Research On Distributed Secondary Control For Multi-VSG Based Islanded Micro-Grid

Due to the flexible and efficient distributed generation and consumption capabilities of new energy resource,the islanded microgrid technology has been widely used and play an important role in smart grid and energy internet.However,subjecting to the low inertia and weak stability characteristic of the islanded microgrid with the traditional droop control,the Virtual Synchronous Generator(VSG)control,which has the inertia support capability,attracts much attention in island microgrid control.In addition,compared with the traditional centralized control,the distributed control based on the multi-agent system(MAS)theory has the following advantages: such as easy to extend,high flexibility and so on.It makes the islanded microgrid has the ability to implement multiple complex control targets in a distributed manner.Thus,it is of great importance and significance to research the distributed control technology of islanded microgrid based on multi-VSG,and this paper focuses on the distributed secondary control strategy of islanded microgrid based on multi-VSG.In this paper,the active power and frequency dynamic coupling oscillation problem,the reactive power distribution control and its coordination with voltage recovery control,and the stability of distributed secondary control strategy under communication delay are studied,thus the stability and the power quality of islanded microgrid can be improved.The main research contents and contributions of this paper are as follows:(1)For the active power and frequency dynamic coupling oscillation of islanded microgrid based on multi-VSG,a concept of distributed VSG stabilizer and a distributed secondary control strategy including the distributed VSG stabilizer are proposed.Firstly,the small signal model of active power-frequency for multi-VSG is built,and the oscillation mechanism is analyzed.Then the distributed VSG stabilizer is introduced in the secondary active power and frequency control,and the differences of response speed between VSGs can be reduced through the active power/frequency second-order consensus control.Thus,the dynamic coupling oscillation can be effectively suppressed and the dynamic performance of islanded microgrid can be improved.Finally,based on the convergence performance analysis,the improvement of the distributed VSG stabilizer to the dynamic performance of islanded microgrid is verified.(2)For the reactive power distribution control of islanded microgrid based on multi-VSG,this paper proposes a distributed adaptive virtual impedance control(DAVIC)strategy.Firstly,with the help of the Lyapunov functions,the asymptotically stability of DAVIC strategy without considering impedance-reactive adjustment characteristics is proved.Then,by linearizing the reactive power transmission equation,the reactive power regulatation characteristics of impedance is qualitatively analyzed,and it is pointed out that there is a critical impedance angle when adjusting the virtual reactance,while it’s not true when adjusting the virtual resistance.Thus,the existing regulatation criterion of virtual impedance in reactive power distribution control are unified,and it can be used to the guide the design of DAVIC strategy.Finally,based on the virtual impedance regulatation criterion and the system decoupling requirement,the proposed DAVIC strategy based on the virtual reactance is derived,and the stability of the proposed DAVIC strategy is improved.(3)For the contradiction between reactive power distribution control and voltage recovery control of islanded microgrid based on multi-VSG,a distributed average voltage restoration control(DAVRC)strategy is proposed.Firstly,the contradiction between the reactive power distribution control and voltage recovery control is eliminated by restoring the average voltage of the VSGs in islanded microgrid,thus the coordination control is realized.Then,with the help of the input-state stability theory,the steady-state convergence conditions of different average voltage observers are analyzed,and the proposed average voltage observer is robust to the sampling error and the plug and play operation of VSG,which increases the adaptability of the DAVRC strategy.(4)For the stability of the proposed distributed secondary control strategy under communication delay,the small signal model of the islanded microgrid is extended to the situation under communication delay firstly,and the stability of islanded microgrid with the proposed distributed secondary control strategy under fixed communication delay is analyzed by eigenvalue analysis method.Then,the concept of delay predictor and the distributed secondary control strategy including the delay predictor are proposed,and the mechanism of the robustness of the distributed VSG stabilizer to the communication delay is proved by comparison analysis.Finally,the sufficient conditions for islanded microgrid to maintain stability under time-varying communication delay are given by Lyapunov-Krasovskii stability theory and Linear Matrix Inequality.