The term High voltage in South Africa refers to any voltage rated above 33kV (36kV U_m) up to about 275kV. On this Topic, I will cover the conductor selection, insulation level, constructional requirements and installation requirements that are necessary to consider when selecting a high voltage cable and it is important to take note that this is just a guide and not in any way conclusive.

At start the cable selection requires the cable operating conditions to be known these include the nominal operating voltage of the system, the highest operating voltage of the system, the nominal system frequency, design lightning and if applicable the switching surge/overvoltage levels of the system (These values are usually on the type tests report (the impulse test conducted uses a lightning waveform of 1,2/50µs, switching waveform not included as part of type test) for a range of cables). Most importantly the rated current of the system or the load which may be specified as continuous operation rating as well as cyclic, emergency loading and overload rating. This allows one to specify a cable in a way that it will not exceed the maximum cable operating temperature. The other fault conditions to consider at design level would be the asymmetrical and symmetrical short circuit for all scenarios for phases to earth and also the consideration for phase to phase of the cable or load and the protection tripping time for both short circuit and earth fault rating as this will help in determining the most economical cable size.

Besides the basic voltage and current ratings it is important to consider the type of earthing and the methods of bonding which can either be single point bonding, double point bonding or include cross bonding as well as confirmation of use of surge limiters to protect the aluminium sheath against transient surges. Dangerous induced sheath voltages build-up is very common and has to be considered when selecting the type of bonding. The type of bonding In other applications such as single point bonded circuits, the running of a parallel bare copper earth conductor will be required to ensure earth continuity, enhance the aluminium sheath protection and improve the earth fault withstand level of the system. An insulated earth continuity conductor should be considered as a means of limiting stray currents from interfering with the operation of the HV system.

The cable conductor size is selected based on the maximum allowable temperature of the cable (dependant mainly on the type of insulation and affected by installation medium) taking into consideration the current and future planned loading of the system as optimally economical as possible. For conductors of generally greater than 1000mm² size special designs as Milliken shaped conductors are required to reduce the impact of the skin effect and improve current carrying capacity of the cable. The other factors are discussed later in this article.

For High Voltage Cables it is necessary to carry out a proper system design and simulation; most Utilities prefer software simulations such as DIgSILENT, ETAP, PSCAD, ERACS and many more for the design of the whole system to determine the operating and design conditions used to select the cable. For a better design of the cable itself another simulation usually done by manufacturers and consultancies to determine the actual rating of the cable based on the installation conditions is done with software such as CymCap and others.

The site Installation conditions have to be taken into consideration when selecting the High Voltage cables. The site information required for cable selection and simulations is as follows:

• The distance of installation or route and terrain.
• The arrangement of cables or grouping in their installation medium.
• The installation medium i.e underground, underwater, Ducts, Pipelines, Cable Ladders, In air (Shaded/Exposed to sunlight), Tunnels, proximity to heat producing equipment and any other application.
• The spacing of the cables and installation format (Flat of trefoil) and spacing between groups of cables.
• Trans-positioning of cables to ensure equal current sharing and to limit mutual induction effects.
• The positioning of the ECC conductor with respect to the cores and trans-positioning to limit the effect of induced voltage and currents.
• The environment where the cable is installed affects the rate of heat loss of the cable hence the soil thermal resistivity, air temperature, depth of burial, conditioning in tunnels and pipes will need to be determined.
• The risk of fire or presence of chemicals in an area has to be determined and the effect of harmful gases or smoke. These will impact the selection of the sheathing material for the cable.
• Water Ingress protection requirements need to be known as there are available options of water blocking cables (Longitudinal or radial).
• Mechanical load capacity of the cable may have to be considered depending on the method and condition of installation.
• Maximum allowable sheath voltage under normal operation (normally 60V) and fault conditions as well as the voltage level at which the SVL should operate under transient conditions.

Allowance has to be made for proper installation, commissioning and structured testing of the cable before putting it into service, as poor workmanship may affect the life of the cable drastically.

Aberdare Cables is one of the leading cable manufacturers globally, offering cables from low voltage to a high voltage of 132 kV, with cables being tested to international and local South African Standards as well as any acceptable customer preference standard. Currently The HV cables are designed and constructed to IEC/SANS 60840 at Aberdare Cables. Aberdare Cables offer technical and design support to their customers on selection of cables as well as simulation for the loading of cables on software as well as providing guidance on installation.