Fundamentals of Microgrid Dead Bus Arbitration

A microgrid is a smaller electric network that can operate independently of a main power grid. An islanded microgrid is typically energized by a generator or inverter and the closing of its associated circuit breaker into a dead bus. A synchronism check is a typical function of programmable protective relays (PPRs) that supervise the closing of these breakers. The source’s voltage and frequency must be in synchronism with the bus voltage prior to closing this breaker. However, in an islanded microgrid energization scenario, a synchronism check cannot be achieved when the bus voltage is dead. Protective relays that attempt to close into a dead bus cannot differentiate between a failed potential transformer (PT) supply and a true dead bus condition. A failed PT supply presents itself as a dead bus voltage condition to the protective relay controlling the generator or inverter breaker. This can create a potentially hazardous situation when a protective relay might close a live machine’s breaker into a live bus that is out of synchronism. This paper explains the problem associated with dead bus arbitration and the various failure modes of PT voltages that necessitate a dead bus arbitration scheme. Typical microprocessor PPR solutions for dead bus close supervision of synchronization systems are also presented. In addition, this paper discusses a scenario with an inverter-based resource (IBR) energizing a dead bus and limitations that need to be considered.