Characteristic Analysis Of Three-phase Four-leg Inverter Based Load Unbalance Compensator For Microgrid

Due to the increasing demands for electricity, distributed power generation is adopted near the load to become both economical and environmentally friendly. When the microgrid system has unbalanced loads, the voltage unbalance and harmonic distortion rate of the system increase,and the power quality of the system is seriously affected. Therefore, suppressing and reducing the unbalanced components and harmonics in the microgrid and improving the power quality of the common connection points are of great importance to ensure the stable, reliable and high quality operation of the microgrid system. The main contents of this paper are as follows:Firstly, the simulation model of PV system, diesel engine power generation system and battery energy storage system are established. Three control modes that contain master-slave control, peer-to-peer control and hierarchical control, are analyzed. At the same time,the distributed energy control methods, including the PQ control, the V/F control and the droop control, are introduced in detail.Then, based on three-phase four leg inverter, a series of control methods for unbalanced load compensation of microgrid are investigated. The reason of voltage unbalance is analyzed,and it is pointed out that the voltage quality of common connection point can be improved if the distributed generation converter can be controlled to absorb or inject some unbalanced current.The mathematical model of the inverter is analyzed at the same time. Based on it, a unbalanced load controller is designed according to the unbalanced load of stand-alone micro grid, and the controller adopts double closed loop control strategy to compensate the unbalanced current. The dual loop control algorithm consists of three parts: the DC bus voltage controller, the current controller and the PWM modulation strategy. Meanwhile, according to the PQ control strategy of diesel engine, a control strategy of constant frequency and constant pressure control of diesel generator system and auxiliary V/F control of energy storage system is proposed in the system.Finally, the simulation model of micro-grid with unbalanced load is established on the PSCAD/EMTDC. The compensation effect of load unbalance compensator and the dynamic response characteristics of the micro-grid are analyzed. The validity of the proposed load unbalance compensator and the feasibility of the integrated control strategy are verified.