Research On Power Balance Control Strategy Of Single Phase Cascaded H-bridge Photovoltaic Grid Connected Inverter

The research on the topology and control technology of multilevel inverter makes it widely used in high voltage and high power system.Among them,the cascade H-bridge multilevel inverter topology is favored for its high voltage and high power conversion ability,high output voltage waveform quality and easy modularization.Pulse width modulation with specific harmonic elimination(SHEPWM)can still output high-quality waveform under the condition of low switching frequency.It is not only conducive to reduce the loss of devices,improve the service life,but also design the filter.This paper studies the power balance of cascade H-bridge when using SHEPWM and CHB multilevel inverter.The main research contents are as follows:Aiming at the problem that the output power of each unit is unbalanced when using SHEPWM in CHB multilevel inverter,this paper proposes a power balance control strategy of SHEPWM based on the equivalent splitting method of fundamental amplitude.Based on the redundancy of output phase voltage of CHB inverter,on the premise that the inverter outputs1 / 4 periodic symmetrical phase voltage waveform and does not change the switching frequency of the device,the fundamental wave amplitude equation of output phase voltage after Fourier analysis is split to control the turn-on time of each unit,so as to realize the power balance control of the inverter.The proposed method is applicable to all multilevel topologies cascaded by H-bridge,and is verified by simulation and experiment of type I and type II CHB inverter topologies.Finally,the weighted total harmonic distortion is used to measure the output voltage waveform quality.The research shows that the output voltage waveform quality is better when the power balance strategy is used under high modulation.Aiming at the control strategy of CHB photovoltaic grid connected inverter system,this paper adopts voltage and current double closed-loop control.Firstly,according to the mathematical model of single-phase CHB photovoltaic grid connected inverter,the control objective is defined.Then,the feedforward decoupling control is introduced in the case of power grid disturbance.In order to track the power grid phase quickly and accurately in the case of power grid fluctuation,the frequency locked loop based on second order generalized integral(sogi-fll)phase-locked technology is proposed.Finally,a simulation model is built to verify the effectiveness of the proposed method.Aiming at the balance control of single-phase CHB photovoltaic grid connected inverter,a power balance control strategy based on feedforward proportional carrier control is proposed to ensure the stable operation of the inverter when the environment changes.Firstly,theoretical analysis of single-phase CHB photovoltaic grid connected inverter in power imbalance state is carried out,and stable operation conditions of the system are obtained.Then,according to the power balance equation,a power balance control strategy based on feedforward proportional carrier is proposed,and the control principle of the strategy is analyzed theoretically.Finally,the simulation results show that the proposed control strategy can ensure the stable operation of the system under unbalanced state.Finally,combined with the control strategy and topology structure,the hardware circuit design with the main circuit and control circuit as the main body,and the software program with the main program and interrupt service program as the framework are completed.On the basis of the above research,a 2k W prototype with TMS320F28335 as the main control chip is made,and the grid connected experiment is carried out on the equipment,and the relevant experimental data are obtained.From the output waveform,the grid connected current and grid voltage are in the same frequency and phase,and the power factor parallel is realized,which verifies the feasibility and effectiveness of the system design.