Approximate Entropy based prevention of third zone distance relay maloperation

Third zone distance relays tend to maloperate during power swings, static load encroachment and voltage instability events. Even during voltage recovery events, there are chances of third zone distance relay maloperation. Prevention of distance relay maloperation during delayed voltage recovery events requires identification of vulnerability of distance relays with the existing method in literature. As long as the impedance trajectory is outside the third zone reach after fault clearance, the vulnerability can be found using relay margin. The third zone distance relay may malfunction, if the impedance trajectory remains inside the third zone reach even after fault clearance due to a delayed voltage recovery event. The approximate entropy based method is proposed to identify the fault occurrence and fault clearance events. The proposed method is applied to the distance relays of the 68 bus system, and it is observed from the results of the case studies that the distance relays do not maloperate during non-fault events and operate only during fault events.