Research On Optimzation Of PWM Modulation Strategy For Three-level Grid-connected Inverter

At present,photovoltaic power generation industry is becoming more and more important under the background of new energy.The key to the performance of grid-connected photovoltaic system is grid-connected inverter.The nonisolated T-type three-level grid-connected inverter has been widely used for its’ high efficiency,high performance price ratio and so on.However,there are still some problems that need to be solved.Neutral point voltage balancing control is related to the reliable operation of three-level inverter.In the nonisolated photovoltaic inverter,the common mode voltage,which changes on the parasitic capacitance between the PV and ground,will produce common mode current,which will increase the network current harmonic and loss,and cause EMC interference and security risks.When the inverter works at high frequency,the switching loss of the power transistor becomes the main part of system’s loss,which affects the efficiency of inverter.This paper mainly studies how to optimize the PWM modulation strategy to solve the above problems,which universal exist in the stable,safe and efficient operation of inverter.First of all,the influencing factors of common mode current are analyzed,the mathematical model of common mode voltage is established,and comparison and analysis of the common mode voltage of 5 kinds of PWM modulation method and modulation method of SVPWM are implemented.A modulation method called dual magnitude and phase shift of modulation waves is studied to suppress common mode voltage.Secondly,the causes of the unbalance of neutral point voltage are described in detail,the midpoint potential fluctuation control model is established,two kinds of neutral point balance control method based on injecting zero sequence voltage(zero sequence voltage analytical calculation and two phase modulation)are introduced.On this basis,the voltage differential predictive control is studied.Secondly,the system loss and several discontinuous pulse width modulation strategies are analyzed,and the minimum switching loss(SLRPWM)method is studied to minimize the switching loss.Finally,the optimized modulation strategy is proposed.According to the importance of the optimization target to the system performance,the priority modulation method is divided into priority.On this basis,the comprehensive optimal modulation method and the block diagram of the grid connected control are given.A simulation system based on PSIM software is built to simulate and verify the comprehensive optimal PWM method.The prototype of T-type three-level inverter,which using STM32F407ZG as core controller,is established to realize the comprehensive optimal modulation.The simulation and experimental results show the rationality and feasibility of the proposed comprehensive optimal modulation method.