Small Signal Stability Analysis Of Multi-bus Microgrid Based On Impedance Model

Stability analysis is an important task for the safe and reliable operation of microgrids.The existing impedance-based stability analysis method usually divides the microgrid into two subsystems.The impedance ratio between the two subsystems is used to judge the stability of the system.However,with the application and development of microgrid technology,the scale of its system is constantly expanding and the topology is becoming more and more diverse,which makes the partition of the system and the overall calculation amount become very complicated.In this paper,the small signal stability problem of multi-bus microgrid based on impedance is studied.The details are as follows:1.According to the difference of the output characteristics of the voltage-controlled inverter and the current-controlled inverter,the corresponding small-signal impedance model is established respectively,and the impedance stability criterion of a single-bus microgrid with two control types of inverters connected in parallel with the same bus structure is obtained.The equivalent subsystem impedance of the single bus subsystem is derived.2.For the microgrid with multi-bus chain structure,a stability analysis method based on single bus subsystem is proposed.It solves the problem that when the traditional impedance analysis method is applied to the multi-bus microgrid,it is necessary to investigate the pole distribution of the impedance ratio.This simplifies the analysis process of the system.Moreover,with the further expansion of the size of the chain structure microgrid system,it is only necessary to apply the proposed method to further analyze the stability on the original basis,without cumbersome subsystem division and complicated calculation.Taking the minimum chain system unit of two busbars as an example,the influence of controller parameters of different control types on the stability of the system is analyzed.3.For the microgrid with multi-bus meshed structure,it is first divided into a reducible structure and a non-reducible structure.(1)For the reducible structure,the whole system is equivalent to a multi-input multiple-output(MIMO)negative feedback system with a forward channel gain as a unit matrix.Therefore,the stability of the microgrid system can be judged by the distribution of the characteristic equation roots of the negative feedback system.Compared with the traditional impedance analysis method,this method reveals the interaction between the various components in the microgrid more intuitively,which has clear theoretical significance.(2)For the non-reducible structure,the closed-loop transfer function matrix of the system is derived.According to the complex frequency domain theory of the linear system,the stability of the whole microgrid system is judged by examining the zero-pole distribution of its Smith standard form.This closed-loop transfer function matrix is determined only by the number of busbars in the multi-bus meshed microgrid system and the line impedance between the busbars,the inverter impedance and the load impedance,which can be applied to the stability analysis of multi-bus microgrids with complex topologies.4.The corresponding multi-bus microgrid system model is built in Matlab/Simulink environment.The simulation results show that:(1)the stability analysis method described in this paper can effectively judge the stability of the system;(2)the controller parameters of the two control types of inverters have different effects on the stability of the microgrid with chain structure.