Design and evaluation of a directional relay for use within a distribution network restoration scheme

Electrical utilities often use automatic fault sectionalization and distribution network restoration schemes to minimize customer minutes lost. Network restoration can be realized using directional relays, switches with voltage and current sensors and a feeder control system that incorporates communication between the switches and a central control point. The current sensors are measurement current transformers (CTs) which are designed for measuring load current and saturate during a fault. They are unsuitable for a conventional directional relaying scheme, which requires protection CTs rated for operation at the highest level of fault current. The paper describes a curve-fitting algorithm that extracts information from the measurement sensors necessary to determine the direction to a fault. Experimental results show that a directional relay based on the algorithm detects the direction to the fault correctly, even when the CTs are deeply saturated.