Intercircuit faults and distance relaying of dual-circuit lines

The main sources of error and other difficulties associated with the distance protection of dual-circuit transmission lines are well known. However, the design or selection of protection schemes usually fails to consider some of the more extraordinary faults that do occur. This paper considers the observed impedances that result from intercircuit faults on a simulated dual-circuit transmission line, where variations in the phasing as well as the impedance ratios have been considered. The simulations performed with the Alternative Transients Program show that intercircuit faults may be undetectable in the instantaneous protection zone depending on the scheme adopted and the impedances of the surrounding network. The observed under-reaching has the potential to lead to a loss of major loads, maloperation of single-pole tripping schemes and even system instabilities based on the critical clearance requirements.