Challenges in Protection of Converter Dominated Medium-Voltage Microgrids

This paper presents a comprehensive review of major challenges in protection of medium-voltage microgrids with large amount of converter interfaced distributed generations (DGs). The challenges are mainly due to the limited fault current contributed by the DGs during the fault. This paper focuses on key issues with overcurrent protection schemes with various configurations of DGs and connection points. It was found that the current magnitude based conventional overcurrent relays would fail to operate in islanded operation of microgrids, while in grid-connected mode, the relays would operate but with extended time delay or would not operate for certain fault locations with high penetration of DGs. Overrated converter interfaced DGs would cause problems such as relay blinding and sympathetic tripping. The problems are examined and discussed in cases with DGs interfaced by rated and overrated converters in islanded and grid-connected operations, as well as in cases of different connection points of DGs, energy storage system and fault locations in the microgrid. The paper also attempts to suggest and discuss a number of protection solutions to the presented issues considering cost and reliability aspects.