Challenges and Solutions For Distance Elements Protecting Transmission Lines With Tapped Loads

In this paper, we provide practical solutions to keep line relay distance elements, as well as line pilot schemes, secure for secondary faults on a load tapped transformer.

When a load tapped transformer is connected near one terminal of a long line, setting a distance element to only reach into a portion of the load tapped transformer may limit distance element coverage for faults along the entire line. To provide distance element coverage for faults along the entire line, the distance relay reach may be set beyond the load tapped transformer. In these cases, a block signal must be provided from the tapped station(s) to keep the line’s distance elements secure. In applications with multiple tapped loads on the line, the line relays must receive a block signal from each tapped station where distance elements overreach.

In the paper, we discuss alternatives to providing dependable protection for the length of the line, while maintaining security for secondary faults on load tapped transformers. This includes solutions when the tapped load is fed from both breakers in a two-terminal line (N-0) or from a single breaker (N-1). For N-0 conditions, in applications in which a line terminal’s distance elements overreach the line and a tapped load transformer, security concerns can be addressed by selecting an appropriate line pilot scheme. If only one-line terminal is overreaching the tapped load, then a permissive overreaching transfer trip scheme with echo logic disabled can be applied to provide security. If both lines terminals overreach the tapped load, then a permissive underreaching transfer trip scheme can be applied if Zone 1 from each terminal provides overlapping protection on the line. These schemes remove the requirement for a block signal to be sent from the tapped load stations. Guidance is provided on when to use each scheme, and compromises on each scheme are discussed.

Often, N-1 contingencies provide a challenge in maintaining fast and dependable line protection while also maintaining security for secondary tapped load faults near the line terminal. In the paper, we offer solutions that can improve the clearing time for end-of-line (EOL) faults by recognizing that unbalanced faults on the secondary side of a load tapped transformer have a higher apparent impedance than the actual transformer impedance due to the commonly used delta-wye transformer connection. This allows us to set distance elements to reach beyond the transformer to cover EOL unbalanced faults but not operate for secondary unbalanced faults. However, three-phase faults on the secondary of a load tapped transformer have an apparent impedance equal to the actual transformer impedance. Therefore, if a distance element is set with a reach beyond the load tapped transformer, supplementary logic is required to prevent overreach for secondary three-phase faults. In the paper, we discuss how far to safely reach through a transformer with supplementary three-phase coordination logic to prevent overreach for unbalanced faults. Further, various methods to maintain coordination for three-phase faults on the secondary tap load transformer are provided.