Modulation And Control Strategies For Centralized Photovoltaic Inverters

Efficiency is one of the most important indicators of centralized Photovoltaic(PV)inverters.Increasing efficiency will not only increase the harvesting power from the sun but also reduce the cost of the PV inverter.In the first chapter of this dissertation,the development status of the centralized PV power system is studied,including topologies,voltage level and the efficiency of the inverter.Then the high-efficiency modulation methods,neutral point voltage control strategies in three-level inverter,and low voltage ride through(LVRT)control methods are reviewed.In order to improve the overall efficiency of the three-phase photovoltaic inverter in the full power range,a hybrid pulse width modulation(PWM)strategy based on continuous pulse width modulation(CPWM)and discontinuous pulse width modulation(DPWM)is proposed in the second chapter of this dissertation.The effects of CPWM and DPWM on the loss distribution of power devices and passive filters are analyzed,and the switching power point between two modulation methods is derived.Then,the influences of modulation ratio and the power factor on switching power points are analyzed.Finally,the proposed hybrid PWM scheme is experimentally verified on a 500kW three-phase PV inverter.It is a cost-effective way to increase the voltage level of the PV inverter.PV inverters with different voltage levels(1000V,1500V and 2000V)are discussed and compared in the third chapter of this dissertation.Power devices,LCL filters,DC capacitors,and other key components are designed and compared respectively,in terms of efficiency,cost and volume.For three-level inverter in PV system,if two PV arrays are connected to the upper and lower dc bus-bar respectively,and each sub-array can be controlled independently and work at its Maximum Power Point(MMP).Then more solar energy will be harvested.An independent control scheme for the dual-input three-level inverter is proposed.Firstly,the mathematical model of the dual-input three-level inverter is established.Then the control loop of the independent control scheme is designed.Besides,the modulation method and control restriction of the control scheme of dual-input three-level inverter are analyzed.Finally,the proposed control methods are experimentally verified on a dual-input three-level PV inverter.T-type three-level centralized PV inverters have problems of poor dynamic performance and unbalance loss distribution during low voltage ride through(LVRT).In the fourth chapter of the dissertation,in order to increase the dynamic performance of the PV inverter during low voltage ride through,coordinated control strategy with multiple loops is analyzed and designed.Based on the loss model and thermal model,the loss distribution of T-type three-level inverter during low voltage ride through is analyzed.The loss is redistributed by improving the PWM scheme.Besides,the dynamic characteristic of neutral point voltage in three-level inverter is considered.Finally,experiment on three-level inverter is implemented to verify the control strategies.The work of this dissertation is summarized at last,and the future works are presented...