A new protection scheme for high impedance fault using adaptive trip and reclosure technique

This paper presents a new protection scheme, which is based on adaptive trip and reclosing technique, for earth fault with high fault impedance. In the technique, to aid the conventional directional earth fault (DEF) protection to overcome the shortcoming that it responds slowly to an earth fault with high fault impedance, the sequence signal of current and voltage components are utilized. These sequence signals are utilized to form two criteria, from which whether or not a fault is inside the protected line section can be derived. To cater for the need to instantaneously clear single phase to earth fault with high fault resistance in some transmission systems with single pole switching, an instantaneous tripping technique has to be employed. This would cause a local relay to make a wrong tripping if an external fault that occurs in the downstream line section can be detected by delayed DEF element. Thus, the local relay needs to give a reclosing command to its associated circuit breaker to correct the wrong tripping with a short delay after tripping. Extensive simulation studies with respect to a typical transmission system show that the proposed technique is able to significantly increase the speed of response of the conventional time grading based DEF protection scheme.