Aging of Polyethylene ADSS Sheath by Low Currents

The electrical environment of all-dielectric self-supporting cables on overhead power lines presents a challenge to cable materials. It is widely acknowledged that the application on lines of system voltages above 150 kV requires specialist sheath materials to prevent damage as a result of discharges on the cables´ surfaces. It is widely held that below this system voltage, polyethylene sheaths are adequate. This is not always the case. However, one such cable which successfully served for 15 years on a 132-kV twin circuit has had its sheath examined. In particular, contact-angle measurements have been made along two span lengths. Even though predicted currents on the sheath are lower than those expected to cause damage, a reduction in contact angle is shown to be closely correlated to predicted current magnitude along the cable spans. An aging process is therefore illustrated, creating changes below levels which would normally be called damage. A 0.5-mA current limit is appropriate for the polyethylene material considered.