Distance protection of cross-bonded transmission cable-systems

In this paper the problems of protecting a cross-bonded cable system using distance protection are analysed. The combination of the desire to expand the high voltage transmission grid and the public´s opinion towards new installations of overhead lines (OHL), more and more transmission cable systems are being laid around the world. Differential protection is often used for the main protection of cables. As a backup protection, distance protection is very often the preferred choice. Therefore, the behaviour of distance protection when applied to cross-bonded cablesystems is very interesting. The basic assumption of a distance relay is that the measured fault impedance is linearly dependent on the distance to fault. For this to be true, the system must be fully symmetrical and only having single-ended infeed. This is not always the case for OHL-systems and never the case for a cross-bonded cable system. The fault current returning from the fault location to the source on a cross-bonded cable system can flow in all three screens and ground. This makes the system nonsymmetrical and the zero-sequence compensation factor (kfactor) will not be able to describe the fault loop consistently for single phase faults any longer as the measured impedance becomes both non-linear and non-continuous with regard to the distance to fault.