A protection and grounding strategy for integrating inverter-based distributed energy resources in an isolated microgrid

Abstract

Reliable protection and grounding schemes have been well established for power systems. With the advent and proliferation of microgrids, however, these subjects need to be revisited as traditional philosophies are no longer sufficient to cope with reduced short circuit levels of distributed energy resources (DERs). A DER dominated microgrid will experience a limitation in the functionality of traditional overcurrent elements. This can degrade protection coordination and selectivity and requires a philosophy that is nonstandard in distribution systems. Furthermore, with most DERs operating as constant current sources and not naturally supplying ground current, performance grounding also becomes a fundamental problem of microgrids. Additional ground sources are required and must be appropriately sized for the needs of the microgrid. This paper proposes a practical protection and grounding scheme for an isolated microgrid that is being retrofitted with a large solar facility and a battery energy storage system (BESS). Much of the theory was developed tailored for this system and serves to reinforce the need for new philosophies that consider practical aspects of real systems.