Assessment of station cable condition using wireless telemetry and diagnostic tests

Following the premature in-service failure of a 15 kV, XLPE insulated cable in a hydraulic generating station, a program was devised to: determine the root cause of failure; assess the condition of the remaining cables in the station; and ensure that the field modifications proposed to rectify the problem were effective. The underlying mechanism driving the failure was found to be overheating of the cable sheath as a result of the inadvertent formation of ground loops in the cable support structure, and hence circulating currents. Physicochemical tests of cable sections recovered from the failed circuit showed that the cable had experienced temperatures in excess of 115 °C. Electrical diagnostic testing of the affected cable circuits indicated that the majority of the cable length was unaffected by the overheating. However, the portion of the cable within the switchgear cubicle may have undergone some ageing as evidenced by higher dielectric loss values. On-line temperature measurements, including those on the current-carrying components using wireless telemetry, confirmed that one of the sources of the high temperatures was the ground structure in the cubicle.