesis,the model predictive control algorithm is applied to the converter with LCL filter.First,the working principle of converter with LCL filter is analyzed,and the mathematical model is derived according to the Kirchhoff’s law of current and voltage and the formula of coordinate transformation in three phase static coordinate system,two phase static coordinate system,two phase rotating coordinate system respectively.This provides a theoretical basis for the analysis of the complex vector mathematical model of converter with LCL filter,the analysis of resonant characteristics and the design of predictive control algorithm.On control of converter with LCL filter,the converter-side current model predictive control（MPC-i₁）is adopted conventionally.The ultimate grid-side current is controlled indirectly by control of converter-side current.Obviously,the control performance can be improved when the grid-side current is directly controlled.Therefore,four predictive current control algorithm（AMMPC-i₁i₂,AMMPC-i₁i₂u_c,FBMPC-i₁i₂,and FBMPC-i₁i₂u_c）which put LCL filter grid-side current vector,converter side current vector and filter capacitor voltage vector in value function is proposed.For AMMPC-i₁i₂ and FBMPC-i₁i₂ algorithm,capacitance voltage proportional feedback active damping method is applied in the high frequency domain,thus the control structure is changed and the resonance is suppressed.Meanwhile based on the LCL filter mathematical model in low frequency domain,the system prediction model and value function are established by two different discretization methods;AMMPC-i₁i₂u_c and FBMPC-i₁i₂u_c algorithm add the predictive value of the filter capacitor voltage to the value function,therefore the state variable proportional feedback active damping is not required.In order to minimize the error of the output value and the reference value of the system at the next moment,the output voltage vector needed at this moment is calculated by the value function.Simulation and experimental results present that the proposed algorithm can improve the current control performance effectively for converter with filter.In view of the unfixed switching frequency of the single vector model predictive current control algorithm and the problem that the grid side output current is controlled indirectly by control of converter-side current,This thesis proposed a novel three vector direct grid-side current model predictive control（GSC-MPC）for converter with LCL filter.Based on timing coordination of forward difference method and backward difference method,a direct connection is established between the grid side output current and the converter side output voltage.Meanwhile a proper mathematical model is built for an improved model predictive control.The exact required voltage vector in the next sampling interval is predicted and calculated by this model.The output optimal voltage vector is modulated by SVPWM technique to control the converter.At the same time,the even harmonic suppression method in the output current of the three vector model predictive control algorithm are analyzed.Furthermore,the proposed GSC-MPC is rather robust to parameter variation.Simulation and experimental results present that the proposed GSC-MPC can improve the current control performance effectively for converter with LCL filter.