Hardware-In-Loop Testing of a Differential Relay Used to Protect Single/Double Circuit Transmission Lines

The results and findings of testing an ABB RED670 current differential relay protecting a 400 kV transmission line that spans a length of 400 km is presented. The relay is configured such that it clears all internal faults and does not issue trip signals for external faults for a single circuit line without series capacitor compensation. The effects of each of the following on the response of the relay are then investigated: the addition of another circuit on the same tower, the insertion of mid-line series capacitors on the single circuit to compensate for 30% of the line inductive reactance and the simultaneous addition of another circuit on the same tower and series capacitors on the now double circuit line. The relay was able to trip for all internal faults and the average time to clear faults was 30 ms (1.5 cycles). The relay was also able to not issue trip signals for all faults initiated on the adjacent circuit