MV Paper-Insulated Cables:
Until the late 1970s, the large quantities of paper-insulated lead covered (PILC) cables used in the UK electricity supply industry for MV distribution circuits were manufactured according to BS 6480. These cables also incorporated steel wire armour (SWA) and bitumenized textile beddings or servings. An example is illustrated in Fig. 1.1.
The lead sheath provided an impermeable barrier to moisture and a return path for earth fault currents and the layer of SWA gave mechanical protection and an improved earth fault capacity. PILC cable continues to be specified by a few utilities; although conversion to XLPE designs are planned.
Following successful trials and extensive installation in the early 1970s, a new standard (ESI 09-12) was issued in 1979 for Paper-Insulated Corrugated Aluminium Sheathed (PICAS) cable. This enabled the electricity supply industry to replace expensive lead sheath and SWA with a corrugated aluminium sheath which offered a high degree of mechanical protection and earth fault capability, while retaining the proven reliability of paper insulation.
The standard was limited to three conductor cross sections (95 mm2, 185 mm2 and 300 mm2) using stranded aluminium conductors with belted paper insulation; although it later included designs with screened paper insulation. An example of PICAS cable is shown in Fig. 1.2. A PICAS cable was easier and lighter to install than its predecessor and it found almost universal acceptance in the UK electricity supply sector. It is still being specified by a few utilities, but as with PILC, a switch to XLPE designs is planned.
MV Polymeric Cables:
High-quality XLPE cable has been manufactured for over 25 years. IEC 502 (revised in 1998 as IEC 60502) covered this type of cable and was first issued in 1975. A comparable UK standard BS 6622 was issued in 1986 and revised in 1999. The following features are now available in MV XLPE cables and these are accepted by the majority of users in the electrical utility sector:
- copper or aluminium conductors
- semiconducting conductor screen and core screen (which may be fully bonded or easily strippable)
- individual copper tape or copper wire screens
- PVC, LSF or MDPE bedding
- copper wire collective screens
- steel wire or aluminium armour
- PVC, LSF or MDPE overs-heaths
Early experience in North America in the 1960s resulted in a large number of premature failures, mainly because of poor cable construction and insufficient care in avoiding contamination of the insulation. The failures were due to water treeing, which is illustrated in Fig. 1.3. In the presence of water, ionic contaminants and oxidation products, electric stress gives rise to the formation of tree-like channels in the XLPE insulation.
These channels start either from defects in the bulk insulation (forming bow-tie trees) or at the interfaces between the semiconducting screens and the insulation (causing vented trees). Both forms of trees cause a reduction in electrical strength of the insulation and can eventually lead to breakdown. Water treeing has largely been overcome by better materials in the semiconducting screen and by improvements in the quality of the insulating materials and manufacturing techniques, and reliable service performance has now been established.
The UK electricity supply industry gradually began to adopt XLPE-insulated or EPR-insulated cable for MV distribution circuits from the early 1990s in place of the PILC or PICAS cables. Each distribution company has specified the best construction for its particular needs. An example of the variation between companies is the difference in practice between solidly bonded systems and the use of earthing resistors to limit the earth fault currents.
In the former case, the requirement might typically be to withstand an earth fault current of 13 kA for three seconds. In the latter case, only 1 kA for one second might be specified and the use of single-core cable with a copper wire screen in place of a 3-core cable with a collective copper wire screen or SWA is viable. Different cross-sectional areas of copper wire screen may be specified depending on the earth fault level in the intended installation network.
The majority of specific designs being used by the UK electricity supply industry are now incorporated into BS 7870-4.10 (for single core) or BS 7870-4.20 (for 3-core). Examples of XLPE cable designs being used or considered by the UK distribution companies are shown in Figs 1.4, 1.5 and 1.6. The latter shows the most commonly adopted design for 11 kV networks, with a similar design used for 33 kV networks although with stranded copper conductors due to the higher load transfer requirements. The MDPE sheaths are specified for buried installations, with LSF used for tunnel applications.
Source: Electrical Power Engineers