Design and Analysis of a Grid-Tied Non-Isolated Three-Phase Unfolder for Energy Storage Systems

This paper proposes a non-isolated bi-directional converter for grid-integration of Energy Storage Systems (ESS). A TNPC-based three-phase unfolder has been used in the DC-AC stage along with a boost and voltage-balancer in the DC-DC stage. As a result, it provides improved power density over existing isolated solutions available in literature and retains the advantages of reduced filter-size and low switching losses inherent to the unfolding action. A detailed analysis for the grid-side filter requirement for a conventional Boost cascaded VSI has been done to highlight the reduced filter requirement in the unfolder system. Passive component design has been discussed and a suitable control strategy has been proposed. Closed-loop simulation has been done to validate the control strategy. Experiments have been carried out on a laboratory prototype upto 2.5 kW and results with resistive load have been presented. The THD of line currents is found to be less than 2% even with a small L-filter at the grid-side.