Analysis of low-frequency oscillation mechanism and optimal control of impedance on the DC side of a two-stage power conversion system

The two-stage power conversion system (PCS), a cascaded system consisting of bi-directional DC/DC and DC/AC modules, is extensively employed in battery storage systems. However, the cascading of modules may result in a less stable system which is vulnerable to disturbances, especially power fluctuations. In fact, the level of the power or the reversal of the power flow can adversely affect the stability of the cascaded system, resulting in a low-frequency oscillation of the bus voltage. Accordingly, to solve the problem of stability degradation after modules cascading, this paper proposes an active damping control strategy with coordinated control of the capacitor voltage. This strategy can reduce the resonant peak of the output impedance of the source converter and increase the phase of the input impedance of the load converter at the same time, so as to avoid the intersection of impedances, effectively enhance the stable operating range of the cascaded system. Finally, simulations and experiments have been carried out for the verification of the analysis and the effectiveness of the proposed control method.