As the penetration of water or the existence of moisture in cross-linked polyethylene (XLPE) cables is an “unacceptable problem” associated with the life span of certain cables, especially under the influence of high electrical stresses, longitudinal waterblocking is an appropriate means of protection for medium-voltage XLPE cables which may experience water ingress as a result of poor handling during installation or damage over its life, says local power cable manufacturer Aberdare Cables.
“Waterblocking, which is the prevention of water spreading along the cable, as well as between the elements of the cable, is achieved by filling the free space within the cable with waterblocking materials,” Aberdare Cables chief engineer Antony Falconer explains.
When water penetrates through a cable’s damaged outer sheath, the waterblocking materials will start blocking against longitudinal water and moisture ingress, by absorbing the moisture.
In longitudinal waterblocking, water is prevented from progressing along the cable length, normally to a distance of less than 1.5 m from the point of entry, under defined test conditions, along any of the layers that make up the cable construction. These layers include the conductors, insulation, metallic screens, laid up cores and armouring, Falconer explains.
Further, he highlights that waterblocking is viewed as a pre-emptive measure, which allows a segment – about 3 m in length – of water-contaminated cable to be isolated and removed after an incident, without water being allowed to spread further along the cable.
Aberdare Cables manufactures medium-voltage XLPE cables with special waterblocking features, and offers full longitudinal waterblocking as an additional feature on these products.
Falconer adds that the “use of tapes and yarns was found to be the most appropriate method of waterblocking from a manufacturing and an application perspective”.
“The company conducts materials selection tests, product trials supported by internal water penetration testing and heat cycling tests, which lead to enhanced designs that are industrialised and type-tested before the product is offered on the market,” Falconer says.
Further, the company is believes that the accelerated ageing test, prescribed in SANS 1339, is the most appropriate measure to evaluate cable materials, manufacturing processes and design in one product test for water treeing – a region of reduced insulation strength, and the only known chemical ageing process for XLPE insulation materials-related failure.
Increased Cable Reliability
“Medium-voltage XLPE cables form a vital link in an electrical distribution system and a large number of electrical users are dependent on these links, by virtue of the high power transfer capability of these cables.
“With an electrical utility measured by the reliability of its distribution network, major effort is normally expended in the selection of equipment, commissioning tests and preventative maintenance practices to ensure that these systems are and remain reliable,” he says.
Full longitudinal water blocking provides engineers with the means to build reliability into their distribution system, which is expected to continue operating in a fault-free manner for decades, he adds.
“A medium-voltage XLPE cable, which is kept free of water, will provide reliable service when manufactured using quality materials and proven processes, qualified by accelerated ageing tests to SANS 1339, installed without damage to the cable, when operated under conditions for which it is designed and qualified by type testing.
“Such well manufactured and qualified cables will have a low risk in the formation of water trees leading to insulation failure, in the presence of water,” he says, adding that waterblocking will prevent corrosion damage to cable components such as the metallic screens and armouring.
Falconer notes that local demand for these cable designs increased with the adoption by one large electrical utility, adding that a second major utility is also soon to introduce water blocked 6.35/11 kV and 12.7/22 kV cable as a standard.
“Further, we are expecting a step-wise increase in demand as more utilities consider adopting this design feature,” Falconer concludes.