| In recent years,with the emergence of new energy industry in China,high-voltage DC transmission technology for long-distance and large-scale power grids has been widely studied.Modular Multilevel Converter(MMC)stands out from many converter devices and is widely used due to its advantages of module integration,high output power quality,flexible operation and easy expansion,and applicability to large-capacity operating conditions,etc.Due to its unique characteristics,MMC will generate internal inter-phase In this paper,MMC is the object of study.In this paper,it is proposed a controller based on Proportional Integral Resonant-Virtual Impedance(PIR-VI)strategy with the objective of reducing the harmonic components in the interphase circulating current.The main work of this paper includes.This paper introduces the basic application background of MMC and the significance of the selection of the subject,gives a brief overview of the current situation of the use of multilevel converters,and analyzes the MMC operation and loop current suppression strategy in more detail.In this paper,the structure of the MMC circuit is analyzed,and the overall structure,internal sub-module structure,operating state,and main component parameters of the circuit are explained through systematic mathematical modeling of its computational derivation.In this paper,the mechanism of circulating current formation and the main components of circulating current are fully analyzed and explained in detail by means of both power energy calculation and electrical quantity calculation for MMC under three-phase balanced conditions.On this basis,a controller based on PIR-VI composite strategy is proposed to effectively suppress the harmonic components of the circulating current in the circuit,and its scientific and rational nature is illustrated through theoretical derivation and mathematical expression.Based on the performance analysis of the controller parameters by Porter diagram,the controller parameters are further optimized by the Particle Swarm Optimization(PSO)algorithm.To ensure the smooth operation of the overall MMC system,this paper designs a dual closed-loop controller with outer-loop power control and inner-loop current control under the mathematical derivation based on the rotating coordinate system.The overall system of MMC circuit containing sampling measurement,control,and control is designed by selecting and optimizing the nearest level approximation modulation strategy,and by sequencing the equalization of voltage and other links.The PIR-VI loop controller and the overall control system proposed in this paper are verified by simulation and analysis sessions in MATLAB platform.And the feasibility and effectiveness of the controller and control system in this paper are illustrated by a lateral comparison with the effect of different methods of circulating harmonic suppression.This paper further analyzes and models the internal electrical structure of the MMC in the context of three-phase imbalance,and optimizes the filtering link designed by the Second Order Generalized Integrator(SOGI)to enable its application to a wider range of scenarios.A constant active power target is selected,and the validity of the method is verified through a MATLAB platform for operating state analysis and steady-state operation,and through cross-sectional comparison. |
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