Research On Stability Analysis And Impedance Optimization Control Of Back-to-Back DC Interconnection Converter In AC Microgrid

The interconnection between microgrids through back-to-back DC interconnection converter can realize fault isolation,active and reactive power decoupling,and two-way power transmission in the microgrid cluster system.Under bidirectional power flow for the interconnection system,without switching individual converter control strategies,negative impedance will sequentially exist on the DC side and AC side of the converter.Therefore,there is a significant stability difference between forward and reverse power flow,that is,the bidirectional power stability difference,which will make the system prone to oscillation and even instability.This article focuses on the above issues.The main contents are as follows:(1)The impedance analysis method is used to establish small signal impedance models of the AC and DC sides of the converters in back-to-back DC interconnection converter.According to the impedance Bode diagram,the difference of terminal impedance in each port under bidirectional power flow is analyzed.(2)Through applying impedance-based stability assessment theory,the system stability of the AC side and DC side under bidirectional power flow is analyzed separately.According to the minimum loop ratio and impedance expressions,the Nyquist curves at different power levels in forward power direction and reverse power direction are drawn.Combined with the Nyquist stability criterion,the bidirectional power stability difference of the AC side and the DC side are analyzed.(3)Aiming at solving the problem of the bidirectional power stability difference on the AC side,an impedance optimization control method is proposed to reshape the reverse negative impedance into a positive impedance,which mitigates the impedance characteristics difference between different power transmission directions and improves the system stability on the AC side.(4)For the problem of the bidirectional power stability difference on both AC and DC sides,a coordinative impedance reshaped control strategy is proposed.The reverse negative impedance on the AC side and the positive negative impedance on the DC side are reshaped to positive impedance,which solves the impedance characteristics difference between different power transmission directions and improves the system stability on both AC and DC side.(5)Based on the above theoretical analysis,a Matlab/Simulink simulation model and a low-power prototype experimental platform were built.Simulations and experiments verified the correctness of the theoretical analysis and the effectiveness of the proposed control strategy.