New Balance Ground Relay for Parallel Four-circuit Transmission Lines

Abstract

A 66kV network is generally grounded through a neutral grounding resistor. In this network a single phase ground-fault current is limited to as small as 100_??_400 A. There are parallel four-circuit transmission lines mounted on the same tower in the 66kV network. In such transmission lines, the load and the fault currents could induce circulating current which flows through the lines. As the circulating current has zero-phase-sequence and negative-phase-sequence components, it could cause an unwanted operation of a balance ground relay using zero-phase-sequence current. However, it is difficult to compensate for the circulating current by the conventional vector compensation scheme.
This paper has presented a new balance ground relaying to deal with the circulating current. In the relay from the ground-fault inception until the first tripping, the difference current Δ3I_2d of negative-phase-sequence current 3I_2d of the differential current between two protected lines is used as an input current. The Δ3I_2d is the difference current of 3I_2d between before and during faults. After the first tripping, the difference current of positive-phase-sequence load current and zerophase-sequence current of the above differential current are used as an input current. Consequently a higher sensitivity of the ground-fault protection for the above lines has been achieved. The correct operation of the new balance ground relay has confirmed when a single-phase-ground-fault occurred in the parallel four-circuit transmission lines, to which the relay is applied.