| With the rapid development of the economy and the increasingly serious energy problems,renewable energy,electric vehicles and energy storage systems are widely used,and the power grid is changing from centralized to distributed.Facing the gradual increase of renewable energy penetration,grid connected technology of distributed energy has been widely researched.However,power electronic converters which generally applied in micro-grid lack the inertia and damping that exist in traditional synchronous generator,the power system is more vulnerable to the power fluctuation and system failure.In order to solve this problem,Virtual Synchronous Generator(VSG)technology has been proposed.The traditional VSG has the problems of slow grid-connection speed,active power/reactive power coupling effect,so it is of great significance to study the efficient grid-connection and power decoupling strategy for the stable operation and popularization of VSG.Focusing on the two technical issues of smooth grid-connection and power decoupling,the main research contents of this paper include the following parts:In the first part,based on the structure and mechanism of ideal synchronous generator,the second-order mathematical model of synchronous generator is extended to the inverter control,and a grid connected inverter system with VSG strategy is introduced.Besides,the VSG is modeled and a control parameter design method is analyzed in detail.The characteristics of VSG are studied under islanding/grid-connected,multi-machines operation modes,the feasibility and superiority of VSG control strategy are verified;In the second part,based on the traditional VSG model,a self-synchronized VSG structure is proposed,which can make use of the synchronous characteristics of VSG and avoid the errors caused by the PLL.Based on the self-synchronized VSG model,a pre-synchronized VSG control strategy using μPMU is proposed,which can ensure the high synchronization of signals collected from different geographical positions and achieve fast and smooth grid-connection;In the third part,the mechanism of power coupling in medium-low voltage power networks is analyzed,a novel second-order VSG power decoupling strategy based on dynamic virtual current is designed.This strategy bases on the idea of dynamic current compensation for dynamic coupling power,which can make up for the lack of traditional virtual impedance power decoupling strategy that can only be applied to small power angle condition.When VSG is operating in a large power angle condition,the novel strategy can ensure a good decoupling of active power and reactive power,especially in droop control,it can also raise the accuracy of power steady-state regulation obviously. |
