Distance relay behaviour on mixed voltage, double circuit lines

Double circuit overhead lines are used to maximise power flow over a given route. The protection of double circuit, parallel lines is onerous. In distance applications, to ensure reach point accuracy for earth faults, mutual compensation is applied to correct for the induced voltages caused by currents from the extra circuit. The investigation of distance protection on double circuit lines having dissimilar voltages is less common. Under phase to phase faults involving phases from different voltages, distance protection is prone to error. This type of fault is, in fact, an earth fault and the effect of the active source in the fault path can lead to reach point inaccuracy in some conditions. This paper will review existing distance protection strategies for double circuit lines and then investigate the effect of mixed voltage applications. Results of a simulation for a 33/275 kV line are presented.