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Intermediate voltage cable (1,9 / 3,3 kV Interdac Cable) has a reduced lifetime compared with standard LV or MV cables, because of higher operating electrical stress and low cost design.
Aberdare’s Interdac range of low cost Intermediate voltage cables was introduced to the South African market to fill a very specialized niche.
Until that time, it was prohibitively expensive to transfer even a small amount of electric power over a long distance at low voltage. Typical application would be bore-hole pumps for farmers, electricity supply for farm workers homes etc.
The implication here is that these are not critical loads, and that failure is of little consequence.
The big advantages over conventional 400 V 3 phase systems, and which was offered by the increased voltage of 3,3 kV were:
(a) no volt drop accommodation over many kilometres. (Typically 10 km.)
(b) use of small conductors. (Typically 10 mm².)
Before the introduction of 3,3 kV, there was a problem with 400 V systems in that they were not able to convey electric power more than a few hundred meters, and even then, very large and expensive conductors were required for limiting voltage drop reasons.
However introducing a higher system voltage reduces the thermal stress on the insulation but increases its electric stress. The relationship between insulation stress and lifetime is well documented – the higher the stress, the shorter the lifetime. In the case of Interdac III, with insulation thickness set to international norms, the stress is several times higher than in conventional 400 V cable. Although this results in insulation material saving, hence lower cost, overall lifetime is reduced as a result.
A typical supply to a farm is 25 kVA, supplied at 400 V 3 phase. (Some farms have 50 kVA or even 100 kVA supplies) This equates to 36 A per phase for the 25 kVA supply. With an allowable voltage drop of 5%, the maximum distance this power can be transferred is 230 m using a 16 mm² x 4 core cable. (The cable incidentally is rated at approximately 80 A!) a 50 kVA supply is rated 72 A / phase, and the maximum distance for this transfer would be 25 0m using a 35 mm² x 4 core cable.
The only way of transferring this power a longer distance, would be to use a larger 400 V cable (because of voltage drop), and this of course is very expensive.
As the alternative, a 10 mm² x 4 core 3,3 kV, Interdac III cable can carry the full 25 kVA supply up to 10 km, before the volt drop limit of 5% is reached. This distance is reached at 5 km in the case of 50 kVA, but still using only a 10 mm² x 4 core Interdac III cable.
The benefits of the 3,3kV system are thus obvious.
As a result, the initial acceptance of 3,3 kV cable was overwhelming. This was due in part by Eskom Agrilec endorsing the use of Aberdare’s 3,3kV Interdac cable, in order to promote the use of electricity in agricultural applications.
As mentioned, Interdac III cable was substantially cheaper than any other cable of comparable capability, because it was:
a) Small conductor size (typically 10 mm²) because of the higher voltage.
b) Without conventional wire armouring.
When compared with an 11 kV system, Interdac III was found to be even cheaper when used to transfer a small amount of power over a long distance. The smallest 11kV cable available is a 16 mm² x 3 core cable, which costs (say) R10 000. Interdac III cable could thus do almost the same job, but at only R2 100 for the same length of 10 mm² size cable. The 16 mm² x 4 Interdac III cable cost is R2 900, still only a fraction of the cost of 11 kV cable. (Costs indicative only).
Recently however, we have experienced greater and greater use of Interdac III cable, in what may be considered inappropriate applications. These cables are now being used by industry, to drive small loads quite far from the main establishment. They are also being used on greater scale by game-lodge owners, to electrify up-market game farms etc. In theory there is nothing wrong with this, BUT the users tend to forget that this is a low cost cable with limitations.
An 11 kV paper insulated cable is designed for a lifetime of approximately 40 years whilst an MV XLPE cable is designed for a lifetime of 30 years. This is not the case with an Interdac III cable, and one wonders what disruption will be caused when these cheaper 3,3 kV cables come to the end of their useful lives in a substantially shorter period than 30 years.
The other problem with Interdac III cable is the difficulty of fault finding. Traditionally on PILC cables, the use of surge generators is standard. When used on Interdac III cables, such use will cause irreversible damage to an already overstressed insulation system. The result, an even shorter lifetime.
Interdac III cables are designed for low power, long distance applications, at low cost but for non-critical loads. It is incorrect to expect the same degree of reliability from a cable costing only ¹/5th of the price of an alternative LV or MV cabling systems.
