Condition assessment of butyl insulated power cable used in generating stations

Condition assessment of an insulated power cable becomes important when it has reached or exceeded its design life and is still in service. It becomes important to evaluate the mechanical and electrical condition of the cable insulation to determine or estimate its remaining useful service life without sacrificing reliability. This work details the evaluation of samples of unshielded 5 kV, 500 kcmil copper conductor cable with butyl rubber insulation and a neoprene jacket. The cable had been in operational service in excess of 30 years at a fossil generating station of DTE Energy, and had shown external deterioration around the jacket extending into the insulation. The tests included visual inspection of the cable samples, tensile and elongation tests to assess degradation of cable materials, dissipation factor measurement in a wet environment to assess dielectric stability, water tree growth, and electrical tests to determine voltage level at which the cable experiences partial discharge. The tests showed that the dissipation factor was stable, with acceptable bulk dielectric characteristics. However, visual inspection showed that there were areas where the neoprene jacket was no longer bonded to the insulation which were also areas of electrical discharge activity. There were however no in-service cable failures.