Application of time domain reflectometry technique in detecting water tree degradation within polymeric-insulated cable

Polymeric-insulated power cables are often subjected to multiple sources of degradation. Generally, the main cause for electrical breakdown in this type of cable insulation is usually due to the microscopic impurities and defects located in the bulk, or even at the interfaces of the material. When the dielectric is subjected to a high electrical stress, imperfections such as protrusions, contaminants and microvoids, will all act as points where the electric field is enhanced; increasing the likelihood that degradation processes will be initiated. The intensification of electric field within the insulating material can cause localized discharge to occur continuously, and thus tree-like channels can be developed in the long-run. This paper attempts to investigate whether the existence of water tree region can be detected within polymeric-insulated cables, and the proposed method for the detection mechanism is the time domain reflectometry (TDR). When water trees are present within an insulation system, the characteristic impedance of the material also changes so this can cause reflection of signal propagating along the cable. It was found that there is a difference in TDR signals between un-degraded cable and water tree degraded cable. It is hoped that the difference in these time domain signals can actually assist in determining the location where the presence of water trees can be considered as significant.