The research on micro-grid mode conversion

Generation and associated loads were viewed as a subsystem or a "micro-grid"; micro-grid can improve the utilization of renewable energy and important load supply reliability. More and more countries began to strengthen the study of micro-grid, power and energy balance, power quality, protection and control are the important research objects of micro-grid. This article will focus on the micro-grid mode conversion control. The Micro-grid has two typical modes - the grid-connected mode and the islanding mode. The micro-grid can operate in parallel to the grid (the grid-connected mode) or can operate in island (the islanding mode), providing uninterruptible power-supply services. The system can disconnect from the utility during large events (i.e., faults, voltage collapses), but may also intentionally disconnect when the quality of power from the grid falls below certain standards. The micro-grid modes conversion including: 1. The planned conversion from the grid-connected mode to the islanding mode 2. The unplanned conversion from the grid-connected mode to the islanding mode 3. The planned conversion from the islanding mode to the grid-connected mode 4. The unplanned conversion from islanding mode to grid-connected mode The fourth conversion can be avoided, but others cannot be avoided. So micro-grid mode conversion control system is needed for micro-grid reliable operation when micro-grid changed the work mode. For the micro-grid mode conversion ,a centralized model controller is set in the micro-grid and the control logic of main-power of micro-grid is optimizated. The unplanned mode conversion is solved by the logic optimization of the main-power, and the planned mode conversion is solved by mode controller and the logic optimization of the main-power. In this paper, a detailed program of the mode controller and an optimization scheme of the main power converter control system will be presented. The battery energy storage system as an example of main power is de- cribed. The control of the micro-grid is "master and slavery" control. The main power works in PQ mode when micro-grid works in grid-connected mode, and the main power works in V-F mode when micro-grid works in islanding mode. The micro-grid work mode is detected through the micro-grid information such as current, voltage and binary. The main power will change the work mode when micro-grid changed its work mode. In the mode conversion process, a series of programs will be used to ensure micro-grid stability. The planned mode conversion is solved by mode controller. The function of mode controller including: 1. The power and energy control of the micro-grid when micro-grid from grid-connected mode to islanding mode The voltage synchronous control of the micro-grid when micro-grid from islanding mode to grid-connected mode Finally, a typical micro-grid model is created and simulated. The simulation results show the correctness and validity of the mode control scheme. The model is created and simulated by PSCAD.