Stability Analysis And Control Of Islanded Microgrid Based On Impedance Model

In a microgrid,the inverters with different control strategies will show different characteristics compared with traditional power system,such as low inertia,low damping,strong time-varying nonlinearity and so on.These characteristics not only lead to the instability of the system,but also lead to the necessary changes of the current stability criterions and design methods.In this paper,focusing on the stability of the Islanded Microgrid with Paralleled Inverters Controlled by Droop/PQ Strategies(IMG-PICDPS),the main works and innovation of this paper are as follows:1.A hybrid impedance model for IMG-PICDPS is derived,in which the inverters with PQ controllers(PQ-Inv)considered as Norton's equivalent circuits,while the inverters with DRoop controllers(DR-Inv)as Thevenin's equivalent circuits.2.Because the derived impedance model of the IMG-PICDPS can be equivalent to a double close-loop system,an impedance-based stability analysis method is proposed for it according to the Nyquist criterion for multi-loop system.The impact of line impedance and the penetration of PQ-Inv on the system stability are analyzed in terms of the proposed stable analysis method.The results show that: 1)The stability of IMG-PICDPS is more sensitive to the line impedance than that where PQ-Invs are connected to a weak grid alone.2)The increase of penetration of PQ-Inv will decrease the stability of IMG-PICDPS.3.In order to improve the stability of the IMG-PICDPS,two different virtual impedance control methods are proposed for DR-Inv and PQ-Inv respectively based on the comprehensive analysis of the different virtual impedance control methods on the characteristics of DR-Inv and PQ-Inv.For DR-Inv,the double virtual impedance control scheme,which contains an inner virtual inductance and an outside virtual inductance,can guarantee that the output impedance of DR-Inv is both inductive in the frequency near 50 Hz and significantly resistive in the resonant frequency.This scheme can facilitate the realization of P/f and Q/V droop control scheme,as well as enhance the damping of the system in the resonant frequency.For PQ-Inv,the inner resistance control scheme realizes the goal of increasing the damping of the PQ inverter in the resonance frequency.4.In order to verify the correctness of the theoretical analysis and the effectiveness of the control method,an IMG-PICDPS simulation model is built by the Matlab/Simulink environment.The simulation results show that: 1)the proposed impedance-based stability analysis method can effectively estimate the stability of the system;2)the resonance of the IMG-PICDPS can be significantly damped by the virtual impedance control scheme and the system stability is improved.