Research On Control Of Grid-connected Three Phase PV Power Generation Based On Quasi-Z-Source Inverter

Photovoltaic(PV)grid-connected inverter,as a bridge connecting solar array and public power grid,is the key to achieve safe,efficient and stable operation of PV power generation system.Compared with the traditional two stage inverter,the quasi Z-source inverte(q ZSI)helps to reduce the cost of PV power generation and improves the conversion efficiency and reliability of the system because it needn't to insert the dead time and achieves boost and inverter functions by controlling its unique through state.When the grid-connected inverter has a certain structure,its performance depends largely on the control strategy of the system.Therefore,this dissertation focuses on the research of efficient grid-connected control technology.Firstly,based on the analysis of q ZSI working principle,an impedance source small signal model is established by using state space averaging method.The steady-state and transient performance of q ZSI is analyzed by zero-pole distribution graph,which provides a theoretical basis for setting the impedance source network parameter and designing system controller.The existing space vector modulation for Z-Source inverter(ZSVM)methods are compared and analyzed to get the ZSVM under which inductor current ripple and capacitor voltage stress of impedance source are minimum and zero vector utilization is maximum.Secondly,the relationship between the shoot through duty cycle and modulation ratio is analyzed,meanwhile,the relationship between them and the grid current amplitude and grid side filter inductance value is also analyzed.The mutual coupling between the through duty cycle and the modulation ratio is obtained.The increase of grid side inductance and grid current amplitude will increase the modulation ratio,which leads to insufficient shoot through duty cycle margin,resulting in abnormal operation of the whole system.To solve these problems,two schemes are proposed.One is changing the system parameters,and the other is using LCL filter.Thirdly,PV output power feed-forward and quasi proportional resonance(QPR)control are introduced to the L filter type quasi-Z source PV grid-connected system.The complete control strategy of the grid-connected system is designed by combining the ZSVM,including PV cell MPPT control,the DC bus voltage control and grid current control.Through MATLAB simulation,it is verified that the QPR control strategy with power feed-forward can make the system transfer to another steady state quickly and achieve no-static-error tracking of a given value in the stationary coordinate,meanwhile,the system is capable of resisting network frequency deviation.Fourthly,considering the inherent resonance of LCL filter and the strong coupling impacts between d-axis and q-axis,the active damping control method based on capacitorcurrent feedback is adopted by combining with QPR regulator in two phase stationary coordinate and the parameters of LCL filter are designed,at the same time,the filter capacitor current feedback coefficient and controller parameters are adjusted.Through MATLAB simulation,it is verified that the LCL filter type quasi-Z-source PV grid-connected system can achieve the control of the grid current and the suppression of LCL filter resonance under the proposed control strategy.The grid current is closer to the sine wave,and the strong coupling relationship between modulation ratio and shoot through duty cycle is weakened.Finally,in order to achieve the control of output power and current quality of q ZSI grid-connected system when the grid voltage is unbalanced,four control objectives,namely the elimination of negative-sequence current,the elimination of active power fluctuation,the elimination of reactive power fluctuation and the elimination of active and reactive power fluctuation are set,meanwhile,the harmonic and overcurrent problem of grid current reference value are analyzed and a current-limited model predictive control(MPC)strategy is proposed in two phase stationary coordinate.At the same time,the online computation amount of MPC algorithm is reduced by calculating the sub-cost function of the inductor current of the impedance network in selecting either the shoot through(ST)case or the other non-ST case.Through MATLAB simulation,it is verified that the proposed control strategy can not only achieve coordinated control of grid power and negative-sequence current,but also ensure the suppression of fault overcurrent.The system has good dynamic and steady-state performance.