Impedance modelling and stability improvement for high frequency isolated power conversion system based on dual active bridge

Abstract

Two-stage high frequency isolated power conversion system integrated with dual active bridges is a preferable choice to serve as a bidirectional interface between the battery packs and utility in battery energy storage applications. When connecting into weak grid, small signal instability caused by the interaction between power electronics converter and non-ideal utility is of great concern for battery energy storage system. In order to carry out stability analysis, the impedance model based method is widely adopted. However, conventional impedance model is assumed as an approximation by omitting the dynamic feature of DC-link and front-stage converter. This is no longer appropriate for two-stage high frequency isolated power conversion system. Therefore, this paper establishes a more complete impedance model for high frequency isolated power conversion system. The impact of front stage dual active bridges converter and DC-link is carefully examined and taken into account. Generalized Nyquist Criterion is employed to determine the stability of the cascaded system. Moreover, by involving Hankel method, this paper presents a grid oscillation frequency detection algorithm and suppression approach based on proportional-resonant controllers integrated with on-line parameter adjustment function. Finally, the theoretical analysis is verified based on a down-scaled three-phase high frequency isolated power conversion system prototype rated at 500 W. The experimental results confirm the correctness of the impedance model and feasibility of stability enhancement algorithm presented in this paper.