PWM Optimization Of CHB Multilevel Inverter And Its Application In Photovoltaic Grid-connected System

In recent years,with the large-scale application of various high-voltage and high-power conversion equipment in the industrial field,multilevel inverters and their corresponding control technologies have also been rapidly developed.Among them,the CHB multilevel inverter is widely used in various fields due to its advantages such as simple control,good output voltage harmonic performance,high reliability,and easy modular expansion.This article takes the CHB multi-level inverter as the research object,thoroughly studies the existing problems of the two traditional multi-carrier PWM modulation technologies and the related issues of the CHB photovoltaic grid-connected inverter.In terms of transformation technology,it has certain theoretical significance and engineering practical value.The main research contents of this article are as follows:Aiming at the problems existing when two traditional multi-carrier PWM technologies are applied to CHB inverters,this paper proposes two optimization and improvement modulation technologies.One is to optimize and improve the CSP-PWM technology.The improved CSP-PWM technology can significantly improve the DC voltage utilization of the output voltage,improve the harmonic performance at low modulation,and the output power of the cascaded units can achieve natural balance,like CPS-PWM technology.The other is to optimize and improve the IPD-PWM technology.The improved IPD-PWM modulation technology is the same as the CPS-PWM technology.The output power of the cascaded units can achieve natural balance.Under the improved IPD-PWM modulation technology,The inverter output voltage harmonic distribution is also the same as when IPD-PWM technology is used,that is,the improved modulation technology retains the characteristics of the optimal output line voltage harmonic performance of IPD-PWM technology.Aiming at the control strategy of single-phase CHB photovoltaic grid-connected inverter,this paper adopts voltage outer loop and current inner loop double closed loop control strategy.Firstly,the mathematical model of the CHB photovoltaic grid-connected inverter is deduced,and the system control target is determined.Then a mathematical model of the system’s double closed-loop control strategy is established.For several grid-connected current controllers,a comparative analysis is made,and the design formula of the voltage outer loop and the parameter selection scheme of the current inner loop are given.Finally,a CHB photovoltaic grid-connected inverter simulation model was established,and the effect of the voltage and current double closed-loop control strategy was analyzed and verified according to the control goals set by the system.The operating conditions of the photovoltaic modules in the cascaded units were inconsistent A simulation analysis was carried out under the circumstances,and the importance of the power balance control strategy was illustrated.Aiming at the problem that the DC-side voltage is unstable due to the imbalanced transmission power between the units of the CHB photovoltaic grid-connected inverter in a complex external environment,this paper mainly studies a power balance control strategy based on power weight control.First,a mathematical model of a photovoltaic module is established,and its output characteristics are simulated and analyzed in different environments.Then based on the double closed-loop control strategy of the CHB photovoltaic grid-connected inverter,the importance of adding power balance was explained in combination with mathematical analysis,and the control mechanism of the power balance control strategy was derived in detail.The proposed power balance control strategy can well balance the input and output power of each unit in a complex external environment,so that the DC side voltage of each unit is stabilized at the voltage of the corresponding maximum power point of the photovoltaic module,which improves the system’s Power generation efficiency.