A Control Strategy for a Standalone PV-Battery System Operating at SoC Boundaries With DC-Link Ripple Management

Renewable-based power generation, such as photovoltaic (PV) systems, are further supported by energy storage systems (ESS) to mitigate the inherent intermittency of renewable sources. Moreover, in the standalone mode of operation batteries provide the power balance of the system. However, one of the key challenges is when the battery state of charge (SoC) reaches its boundaries. This limits the battery availability, and the system must continue operating even in this condition. Additionally, in three-phase systems an oscillation at the second harmonic appears in the dc-link voltage when supplying unbalanced loads, which are common at the distribution level. In this work, a control strategy is developed to handle edge SoC conditions in the standalone mode of operation. For the upper SoC limit two transition approaches are introduced. The first is the battery disabling method. The second one, a zero battery current mode (ZBCM), allows the battery to support the dc-link voltage regulation during transients. Besides, the management of the dc-link ripple is incorporated by the use of cascaded-comb integrator (CIC) filters and feedforward terms. Regarding the lower SoC limit a load-shedding mechanism is included. Real-time simulations performed in the RT Box hardware-in-the-loop (HIL) platform from PLECS, demonstrate that the proposed methods maintain the power supply under unbalanced and dynamic load conditions. Both methods are compared in terms of overshoot and ripple content of the dc-link voltage and battery current at the second harmonic.