Applications of NGRS in Multiple Source MV Power Systems

Neutral grounding resistors (NGRs) are commonly used in medium voltage (MV) distribution systems for industrial facilities. The application of NGRs becomes considerably more complex when multiple power sources, including on-site generation, are involved. Industry standards and best practices have evolved over time, and the sizing of each NGR cannot be performed in isolation. Coordinating NGRs throughout a distribution system is critical for the protection of generators, transformers, and feeders during ground faults, especially since each individual NGR affects the fault current seen by the entire system. Additionally, introducing NGR monitors capable of verifying continuity of the ground connection when decoupled from the bus, while remaining compatible with multiple sources, is an important consideration. This paper aims to offer recommendations and application guidelines to assist engineers when sizing NGRs for the overall MV power distribution system, including generators and transformers, to ensure comprehensive protection of all equipment within the specified system and application. NGR sizing aims to minimize equipment damage and ground-fault current, reducing the risk of ground faults escalating into phase-to-phase arcing faults and enhancing personnel safety. NGR monitoring is also discussed since it is a requirement in some jurisdictions. An applicable case study for a paper mill is included that demonstrates the NGR sizing process, calculations, and considerations.