Sympathetic trip prevention by applying simple current relays

For any type of shunt faults in power system, all parts of the network will experience voltage sag (voltage dip) for the duration at least equal to the fault clearing time. The percent of this voltage sag at a given point depends to its distance to the fault and fault resistance. If a bus bar supplying a group of induction motors is exposed to such voltage dip, which may be due to a fault at another feeder or higher voltage level, the excessive phase current drawn by the group of motors may cause operation of over current protection of the supplying feeder. When the remote fault is unbalanced, or if the load consists of different size/type of single phase motors, earth fault protection of the feeder may also trip because of zero sequence current created due to unbalance load. These kinds of relay operation which results unwanted tripping of a healthy feeder due to behavior of its specific supplying load in response to a fault elsewhere in the system are called sympathetic tripping. The subject has been discussed in several papers in the past and different solutions proposed which are reviewed shortly in this paper. This paper studies current profile of induction motors load both on-event or during voltage dip, then post-event or right after fault clearing, and describes a solution by implementing a logic in simple current relays to prevent sympathetic tripping based on differences between profiles of internal fault current and such particular load current. A real world case resulted to sympathetic tripping was simulated and stability of an overcurrent relay using the logic was tested successfully.