Drying of medium voltage cables-a chance to restore the electric strength?

Many polymer insulated cables suffer from water trees when they are buried in the ground without a watertight shield. After some years in service the water trees may lead to a marked increase of the loss factor and to a decrease of the breakdown strength of the cables. To obtain informations on the drying and refilling phenomena the author took 10 m-pieces of field aged medium voltage cables (from three different manufacturers) containing water trees and examined the variation of the loss factors and the capacitances under various conditions over a period of more than one year. To simulate realistic conditions as close as possible, tap water was applied under the mantle of the cable but no water was injected into the conductor. This is a realistic simulation of a local damage of the jacket, whereby it is known that water flows very quickly along the cable. The applied voltage was 20 kV_RMS, the frequency 50 Hz, and a conventional bridge was used for the measurements