Research On Three-level Grid-connected Inverter With LCL Filter

In recent years,the photovoltaic technology has been widely concerned at home and abroad with research and application due to its advantages of high efficiency,renewable energy source and zero pollution.As the key role in the photovoltaic technology,the grid-connected inverters influence directly to the reliability of the system and power quality.Furthermore,its performance restricts the development as well as business marketing of the photovoltaic technology.So,it is quite important to research deep of the grid-connected inverter.The main descriptions of this dissertation are as follows:This dissertation uses the neutral-point-clamped three-level inverter with LCL filter as subject.Firstly,this dissertation analyzes how this type of inverters work and its SVPWM algorithm.Also,considering the inherent neutral balancing problem of the NPC inverter,a neutral balancing strategy is given in this dissertation.The mathematical models of NPC grid-connected inverter with LCL filter under the ABC three-phase stationary,?? two-phase stationary and dq two-phase rotating coordinates are established.Moreover,as the LCL parameters couple in dq two-phase rotating coordinate,this dissertation deduces a full decoupling method via block diagram equivalent.According to the operation mode of the NPC inverter and the photovoltaic grid-connected criteria in China,this dissertation explore a LCL filter design method which can be applied in NPC three-level grid-connected inverter.After analyzing in the NPC three-level output current ripple and current harmonics effected by each component in LCL filter,the LCL parameters are chosen through 3-D graphic analysis.The passive damping and active damping methods are introduced in the face of the inherent resonant problem of LCL filter.Based on this,double closed loop control strategy of capacitor current and grid connected current is proposed in this dissertation.Meanwhile,the parameters design of PI and P controllers in the double closed loop are given.Based on the analysis above,this dissertation establishes simulation models in the Matlab/Simulink software to verify both the LCL filter parameters and strategy of grid connection.A 1.5kW LCL type NPC three-level grid-connected inverter prototype is built,and the hardware design(including main circuits,IPM drive circuits,signal detection and conditioning circuits and slave system.)and software design(including SVPWM generation,AD sampling,control algorithm loops and serial communication)are introduced as well.In the end,according to the simulation and experimental results,the effectiveness and feasibility of three-level SVPWM,LCL decoupling,LCL filter parameters design and double closed loop control strategy of capacitor current and grid connected current are verified.