Stability Enhancement of Cascaded Power Converters Using Parallel Virtual Impedance Via Output Impedance Shaping of the Source Converter

Abstract

This work addresses the problem of negative incremental resistance behaviour that arises due to the impedance interactions between the power electronic converters that consists of an LC filter and a tightly regulated DC-DC buck converter. Failing to address this problem renders the cascaded system prone to instability. This paper introduces three different parallel virtual impedances, connected across the capacitor of the source side converter for restoring the stability and also for improving the dynamic performance of the load converter, during the external disturbances. The specific advantage of the proposed approach is that it requires the sensing of voltage across the LC filter rather than the current through the inductor in LC filter for designing the required virtual impedance. All these impedance configurations not only provide stability but also ensure a good dynamic response both across the LC filter and at the load converter's output voltage terminals. The response from the MATLAB/SIMULINK based simulations reveal the stability at steady state and the dynamic performance shows the improvement over a second order virtual impedance and an optimal method. Among the chosen impedance network configurations, the series RL network exhibits superior dynamic response to external disturbances. Analytical and simulation results of the work are included.