Efficient Improvement of Photovoltaic-Battery Systems in Standalone DC Microgrids Using a Local Hierarchical Control for the Battery System

The conventional control methods for the battery systems of photovoltaic (PV) battery systems in standalone dc microgrids are designed to stringently regulate the bus voltages at the maximum power points (MPP) of PV modules while the state of charge (SOC) of the battery packs is regulated within the tolerances. In this paper, a local hierarchical control (LHC) is proposed for the battery system to improve the energy efficiency of the entire PV-battery system at the MPP of PV modules while the SOC of the battery pack is still regulated within the tolerance. Specifically, by allowing the dc bus voltage to deviate within a preset allowable tolerance, the secondary control of the LHC is employed to compute real-time optimal references to its primary control, such that the energy conversion of the entire PV-battery system can be optimized. Simulation studies exhibit significant efficiency improvement of a 12-PV-battery system under both uniform and nonuniform insolation conditions on a cloudy day and a 600-kW PV-battery system on a sunny day using the proposed LHC. Experimental results validate that the energy efficiency of a single-PV-module-battery system controlled by the LHC can be enhanced using shortened sunny-day and cloudy-day irradiance profiles for various PV modules. The proposed control scheme can be easily implemented in digital controllers without additional hardware costs.