Research On Self-Synchronous Inverters Based On Hybrid Droop Control In Microgrid

In recent years, the grid-connected technology of MG inverter has become a hot research problem. At present, MG inverter control modes are mainly single mode control method and dual mode control method. Single mode control method can realize seamless switching when MG is grid-connected or unplanned off-grid, avoiding the risk of the mode switch failure that double model control method may lead to. Traditional droop control can realize single mode control, but there exist deviations between the output voltage amplitude, frequency and rating value when MG works in island mode, in the meantime, grid-connected operating is not allowed. Phase locked loop technique is used to achieve quasi synchronous control. For the shortcomings of the traditional droop control, a hybrid droop self- synchronous control method is proposed in this thesis based on traditional droop control, and the system frequency is constant. Furthermore, grid-connected operating is available without synchronous control process by PLL, "plug and play" can be achieved perfectly.Firstly, this thesis introduces the implement methods for droop control under different coordinate system, compares the characteristics of three closed-loop droop control and single-loop droop control, analyze the quasi-synchronization control strategy for grid inverter.Secondly, the self-synchronous control method proposed in this thesis is analyzed, and then small-signal models of grid connection and island are established by small signal analysis method, the root locus diagram is drawn according to the system state space equations. In order to determine the main control parameters affecting the system and provide theoretical basis for selecting system control parameters, the root locus of the system are needed to be analyzed. Back-stepping control method is used to design the nonlinear supplementary voltage controller, which can ensure the smooth transition of the system during grid connection and islanding and improve the system anti-interference ability. There need to choose main circuit and the controller parameters for the simulation system, and build simulation model using standalone, parallel island, grid connection, unscheduled off-grid mode and the load mutation. The simulation results show that the self-synchronous control method is reasonable and feasible.Finally, in order to further to prove the validity of the self-synchronous control method in this thesis, an experimental platform is built which takes DSP F2812 as the control core, both the hardware and software for the system are designed, and software program for the control system is written in C. The self-synchronous control method can get good control effect according to actual data and experimental waveforms from the system.