Deploying correct fault loop in distance protection of multiple-circuit shared tower transmission lines with different voltages

Combined faults occurring between different voltage levels in overhead lines present a challenge for distance protection. Previous work has shown that such faults most often appears as single-phase-to-ground (SPTG) faults in a normal type of overhead line. However, it is not obvious that distance relays will identify and select the correct fault loop according to being similar to SPTG, as all six fault loops get excited when combined faults occur. A study where two distance relays of different manufactures are tested using transient replay and secondary test equipment is presented, in order to reveal which fault loops are activated and whether a safe trip for combined faults happens.