Understanding the challenges related to the continued use of paper cables and the need to improve the life of XLPE cables, especially when used in high-water-level areas, Aberdare Cables has extended its range of products to include radially water-blocked medium-voltage cables using aluminium laminate technology.
The product is available for sale in Africa and globally as well.
Looking at XLPE
High-performance cross-linked polyethylene (XLPE) is the most common insulation used in medium-voltage (MV) cables between 6.6 kV and 33 kV, primarily due to its impressive mechanical and electrical properties.
XLPE's properties are drastically impacted if exposed to water and moisture, resulting in water trees; it then becomes very important in design to ensure that the cable structure offers water ingress protection to the XLPE insulation.
The life of an XLPE cable is usually estimated to be above 30 years, but once the XLPE material is exposed to water or moisture ingress, the life of the cable is affected. Hence, it becomes essential that cable production processes are well controlled from the start of manufacturing to prevent any moisture or water ingress. (This is why cable users must consider buying their cables from reputable cable manufacturers.)
It is also important to prevent moisture ingress during cable operation by ensuring that the jointing and termination workmanship is up to standard and that the cable will withstand the conditions under which it is installed.
Specified radial water blocking
SANS 1339 is a compulsory standard in South Africa covering MV cables with voltages rated from 6.6 kV to 33 kV. This standard specifies longitudinal and radial water blocking for MV cables when required by the cable user.
Longitudinal water blocking is the most deployed water blocking method as 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 offer lasting water blocking capabilities of a cable in some environments, especially in areas with high water levels in the ground. Offering radial water blocking then becomes key to cables to ensure enhanced water blocking capability of MV XLPE cables at both the design and manufacturing level.
Radial water blocked cables are not very common in South Africa for MV cables; they are common for high-voltage (HV) cables of 44 kV–132 kV rated cables using corrugated seamless aluminium (CSA).
How longitudinal water blocking works
Using moisture absorption barriers installed within the structure of the cable along its length, longitudinal water blocking is intended to prevent the water ingress and spread in the areas between cores, around armouring, metallic screens and insulation. Water block yarns and tapes are commonly installed in the interstices of the multicore cables or around the bedding layers of single-core cables. If exposed to moisture, the yarns will absorb and swell, preventing further ingress of water or moisture inside the cable, hence offering protection.
As per SANS 1339 of 2020, the cable will be type tested for conformance, which becomes vital for the users to ask for these type test reports when purchasing all the cables claimed to have such features.
Radial water blocking technology
Because radial water blocking features are not very common in the industry in South Africa, 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 HV cables is achieved through the CSA structure, which then offers mechanical protection, earthing, screening, and radial water blocking for the cable. For MV cables, the common radial water protection material globally includes lead sheathing, aluminium-laminate tape and smooth welded aluminium tape.
Applications in high-moisture environments
The cable that offers both longitudinal and radial water ingress protection has considerable use in areas with high water levels, mostly the coastal areas in South Africa, as well as any wet land. For such applications, the paper cable with lead sheath has always been the best option which is the main reason why most coastal areas still prefer MV paper lead sheathed cables 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 in comparison to XLPE MV cables. Paper MV cables may also introduce environmental issues related to the impregnate oil and lead, hence XLPE is the better way to go especially if it has radial water blocking features.
