Using Sequence Decomposition Control To Parallel High Frequency Link Matrix Three-phase Four-leg Inverter With Unbalanced Load

In order to provide more technical support in the new period of energy industry transformation,the inverter system as an important interface circuit of the distributed micro-grid system,not only meets the requirements of small-lightweight and redundancy for clean and renewable energy generation,but also ensures the quality of power supply under unbalanced conditions.The high-frequency link matrix inverter insulate source and load through high-frequency magnetic coupling,which reduces the volume and weight of the equipment while ensuring the safety of power supply,also makes the installation and debugging simpler.Since an extra bridge arm appear in three-phase four-leg inverter,one degree of freedom added in its control corresponding,which is convenient for controlling the zero-sequence current under unbalanced conditions,this can enable three-phase output voltage balance,especially suitable for the control under island mode.Therefore,this paper focuses on the parallel high-frequency link matrix three-phase four-leg inverter system,mainly studies methods of the control under unbalanced conditions and the power matching strategy between inverters.Firstly,the control of a single high-frequency link matrix three-phase four-leg inverter is studied.In order to weaken the common mode interference problem in the kind of system with neutral line such as three-phase four-leg arm inverter,a decoupling integrated modulation method suitable for the topology structure is adopted to reduce the common mode voltage;In order to improve the output voltage imbalance under unbalanced conditions,the independent control of the fourth bridge arm is realized by analyzing the flow path of the zero sequence component,and the influence of the zero sequence output impedance on the system is analyzed.Secondly,in order to realize the matching of the load power between the inverters of the parallel system,based on the comprehensive analysis of the sequence control method in different coordinate systems,a virtual impedance matching drooping scheme without coupling under two-phase stationary coordinate is proposed for the first three bridge arms,while a separate virtual resistor control scheme used by the fourth bridge.At the sametime,the traditional PI closed-loop controller is improved.The improved PI control removes the coupling between the axes and facilitates the design of the closed-loop controller.The proposed control scheme optimizes the design structure of virtual impedance,can independently design the virtual impedance according to requirements of different sequence components,that improves the output voltage quality,and can realize the accurate distribution of the capacity between the inverters of the parallel system.Finally,the effectiveness of the proposed control strategy is verified by establishing a simulation model,then the design of hardware circuit and software program proceed immediately.The DSP and CPLD joint controller is used to control the parallel high-frequency link matrix three-phase four-leg inverter experimental platform,which verifies the feasibility of the proposed strategy.