The high-performance cross-linked polyethylene (XLPE) material is the most common insulation used in medium voltage (MV) cables, from 6.6 kV to 33 kV, mainly because of its mechanical and electrical properties. However, the properties of XLPE are significantly affected if the material is exposed to water and moisture, resulting in water trees.
From top: Aberdare Paper Armoured Cable, Aberdare Vultex 1C Unarmoured Cable and Aberdare Vultex 3C Unarmoured Cable.
Donemore Torerwa, electrical engineer and GM: Product Development and Applications at Aberdare Cables Pty Ltd, says it then becomes important, in design, to ensure that the cable structure offers water ingress protection to the XLPE insulation.
The life of XLPE cable is usually estimated to be above 30 years, but once XLPE material is exposed to water or moisture ingress the longevity of the cable is affected. Hence it is important that, from the start of manufacturing, the cable production processes are effectively controlled to prevent any moisture or water ingress. (This is also a good reason why cable users should choose to buy their cables from reputable cable manufacturers.)
Torerwa emphasises that it is also important to prevent moisture ingress when the cable is in use, by ensuring that the jointing and termination workmanship is up to standard, and the cable will withstand the conditions under which it is installed.
SANS 1339 is a compulsory standard in South Africa covering MV cables from 6.6 kV to 33 kV. The standard specifies longitudinal and radial water blocking for MV cables when required by the user of the cable. Longitudinal water blocking is the most commonly deployed water blocking method, applied by many cable manufacturers in the local market, mainly because it is relatively easy to implement. Longitudinal water blocking alone, however, may not be enough to provide lasting water blocking capabilities for the cable in some environments, especially in areas with high water levels in the ground. Radial water blocking then becomes key, in design and manufacturing, to ensure enhanced water blocking capabilities for MV XLPE cables. Radial water blocked MV cables are not common in South Africa but are more common in high voltage (HV) cables, rated for 44 kV to 132 kV, making use of corrugated seamless aluminium (CSA).
Longitudinal water blocking is intended to prevent the ingress and spread of water in the areas between cores, around armouring, metallic screens and insulation. This is done with the use of moisture absorption barriers installed within the structure of the cable along its length. Typically, water block yarns and tapes are installed in the interstices of the multicore cables or around the bedding layers of a single core cables. If exposed to moisture, the yarns absorb and swell, preventing further ingress of the water or moisture inside the cable and thus offering protection. In terms of SANS 1339 of 2020, the cable will be type tested for conformance and users should ask to see these type test reports when purchasing cables that are claimed to have such features.
Radial water blocking features are less common in the industry in South Africa and the SANS 1339 standard does not go into detail in specifying the requirements for radial water blocking. Radial water blocking technology relies on an impermeable concentric layer around the cable preventing radial water ingress. Radial water blocking in high voltage cables is achieved through the CSA structure, which offers mechanical protection, earthing and screening as well as radial water blocking for the cable. For MV cables the common radial water protection materials used globally include lead sheathing, aluminium laminate tape and smooth welded aluminium tape.
The cable that offers both longitudinal and radial water ingress protection can be used in areas with high water levels, mostly the coastal areas in South Africa as well as any wetland. For such applications, the paper cable with lead sheath has always been the best option. This is the main reason why MV paper lead sheathed cables are preferred in most coastal areas, as opposed to XLPE MV cables.
It is important to note that due to the thermal performances of XLPE MV cables, paper MV cables have much lower current or load rating than XLPE MV cables. Paper MV cables may also introduce environmental concerns related to the impregnated oil and lead. This makes XLPE a better option, especially if it the cable has radial water blocking features.
Understanding the challenges related to the continued use of paper cables and the need to improve the life of the XLPE cables, particularly when used in high water level areas, Aberdare Cables, the leading manufacturer of cables in South Africa, has extended its range of products to include radially water blocked MV cables, making use of aluminium laminate technology. These MV cables with radial water blocking are available for the Africa market as well as globally.
For more information visit: www.aberdare.co.za