Research Of Grid Connection Technology Based On Virtual Synchronous Generator

In recent years, with the development and application of new energy technologies such as wind energy, solar energy and other new energy technologies, micro grid has been paid more and more attention. Micro grid, which has distributed structure, can improve the reliability of the power grid; it also contains a variety of DC power sources. The DC sources provide energe for the load after being transferred to AC power by DC-AC inverters.Thus, active and reactive power allocation of each inverter becomes the key technology of micro grid technology. We call the inverters that mimic synchronous generator(SG) synchronverters.The real and reactive power delivered by synchronverters connected in parallel and operated as generators can be automatically shared using the well-known frequency-and voltage-drooping mechanisms.Based on this,the virtual synchronous generator technology is gradually drawing people’s attention and research.In-depth research on virtual synchronous generator (VSG) grid technology is carried out in this paper, which based on "Virtual Synchronous Ggenerator Prototype Research" project by Chinese Electric Power Research Institute.First of all, we finished the mathematical modeling of the VSG and analyzing the mathematical models of synchronous motors and the three-phase four-leg inverter.By calculating the rotor motion equation and the flux linkage equation of SG, the Electromagnetic transient relationship has been explained.The active-frequency controller and the reactive-voltage controller were designed and optimized.Then, a preliminary design of the prototype is carried out according to the engineering practice, including its main circuit, inverter modulation strategy and the control algorithm of grid-connecting. The main circuit contains three parts:the energy storage battery, the bidirectional buck-boost converter and the three-phase four-leg inverter which is modulated by 3D-SVPWM strategy.Then, Simulink modeling and simulation analysis are carried out on the research objects. Through the simulation analysis, virtual synchronous machine with virtual inertia, variable frequency and variable voltage characteristics have been proved.The transient process is slow, and it can reflect the synchronous generator rotor storage corresponds to the characteristics and track the load changes, sharing load independently. In addition, due to the self synchronization ability of virtual synchronous machine, the steady state frequency tracking is accurate.Finally, a prototype of lOkW is built, and the prototype is tested and connected to the grid. Test results show that the feasibility of virtual synchronous generator and the network technology, and it has grid-friendly and voltage frequency characteristics and so on. The bidirectional flow of energy between the energy storage part and the grid is realized, and the seamless switching between the grid on and the grid off mode is also realized.The experimental and simulation results show that the virtual synchronous generator can simulate the synchronous generator to participate in the grid-connecting/island modes, which can realize the bidirectional flow of energy, and achieve the same voltage regulation and frequency regulation characteristics just like SG. And the the phase locked loop is not necessary for virtual synchronous generator.