Output-Impedance Shaping of Bidirectional DAB DC-DC Converter Using Double-Proportional-Integral Feedback for Near-Ripple-Free DC Bus Voltage Regulation in Renewable Energy Systems

This paper investigates the design and implementation of virtual-output-impedance shaping on an inverter-loaded, fuel-cell-battery-powered dc-dc converter system for achieving near-ripple-free dc bus voltage regulation. The method is based on the insertion of a second output-voltage feedback loop as can be inferred from the Mason's gain formula. Three basic modes of virtual-output-impedance shaping (proportional, derivative, integral) are discussed and the closed-loop output-impedance characteristics due to each of them are analyzed in detail and with their Thévenin equivalent circuits derived. Despite the suitability of integral feedback for minimizing converter's output impedance, it can give rise to an unwanted resonance peak near the converter's crossover frequency, thus potentially destabilizing the system. The solution to the problem using combination of basic virtual-output-impedance shaping modes and its practical implementation are discussed. When implemented on a fuel-cell-battery-powered dual-active-bridge dc-dc converter, the second-harmonic distortion of dc bus voltage is shown to have been reduced by 85.5% compared to a conventionally PI-compensated system.