A functional auxiliary module for 1st-zone conventional distance relays of double-circuit lines to eliminate high fault resistance-introduced under-reach

• The proposed module effectively removes distance relays underreach in case of HRFs.
• The proposed module is applicable to double-circuit lines regardless of line length.
• In 95% of the 1st-zone HRFs are tripped by the proposed module within 12–17 ms.
• The proposed module functions accurately in various operating and fault conditions.
• The security of the proposed module is preserved in case of irrelevant-circuit faults.

This paper presents design steps of an auxiliary module intended to function in conjunction with under-operation 1st-zone conventional distance relays of double-circuit transmission lines. The proposed module pursues the objective of eliminating the underreach of associated ground elements, arising from single-line-to-ground faults of high resistance. It is founded on the online estimation of the fault distance during the faults, through the roots of a non-linear algebraic equation developed in the sequence networks. The equation is mathematically discussed and a fitting numerical algorithm efficiently finding its roots with no concern about the computation burden is chosen. In order to be applicable to the lines of any lengths, particularly long lines, the proposed module is developed through the distributed-parameter based model of double-circuit lines. In addition, it employs only the data acquired at the relaying location and is independent of the network configuration. Important considerations are made for the proposed module, making its function both dependable and secure. Simulation studies conducted in MATLAB and PSCAD/EMTDC examine the proposed module in a variety of influencing situations, and confirm that it accurately fulfills its intended objective within the time frame of the distance protection.