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Introduction
The following types of power cable are mainly used for distribution purposes in the medium-voltage (MV) range and will be the focus of discussion in this section:
• Paper-insulated lead covered (PILC) cables
• Cross-linked polyethylene (XLPE) cables.
PVC cables are used mainly in the low-voltage (LV) range, and will be briefly discussed, as well as elastomeric trailing cables, which are used for specialized applications, for example the mining industry.
Overhead lines are mainly used for transmission purposes and will only be briefly mentioned.
Paper-insulated lead-covered (PILC) cables
The PILC cables are manufactured by using layers of paper impregnated with
a compound mineral oil as insulating medium, both as individual core and overall
insulation. A lead sheath is constructed as an outer core layer to mainly
provide a seal for the compound in the paper layers, and also for excellent
corrosion protective properties as well as to provide additional mechanical
protection. A steel tape layer (often a double layer) or steel wires are
used for the main mechanical protection and it may also be used as a return
path for ground currents. The outer sheath may be a PVC layer or other type
of insulating and waterproof material.
DSTA = Double steel tape armored
SWA = Steel wire armored
Earlier PILC cables displayed the phenomenon of compound migration, meaning the compound tended to drain toward the lower part of the cable when installed vertically or against slopes. Non-draining compound are used in modern manufacturing techniques, eliminating this phenomenon.
PILC cables are generally used for 66 kV up to 33 kV applications. The designation 6.35/11 kV, For example, means that the cable has an insulation level of 6.35 kV between each core and ground, and 11 kV between phases. Cables rated 6.35/11 kV may be used for grounded systems (transformer neutral grounded), whereas 11/11 kV rated cables should be used for unearthed (non-grounded) systems. The same applies for other voltage levels.
Detail specifications of PILC cables are given.
Cross-linked polyethylene (XLPE) cables
XLPE, PEX and PVC are used as conductor insulating materials in these cables. XLPE is a semiconductor, and provide partial insulation as well as electrical stress relieving. The conductors, with their XLPE layers, are embedded in PVC to provide total insulation.
Steel wires are used for mechanical strength, and may also be used to provide the return path (or part thereof ) for ground fault currents. The outer sheath is normally a PVC sheath, to provide insulation and waterproofing.
XLPE cables are used from low voltage (600/1000 V) to 132 kV applications. Pure PVC cables, in which PVC replaces the XLPE as conductor layer, are used up to a maximum of 6.6 kV. Detail specifications of XLPE and PVC cables are given.
Elastomeric cables
These types of cables use a rubber-type elastomeric material for insulation to provide mainly greater flexibility as well as high corrosion resistance against harsh environmental conditions and ultraviolet radiation. The outer sheath is usually reinforced with a mesh braiding.
Elastomeric cables are mainly used as trailing cables in mining operations and other specialized applications.
Aerial bundled conductors (ABC)
This type of cable is used for LV distribution of power in residential areas. The three insulated conductors are rapped around a steel rope and strung between overhead live poles. This reduces the risk of open conductors touching, and trees causing faults.
above: Triplex ABC Cable for Service Drop Cable Applications
Overhead aluminum conductors
Aluminum conductors have gained wide acceptance all over the world for use
in overhead transmission and distribution lines. Generally a steel core
is used with the aluminum to give the conductor mechanical strength, termed
aluminum conductor steel reinforced (ACSR). Aluminum is preferred as conducting
medium in overhead lines above copper, mainly based on economic considerations.
Aluminum is considerably lighter than copper, even when allowing for less
current density of aluminum compared to copper. This has a considerable economic
advantage when designing overhead lines, as span lengths can be greater and support
masts lighter. Aluminum also has superior corrosion-resistant properties,
which is a great benefit for overhead lines, as no insulating material is
present to protect the conductors against corrosion, as is the case in cables.
The design and application of overhead lines are very specialized, and they are nowadays used mainly for transmission purposes. Therefore, a detail discussion of overhead lines falls outside the scope of this manual.
Refs:
Notes:
USA/Canada: Medium-voltage power cable. 5-46k -- Also: 2.4-35kV
India: 3.3kV - 33kV
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