Research On Virtual Synchronous Generator Based Control Strategy Of Inverters And System Stability For Microgrid

In order to make full use of the distributed generation (DG) and to avoid its negative effects, microgrid was proposed. Microgrid which contains energy storage element, loads, DG and control devices has two operation modes:one is grid-connected operation mode in which the power electricity can flow between the grid and microgrid, the other one is islanded operation mode in which microgrid can provide high quality and uninterrupted power supply for loads. The purpose of this thesis is to introduce the virtual synchronous generator (VSG) into the microgrid so that it can work like synchronous generator while in the grid-connected operation mode. In this paper the microgrid inverter is controlled as VSG and based on the principle of VSG, the access to the grid, the control of voltage and frequency, the distribution of load power while microgird is under the grid-connected operating mode are presented in detail and validated in the simulation. The specific contents are as follows:(1) The concept relevant to microgrid, present research situation and advancement of microgrid technology are presented in detail, the control structure and strategies of microgrid, the concept of VSG are given.(2) The mathematical model of the typical microgird inverter is established, based on this, the stator and rotor equation of VSG are derived, then the consistency analysis between SG and VSG is made.(3) A controller model of the VSG is given, then the key parameters and the microgrid grid-connected/islanding problem are analyzed.(4) Based on the principle of VSG, the primary frequency modulation, secondary frequency modulation and the primary voltage modulation and secondary voltage modulation are designed under the hierarchical control structure; the load distribution issue is analyzed while the microgird is under islanded operation mode.(5) The simulation model of a single VSG with load and two VSG working under grid-connected operation mode are established separately in Matlab. The effectiveness of the stable control of grid voltage and frequency using the VSG strategy is tested through the simulation example.