A novel cascaded modular photovoltaic energy storage system for partial shading conditions

Abstract

To satisfy the grid-connected voltage level, both photovoltaic modules and energy storage modules are connected in series. However, the multiple photovoltaic modules often fall into local maximum power point under partial shading conditions during practical operation, and the multiple energy storage modules may suffer from a reduction in the effective capacity caused by characteristic differences among modules. To solve this problem, a novel cascaded modular photovoltaic-energy storage system is proposed in this paper. In the proposed topology, the energy storage modules achieve maximum power point tracking of the corresponding distributed photovoltaic module, and the proposed energy optimization strategy based on particle swarm optimization can ensure the efficient constant active power transmission from a photovoltaic energy storage (PV/ES) system to the grid in a certain time period under capacity constraints. Compared with conventional photovoltaic systems, the proposed scheme can avoid hot spots or the hot strings phenomena for PV modules and the large current and voltage stresses for DC/DC converters. Furthermore, the proposed energy optimization strategy for the coordination of all ES modules can realize the independent MPPT of each PV module and the constant active power between the PV/ES system and the grid under inconsistency of the light intensity under partial shading conditions. A hardware-in-loop photovoltaic-energy platform is established to verify the feasibility and effectiveness of the proposed topology and control strategy, and the proposed system achieves efficiency of about 97% under partial shading conditions, thus providing an effective and practical solution for power generation system.