Research On Grid-connected Seamless Transfer Control Strategy Of Inverter In Micro-grid

Exploitation and utilization of renewable energy are basic requirements of implementing scientific development, constructing resource saving society, and realizing sustainable development, which also are essential measures to protect environment and cope with climate change. Distributed power supply has become the dominant form of new generation because of its special characteristics. Micro-grid is a new technology set up on basis of distributed energy generation, and greatly improves the efficiency of distributed energy supply.Micro-grid operates in grid-connected mode or in isolated mode. Therefore,inverters in micro-grid need to work in the same mode cooperating with medium and low voltage distribution, or supplying electric power to local loads. Loss of synchronization of distributed energy to the grid, and discrepancy of control methods, will both lead to over voltage or over current at the moment when inverter connects to the grid, endangering stability of micro-grid. Nowadays, there is a shortage of deeply research of dual-mode inverter and its seamless transfer control method, restricting further development and application of micro-grid.This paper proposes grid-connected inverters in micro-grid and their seamless control methods, details are as follows:(1) By introduction of inverters in micro-grid, one topology of three-phase inverter and its equivalent circuit diagram are presented. Several basic control structures are described and analyzed, and because of strong interference immunity, digital phase-locked loop(PLL) and its typical algorithms are elaborated.(2) Aiming at the inrush current when an inverter connects to the micro-grid, this paper puts forward a dual-mode control and seamless transfer control method of the three-phase inverter. The control scheme combines steady-state control and transfer control. The transfer control consists of soft-start virtual impedance and single loop current control. Soft-start virtual impedance is used for reducing current impact and enhancing stability of the inverter, specific modality is given and parameters are calculated. To refrain from reverse energy flow, phase advance regulation of droop control with its frequency scheme is presented in process of grid-connecting and grid-steady control. The method can distinctly restrain the impact of the inverter output current and the grid-injection current when inverter connects to the grid, realizing seamless transfer.(3) With introduction of virtual synchronous generator model, control strategy is given which makes up of power frequency control, excitation control, and synchronous generator algorithm. To repress the impact of current when inverter connects to the grid, the measures of phase advance regulation and soft-start virtual impedance are taken in, which have been adjusted in the virtual synchronous generator by adding frequency, excitation and phase adjustment parts. The control method refrain the inrush current, achieving the goal of seamless transfer of virtual synchronous generator.(4) Design the hardware and software of the system and validate the control methods in prototype. Select components of the inverter, calculate parameters of the circuit, i.e. capacitor on DC side, inductance and capacitor of LC filter, and design peripheral circuits. DSP resource allocation and software of main program, interrupt program, and LCD display program are given. An experimental system of photovoltaic three-phase inverter is established in the laboratory. The prototype experiments verified the proposed control schemes are consistent with the simulation results. Simulation and experimental results demonstrate the feasibility and effectiveness of the proposed methods.