Adaptive Direct Underreaching Transfer Trip Protection Scheme for the Three-Terminal Line

The direct underreaching transfer trip (DUTT) scheme used for the protection of the three-terminal transmission line operates when at least one of the three zone 1 elements detects an internal fault. This scheme uses a dedicated communication system to transfer the decision from one end to the other ends. The DUTT scheme is reliable until the overlappings between zone 1 settings are ensured. The infeed through the tapped line reduces the overlappings between the zone 1 elements in the DUTT scheme. At times, the zone 1 overlappings of the three distance relays vanish, which leaves an unprotected region in the scheme. In this paper, an adaptive DUTT scheme is proposed where overlappings of zone 1 settings are calculated using prefault synchronized data, and relay settings are updated whenever required to ensure a proper protection decision. The proposed adaptive method is tested for a three-terminal line simulated in EMTDC/PSCAD and the results show its high accuracy.