A Combined Three-Port LLC Structure for Adaptive Power Flow Adjustment of PV Systems

This article proposes a scheme to achieve improved efficiencies for a standalone photovoltaic (PV) system. Specifically, a combined three-port LLC structure is utilized to adaptively regulate the power sharing between the PV and the energy storage battery. This is fulfilled by adjusting the equivalent resonant conditions of two phase-controlled resonant tanks through a shared resonant capacitor. Thus, the overall efficiency is improved by simultaneously reducing the power conversion stages and optimizing LLC converters with fixed-frequency operation. Because an extra power flow path is added to discharge the battery, the nonisolated converter used for battery charging does not need to be bidirectional. Also, since each resonant tank can share part of the load, the new circuit does not add any extra costs of power components. In addition to detailed theoretical analysis, experimental results verify the effectiveness of the proposed concept.