The effect of water treeing on the electric field distribution of XLPE. Consequences for the dielectric strength

Dielectric characterization and conduction modelling of a water tree degraded LDPE

“…The relative permittivity of water trees in LDPE continuously increased from its value for non-degraded PE, ε 2 =2.3, up to a maximum value ε 1max =4.1, for an ageing period of 272 h and an average water tree length of 373 µm (g=74.6%); it supplies a permittivity amplification factor A ε =1.8, a little bit higher than the value found for XLPE in [10], A ε =1.6 . From polarization current measurements we could determine the evolution of coefficients {I 0 ,m} of the Curie-von Schweidler law with the ageing time.…”

“…From the point of view of accelerated ageing, the typical degradation of cable polymeric insulations known as "water treeing" was simulated at laboratory using a planeplane electrode configuration, a 0.1 M NaCl solution and an ageing voltage of 5 kV rms at 1500 Hz. Once the laboratory specimens were assembled from disks, and previously to the measurement of polarization current with a Keithley 6517A electrometer, I(t), the evolution of capacitance, C, and dielectric losses, tanδ, [4,10] was also controlled for the same scheduled periods of accelerated ageing. In parallel with these experiences, a destructive microscopic analysis of the water tree aged specimens was also performed, which allowed us (once the water tree kinetics for the LDPE under study was completed) to establish a continued correlation between the evolution of the above electrical variables and the actual treed thickness.…”

“…This abrasive-paper method, at least in LDPE, revealed as less aggressive and more reproducible than the method of sandblasting [27], especially because we could choose a grain width which produces a kind of defect deep enough for initiating water trees without involving a direct risk of dielectric breakdown when applying the ageing voltage. The laboratory specimens [4,10] (see Figure 1) were placed on a metallic recipient, which acted both as ground electrode and as a container of a small quantity of silicone oil. It helps to avoid partial discharges in the air by filling the occasional small cavities that usually appear between the LDPE disk and the metallic plane of the ground electrode.…”

“…Parameter b(x) which is related to the permittivity of the material depends on the position as, Figure 11. Model for the variation of parameter b=b(x) within the LDPE plane-plane sample from the hypotheses for the variation of permittivity in a water treed specimen [30,10]: lw=gL/100 ≡ degraded width corresponding to a continuous layer of water trees.…”

“…Water trees grow during the service life of power cables from defects where the electric field is amplified [3,4,6,7,8]. The longer the water tree, the more dangerous it becomes, because the insulation breakdown voltage decreases with the increase of the length of water trees [4,9,10].…”