Strategies to Deal with Ground Faults in Grid-Connected Transformerless Photovoltaic Converters with Battery Energy Storage System

Abstract

Grid-connected photovoltaic systems with energy storage, also called PV hybrid mini-grid system (PVHMS), operate in both grid-tied and stand-alone modes and are expected to play an important role in distributed generation. Transformerless photovoltaic converters are most preferred for these systems due to their higher conversion efficiency in comparison to insulated converters, increasing autonomy of the battery energy storage system (BESS). Safety in transformerless photovoltaic converters is a critical issue due to parasitic capacitance between PV modules and ground that could result in high leakage current. Existing safety requirements for grid-tied PV inverters may not be sufficient for PVHMS converters since they have multiple leakage current paths. This study analyzes some leakage-current-related faults on transformerless PVHMS converters, and proposes relay opening sequences to avoid unnecessary interruptions of power supply for the local loads. The following situations are analyzed: i) fault at dc side, ii) fault at ac load side, and iii) commutation between on-grid and off-grid operation modes. These faults have been studied for a transformerless PVHMS converter with a single dc-ac stage. Experimental results are presented to validate the proposed schemes and a table summarizing the proposals is presented.