Research On Control Technology Of Photovoltaic Grid-Connected Inverter Based On Virtual Synchronous Generator

In recent years, distributed power in the power system in the increasing penetration rate, but also to the power system security and stability has brought great challenges.Because distributed power supplies based on power electronic inverter interfaces do not have damping components and rotational inertia that are conducive to maintaining system stability, these factors are inherent in synchronous power generators in conventional power systems. As a method to make the distributed inverter power supply have similar characteristics with the synchronous generator, the virtual synchronous generator control technology plays an important role in improving the stability of the whole power system.Based on the analysis of distributed power system modeling and control strategy, this paper puts forward the control strategy of virtual synchronous generator (VSG) . Main research contents:(1) The working principle,mathematical model and equivalent circuit of PV system are studied, and the maximum power control (MPPT) is studied. Finally, the simulation model is built in Matlab/Simulink, and the output characteristics of PV cells and MPPT are analyzed and verified.(2) Focus on the distributed inverter power supply control methods were studied and analyzed, including the inner ring control and external control two. For the inner ring control, mainly introduced dqO rotation coordinate system control, ??0 still coordinate system control and abc natural coordinate system control three. While the outer ring control is focused on the PQ control, V/f control and droop control three. In this chapter, the principle and control block diagram of different control methods are given. At the same time,the different control methods are compared and analyzed, which lays the foundation for the control strategy of the following virtual synchronous generator.(3) Mathematical modeling of virtual synchronous generators. The mathematical model of the synchronous motor and the three-phase four-leg inverter is analyzed. The virtual synchronous generator is modeled by the rotor motion equation, the flux linkage equation and the electromagnetic transient relationship of the synchronous motor. The active-frequency control of the inverter And the reactive power - voltage controller are designed and optimized to set the three - phase four - leg inverter as a virtual synchronous generator.(4) Research on Synchronous Grid Connection Control of Virtual Synchronous Generator. Two different VSG synchronous grid schemes (the difference is whether to add the phase locked loop) is designed by using the principle of grid connection control of synchronous generator,and the advantages and disadvantages of the two grid schemes are analyzed. Phase network control scheme to achieve the VSG smooth and grid.(5) virtual synchronous generator operating characteristics analysis. This paper studies the off - grid operation,networking operation and grid - running characteristics of virtual synchronous machine, and designs the control strategy to adapt to different operating characteristics to realize the optimal control of different operation modes.(6) In order to verify the correctness of the control scheme designed in this paper, a simulation model of off-grid operation, grid-running and synchronous grid-connected PV system based on virtual synchronous machine is built in Matlab / Simulink.