Research On Switching Control Strategies Between Grid-connected And Islanding Operation For Distributed Power Systems

With increasing energy consumption and severe environmental problems, the development of new energy and clean energy is imperative. In this context, more and more wind energy, solar energy and other new-clean energy are connected to the grid through the formation of a distributed power system. In grid-connection operation, the distributed power systems are usually controlled as a current source to exchange power with the grid. Power outage will occur owing to external forces or one’s mistake, and distributed power systems will be transferred into the islanding operation. Due to the sudden of the unintentional islanding, higher distortion of PCC voltage will occur. Therefore, it is necessary to research the distortion and the switching control strategy between grid-connected and islanding operation for distributed power systems.Firstly, the existing switching control strategies are studied. Expressions of output current of DG and voltage of PCC are deduced at power outage by the single-phase equivalent circuit model of grid connected inverter. And expression of output voltage of DG for seamless transfer is deduced. Expression of output current of DG for voltage amplitude fluctuation of grid is also deduced. All expressions are analysed.By analysing power exchange between inverter and grid, two ideas are proposed for seamless transfer. One is changing the reference value of output current of inverter directly, the other is changing it indirectly. Then all control variables of power stage are arranged and analysed to find out possible solutions for transfer. And positions of control variables’ effect are also given. Based on three schemes of upcc, iLref-iC-iLL-ig, iC, three new switching control strategies are designed to realize seamless transfer between grid-connected and islanding operation. The three control strategies are effective solutions to voltage distortion of PCC for unintentional islanding.At last, the principle and design method of limiters of switching control are given detailedly. And the controllers of three control strategies are designed. Effects of three controls are verified by simulations.