PV/Battery Grid Integration Using a Modular Multilevel Isolated SEPIC-Based Converter

Abstract

Photovoltaic (PV) plants can be built rapidly when compared with other conventional electrical plants; hence, they are a competent candidate for supplying the electricity grid. The output power of the PV modules can be used in plug-in electric vehicles (PEVs) DC charging stations to reduce the burden on the electricity grid, particularly during peak load hours. To integrate PV modules and electric vehicles (EVs) with the electricity grid, the modular multilevel converters (MMCs) topologies producing staircase voltage waveforms are preferred as they are able to deliver less total harmonic distortion (THD) and higher efficiency in addition to lower voltage stress on semiconductor switches. In conventional centralized MMC topologies, a direct connection to a high-DC-link input voltage is required which is not appropriate for PV plants. A new MMC topology for PV/EV/grid integration is proposed in this paper, where the individual PV arrays are directly connected to each phase of the AC grid to harvest the maximum available power point. A current-source converter (CSC) based on a single-stage isolated SEPIC converter is adopted as the submodule (SM) for the proposed MMC topology given its outstanding features, such as low input ripple current, high efficiency, high power factor, and flexible output voltage higher or lower than the input voltage. The single-stage SMs can operate in both DC/DC and DC/AC operating modes. Proper controllers for each mode of operation are designed and applied to supply constant current from either the PV modules or the battery cells by eliminating the second-order harmonic component. The performance of the proposed converter is verified by simulations and a downscaled prototype controlled by TMSF28335 DSP.