Sheath insulation fault test technique for high voltage power cables

Healthy sheath insulation is very important for the safe operation of power cables, especially for single core cables. If there are insulation faults in the sheath, the cable´s life will be obviously shortened, and many online cable insulation-monitoring systems will fail to operate. An effective test system to locate and position insulation faults in cable sheath is explored in this paper. Measuring the metal shield resistance between the cable terminal and the fault point, and then comparing it to some parameters based on the normal shield resistance will be helpful to evaluate the distance to the fault point. The influence of the intrinsic and contact resistances of the test leads can be completely eliminated. After the fault distance is located, it is necessary to find the accurate position by so-called positioning methods. The modified pace voltage method is developed in this paper. Other than the traditional constant voltage method, a periodical high voltage surge is applied to the metal shield of the tested faulty sheath, and the pace voltage is recorded by a digital voltmeter instead of analog pointers. Such a testing system has showed great convenience and accuracy in practices. Some actual cable sheath fault locating and positioning test cases are also listed and analyzed in this paper.