A theoretical investigation for the development of a water tree dielectric response model

Underground medium voltage XLPE cables are at risk of failure from water tree degradation and it is vital that water tree degraded cables be detected in a power system network before they can cause outages. Time domain and frequency domain diagnostic techniques can be used to assess the insulation condition but the interpretation of the results from these measurements, and how they are correlated with degradation indicators such as water tree density and length, is unquestionably difficult. This paper intends to investigate the polarisation and conduction mechanisms within water tree degraded cables, paying particular interest to the effect of space charge accumulation at the water tree tip. It is postulated in this paper that the electric field at the tree-tip will be strong enough, from the contribution of space charge, to induce non-linear effects in the local surrounding, healthy XLPE. It is predicted that this non-linear effect will act to ´amplify´ the healthy XLPE response to a higher magnitude, resulting in higher losses, polarisation and conduction.