A Novel Fault Current Control Scheme to Reduce Synchronous DG's Impact on Protection Coordination

Synchronous-machine DGs are well known to cause miscoordination of distribution system protections because of the generator's ability to contribute large fault currents to the fault. This paper proposes a field discharge circuit to limit the generator's fault current, thus leading to a synchronous-machine DG with little impact on distribution system protection. In this paper, the operation of a solid-state switch-based field discharge circuit is studied, and its effects on the generator's output current during the fault are investigated. It is shown that the proposed scheme removes the steady-state component of the fault current and accelerates the decay of the transient ac component of the current. The results demonstrate that the proposed field discharge circuit is sufficient to prevent miscoordination of the feeder protections when short time-delay and/or inverse-time overcurrent relays are involved in the protection scheme.