A SEPIC-based three-port converter system using a mode-specific power flow management control for solar energy harvesting

Abstract

The growing demand of renewable energy sources in the energy sector has a considerable impact on the development of stand-alone solar photovoltaic (PV) systems. Three-port converter (TPC) topologies are widely used as a gateway between solar PV, energy storage, and loads. This paper proposes a three-port converter with buck and boost operating modes. The topology is SEPIC-based rather than the conventional buck-boost or Cuk circuit, hence eliminates the inverting output and reduce the input current ripple. The series capacitor in a SEPIC converter endows the TPC with inherent protection against short-circuit output. The TPC has four modes of operation: Dual Output (DO) mode, Dual Input (DI) mode, Single Input Single Output-1 (SISO-1) mode and Single Input Single Output-2 (SISO-2) mode. The power flow management algorithm employs a mode-specific voltage regulation strategy that distributes the duty cycle for each switch via time-sharing control scheme. Small signal model of the TPC is developed in each operating mode and based on this, voltage controller parameters are derived. When the TPC changes modes, the algorithm modifies the controller parameters to the mode-specific values. The proposed system is implemented by utilising the FPGA Cyclone-III comprising the Nios-II processor core with necessary hardware intellectual property (IP) forming a System on a Programmable Chip (SoPC). The system is evaluated in real time using a 120W solar PV as the energy source and lead-acid battery as the storage.