Research On Control Strategy Based On Parallel Modular Multilevel Inverter

With the continuous increase of wind power unit capacity,the composition of high-voltage and high-power wind turbines has become a development trend.Inverters are an important part of ensuring the stability of wind turbines.Parallel modular multi-levels can not only achieve high-voltage and high-power requirements but also meet high-current operation,but there are also some problems due to its unique structure.This paper focuses on the control strategy of parallel modular multi-level inverters,and mainly studies the suppression of internal circulating current and the suppression of parallel modular multi-level external zero-sequence circulating current in the control strategy.The specific research content is as follows:1.First,analyze the significance of the development of wind power and the necessary trends in the development of large-capacity wind turbines,and put forward the importance and research significance of modular multi-level inverters in wind power grid integration.,The establishment of the mathematical model and the analysis of the working principle of the sub-modules,the block diagram of the general control strategy of the parallel modular multi-level is proposed,and the selection of the modulation strategy and the sub-control neutron-module capacitor voltage balance control strategy are analyzed.2.Next,the design scheme of the internal circulating current suppressor of the modular multi-level inverter is given.First,analyze the generation mechanism and main components of internal circulation,and construct a closed-loop model of internal circulation suppression.The traditional PI controller can only suppress the second harmonic of the circulating current harmonics,and proposes an improved quasi-proportional resonance controller to suppress the circulating current.The control method is changed to simultaneously suppress the second and second harmonics of the circulating current harmonics.Fourth harmonic,and carry out simulation comparison analysis.3.Then,for the analysis and control of the parallel modular multi-level external circulation,firstly,the mechanism of the external circulation is given,and the main component of the external circulation is the zero sequence circulation,and then the double second-order generalized phase-locked loop is designed.Technology,and use a quasi-proportional resonance controller to suppress the external zero sequence circulating current,and establish a simulation model for verification.4.Finally,the main control strategy of parallel modular multilevel inverters is analyzed,and a parallel modular multilevel simulation model is built on the Simulink platform.And based on RT-LAB and real controller,a CHIL(controller hardware-in-the-loop)semi-physical simulation platform with a total inverter power of 20 k W is built.Finally,the effectiveness of the control strategy proposed in this paper is verified through experiments.