The Research Of Circulating Current Suppression And Power Sharing Among Multiple Virtual Synchronous Generators Operation In Parallel

Distributed power generation technology is benefit to large-scale application of new energy and renewable energy which can be a good solution to current energy crisis and environmental pollution. Distributed power is generally controlled by inverters which are made up by power electronic components. It has the characteristics of fast response and it is difficult for the conventional inverter control strategy to achieve inertia and damping characteristics which large power grid has. Thus, accessing to a large number of distributed power grid greatly reduces the power system stability. Virtual synchronous generator(VSG) control algorithm uses inverter to simulate synchronous generator(SG). Virtual synchronous generator not only provides frequency and voltage support for power system, but also provides the inertia and damping characteristics so as to greatly increase the grid stability. Usually distributed power has a relatively small capacity, while power system has a large capacity requirements, thus distributed power systems operating in parallel can be used to expand capacity. In this paper, circulating current suppression and power sharing among multiple virtual synchronous generators operation in parallel is studied, multiple virtual synchronous generators in island micro grid is studied as well.Firstly, for single inverter control, the conventional inverter control algorithms are summarized, VSG algorithm is chosen as the inverter control algorithm. By analyzing the mathematical model and control principles of VSG thoroughly, we designed excitation unit controller and frenquency unit controller correspondingly. A single VSG simulation model is built with MATLAB/Simulink and influences on the dynamic responses of the system caused by frequency and voltage regulating properties and inertia are verified.Secondly, when multiple virtual synchronous generators are operated in parallel, we studied the issues of circulating current suppression and power sharing. The cause of the circulating current in parallel system is analyzed minutely. A voltage loop control strategy based on VSG virtual impedance is proposed, which can use virtual impedance in the VSG algorithm to simulate a large hardware inductance.This strategy not only can suppress circulating current, but also each inverters in parallel system can be controlled as the SG which have the characteristic of large inductive output impedance.A parallel system simulation model with two the same capacity VSGs is built with MATLAB/Simulink, and the purposed strategy is verified.The characteristics of transmission power of each VSG in parallel system is analyzed minutely. A power sharing control strategy among multiple virtual synchronous generators operation in parallel is proposed, which can suppress circulating current and accomplish accurate power sharing. The inertia in VSG algorithm can slow down frequency and suppress power fluctuations when load changes. A parallel system simulation model with two the different capacity VSGs is built with MATLAB/Simulink, and the purposed strategy is verified.Finally, the working condition of the proposed power sharing control strategy is expand into island micro-grid mode, and the purposed power sharing control strategy is verified that it can work in island micro-grid mode. A micro-grid system example from relevant paper is selected under full consideration of the characteristics of China’s power system situation. Using given micro-grid structure and circuit parameters, the island micro-grid mode simulation model with multiple VSGs is built with MATLAB/Simulink, and it is verified that the purposed power sharing strategy can work in micro-grid mode.