The Performance of WTR-XLPE Insulation Systems Against the Requirements of DIN VDE 0276-605/A3

by M. Aarts, J. Kjellqvist, A. Mendelsohn and K. Vaterrodt

Polyethylene was introduced as an insulation material for electric power cable more than 40 years ago. Its inherent characteristics of toughness, resistance to chemicals and moisture, low temperature flexibility and excellent electrical properties, along with low cost and easy processability, make it a very desirable material for insulating low, medium and high voltage electric cables. There have been a number of significant advances in materials technology that have contributed to polyethylene's continued worldwide position as the material of choice for insulating cables.

The use of water tree resistant (WTR) insulation is predominant in North America, while the European market has diverged into the use of homopolymer and copolymer systems. The use of copolymer systems in Europe has been governed by the two-year wet aging requirements of VDE (Verband der Elektrotechnik). The use of homopolymers is more often used in dry designs, where a more complex cable design attempts to avoid ingress of water into the cable core.

The harmonized standard CENELEC HD605 S1 HD 605 S1:1994/A3:2002 "Electric cables - Additional test methods" is currently in place. It contains the two-year aging test in water and could drive a more uniform approach to the use of insulation materials. The current study evaluates the performance of WTR insulation materials against critical European test requirements.

Three medium voltage cables, with dimensions very close to 6/10 kV cables according to DIN VDE 0276-620, were extruded with a triple layer die and were cured under nitrogen atmosphere in a continuous vulcanization process. A homopolymer, a copolymer, and WTR insulation were used. For one year, these cable cores were conditioned and exposed to water on both sides, in and out, and tested under field strength of 6 - 6.7 kV/mm, analogous to 20 kV medium-voltage cables. Microscopy was used to identify the development of water trees with aging. The loss factors of the cables have been measured in the range up to 18 kV (3Uo) as a function of temperature up to 130° C.

The AC break down strength of the virgin cores is the highest with the WTR material. It has been found that the electrical break down strength of the WTR material shows the best performance after both 1 and 2 year aging.

All numbers in kV/mm Virgin Cores After 1 year After 2 years
Homo-polymer 57 33 30
Co-polymer 57 36 39
WTR 61 51 45

Table 1. AC break down strength for different insulation materials after conditioning and water aging.

The paper discusses the mechanism by which the tree retardant formulation resists the degradation of AC breakdown during aging. The final report includes measurements for the intermediate exposure intervals and up to two years of exposure with an interpretation of the observed trends.

The data also indicates that the use of WTR materials is an excellent choice for passing stringent requirements, after conducting the new CENELEC HD 605 S1 two-year test. WTR materials and "dry designs" benefit from more than 20 years of experience in North America, with both strippable and fully bonded insulation screens. Even in "dry designs" additional security can be expected in case of physical damage to the water stop structure.

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