Negative-sequence overcurrent considerations for induction motor loads

Negative-sequence overcurrent (51Q) elements can add sensitivity to transformer and feeder protection. Because these elements do not respond to balanced load current, they can be set below phase overcurrent elements. However, certain loads may cause higher than expected negative-sequence current to flow because their negative-sequence impedance is quite low when compared to their positive-sequence impedance. This paper describes an event in which a 51Q element operated for a fault on an adjacent feeder, discusses the impact of voltage unbalance on induction motors, and examines induction motor load modeling and sequence networks. The analysis is supported by a PSCAD model developed for this event. The paper also shares specific lessons learned to assist engineers to securely set 51Q elements in the presence of induction motor loads.