Research Of Frequency Control In Microgrid Based On Virtual Synchronous Generator

In order to solve the energy problems which have become increasingly severe,new energy generation technology based on renewable energy such as photovoltaics and wind power have attracted more and more attention due to their green and low-carbon characteristics.Microgrid is one of the research hotspot of current research as the main consumption method of distributed renewable energy sources.As a small autonomous system,the ability of island operation of microgrid is very important.However,the interfaces of distributed energy are mostly inverter type,which are lack of necessary inertia and damping so that the frequency stability of the microgrid is very poor.Virtual synchronous generator(VSG)control method is proposed to increase the inertia and damping of distributed energy by simulating the characteristics of synchronous generator,which can support the frequency of the islanded microgrid.In this paper,the frequency problem when multiple VSGs operate in parallel is studied under the framework of the island microgrid.The main research contents are as follows:(1)The basic principle of virtual synchronous generator including virtual excitation system and virtual speed regulation system is studied.The operation control mode of virtual synchronous generator is given,which includes virtual synchronizer outer control loop and voltage-current inner control loop.A small-signal model of multiple VSGs operating in parallel is established.The key parameters of virtual synchronous generator are analyzed to assess system stability and control performance.(2)The frequency recovery control strategy which is suitable for virtual synchronous generator is studied.A series of problems such as integral error accumulation are pointed out only by using the integral module to achieve frequency recovery.The frequency coordinated control with delayed switching characteristics is proposed in this paper.The primary frequency control and secondary frequency control are decoupled by designing delay switch function.In addition,the control strategy does not depend on communication,which can both ensure accurate power sharing and frequency recovery.(3)The frequency characteristic of multiple VSGs operating in parallel is studied,and the active frequency oscillation mechanism in parallel operation is analyzed.A control strategy based on mutual damping under distributed communication(MDDC)is proposed,with which the power oscillation is restrained effectively and the dynamic characteristics of VSG is improved.Furthermore,the stability of the improved system is analyzed and proved by Lyapunov function.The results show that the mutual damping reduces the transient energy and enhances the stability of the system.(4)The real-time simulation technology based on RT-LAB is studied.A real-time simulation platform with hardware controller to realize the proposed control strategy above is established.Besides,a information interaction platform for VSGs in microgrid is established,which can realize distributed communication between virtual synchronous generator.Moreover,the platform has the ability to simulate the communication delay and communication fault in microgrid system.Finally,the proposed strategies of this paper is verified by the real-time simulation platform.