Interpretation of the relation between residual charge and water trees in XLPE cables

Crosslinked polyethylene (XLPE) insulated cables working in water are known to be easily degraded by forming water trees. This is one of the most serious concerns among users of XLPE cables. The possibility whether measurement of residual charge is usable as a tool to detect water trees in XLPE is discussed. Two types of samples were used; one is an XLPE cable with water trees and the other is an XLPE sheet with water trees grown by ac voltage application, The sheet sample was used since we thought that we could easily check the growth of water trees and could also use a generator of a small capacity, which enabled precise experiments. The results obtained are summarized as follows. (1) Residual charge was not measured in the sheet sample because of its small electrode area. (2) Residual charge was detected in the cable sample. (3) A clear frequency dependence of the residual charge was not observed. A model of the residual charge measurement is proposed, where the charges accumulated in a water tree are oscillated by ac electric field and are diffused through the water tree path with a high conductivity. The frequency dependence of the residual charge is discussed by dividing it into the contribution of the electric field distribution, that of the detrapping process, and that of the carrier transportation process into the electrode. As a result, it is confirmed that the weak frequency dependence of the residual charge is reasonable.