Research On Control Method For Parallel Three-phase Inverters

Micro-grid technology and distributed generation technology with the rapid development of renewable energy become increasingly important.The distributed generations in AC Microgrid can be connected to AC bus by three phase inverter. The control method of parallel three-phase inverters is researched in the paper. Parallel three-phase inverter control objective is sharing the output current of the inverters and power flow of load.Firstly, the mathematical model of the three-phase inverter based on the rotary coordinate system is established. On this basis, the voltage and current double closed-loop SVPWM control of the inverter is realized. Then circulating current and the main circuit of the parallel inverters are analyzed.Without current sharing control method the circulating current is researched on the condition of both open loop control and double loop control parallel inverters.Secondly, Master-slave control method of parallel inverters is researched. Master-slave control method with public voltage regulator is studied. Master inverter is controlled by voltage and current double closed-loop while slave inverters are controlled by single current loop which is following the output of the master's voltage loop.Thirdly, droop characteristic control method is studied. By analyzing the characteristics of the traditional control of the basic principles of droop characteristic control, many drawbacks are found such as slow dynamic response and steady state deviation of output voltage and frequency.So this paper proposes an improved droop characteristics control method which improves the dynamic response by adding differential term to the droop formula and eliminate the steady state deviation by adding a high pass filter.This paper also researches on the converters'switching control method of grid-connected mode and standalone mode. The micro grid converter is controlled as a current source on grid-connnected mode while it is controlled as a voltage source on standalone mode.At last these theoretical analysis and control methods in this paper are verified by simulation based on Simulink.