Research On Parallel Control Strategy Of Grid-Connected Converters Based On Virtual Impedance

As grid-connected application method of clean renewable energy, microgrid provides newideas for regional power supply and new energy consumption, which has been studied widelyand deeply. As a grid-connected converter for the clean energy grid-connected interface, droopcontrolis usually used to provide voltage support for isolated island microgrid, which can achieveno communication synchronization between parallel converters, distribute the load according tothe capacity ratio, and "plug and play" power supply. However, due to the weak gridcharacteristics of microgrid, the large impedance ratio of the converter grid-connected line willcause the converter output power to be uncoupled, which affects the accuracy of droop control.In addition, the uncertainty of the impedance of the grid-connected line wni also cause thereactive load shared between the converters not to be proportional to the capacity of theequipment, or even a large reactive circulating current between the converters.In order to overcome issues mentioned above, the improved droop controlis proposed. Firstly,the mathematical model of the main circuit and control part of the converter is established, andthe parameters of droop control and double loop controller are designed. Based on the establishedsystem model of grid-connected converter, the factors that cause the output power cannot beapproximated decoupled, the reactive power circulation and the reactive load cannot be allocatedaccording to the capacity are analyzed. For these reasons, a virtual impedance is introduced intothe control loop of converter, and a system impedance model including the converter outputimpedance, line impedance, and virtual impedance is established. By properly designing thevirtual impedance, the system impedance frequency characteristic compensation of the converteris completed,so that the system impedance can simultaneously satisfy the requirements of power decoupling,suppression of reactive power circulation and improvement of reactive power distribution accuracy.In addition,the virtual impedance may cause the amplitude of the converter output voltage drops?Hence,the adjustment of the voltage frequency and amplitude is added in the improved droop control,so that the output voltage of the converter meets the grid requirements.The simulation model is built under Matlab/Simulink to verify the improved droop control strategy.A parallel experimental platform of three-phase voltage source converter based on TMS320F28335 controller is built,and the control strategy is verified.Both simulation and experimental results demonstrate the effectiveness of the improved droop control based on virtual impedance.