SCFF cable is the type that preceded XLPE (cross-linked polyethylene) cable. SCFF cable is insulated with layers of lapped paper or laminated polypropylene paper (LPP) tapes and is completely impregnated with a low viscosity dielectric fluid (a synthetic, low viscosity hydrocarbon). SCFF cables were originally impregnated with a blend of mineral oil and were then called low pressure, oil-filled cables. The first 400 kV 2,000 mm2 large conductor, long length cable circuits containing joints were installed in the UK in the late 1960’s in direct buried applications. A 400 kV 2,600 mm2 long length tunnel circuit was installed in the early 1970’s[12].
Figure 48. SCFF 525 kV 1,000 mm2cable commissioned in Grand Coulee Dam in 1976 Notable applications of SCFF cable at 500 kV and above are:
Grand Coulee Dam, Washington, in tunnels. Two manufacturers supplied the paper insulated 525 kV cables, Figure 48, which were installed in tunnels. The conductor sizes are 1,000 mm2and 1,250 mm2. The circuits were commissioned in 1976. Long term Proving Tests were performed[13] [14] [15]
.
Development tests on 750/1100kV paper insulated cables reported in 1979[16].
Vancouver Island. A subsea crossing of 39 km total length was commissioned in 1984[17], comprising two 1200 MW circuits of 525 kV, 1600 mm2 paper insulated submarine cables. A distributed temperature system was retrofitted to the cables and is described in a CIGRE 2006 technical paper[18].
Honshu-Shikoku Interconnecting Line[19] is a major land and bridge link across Tokyo Bay. The conductor size is 2,500 mm2 and it is insulated with LPP
installed in two stages in 1987 and 1993 and were commissioned in 1994. An LPP cable is shown in Figure 49.
Canada, IREQ, long term tests on 800 kV LPP cable with accessories. The conductor size was 2,000 mm2. The tests were performed in 1991.
On technical grounds SCFF cable is a prospective second choice to XLPE for the Edmonton region of Alberta as it has proven 500 kV and low temperature operating capabilities. SCFF cable is more forgiving of short term high temperatures than XLPE even though it may suffer shortened life in the longer term.
The low temperature capabilities are limited only by the temperature limit of the O-ring rubber seals in the accessories and by the temperature at which the standard hydrocarbon impregnant begins to thicken, this being around -40oC. The paper and LPP tapes are capable of operating at cryogenic temperatures (e.g. -270oC) in combination with a liquid helium or liquid nitrogen impregnant.
Figure 49. SCFF LPP 2,500 mm2cable, similar to that commissioned in Japan in 1994
The substitution from SCFF cables to XLPE cables at 400 kV and 500 kV first began in the late 1980’s for short cable circuits with small conductor sizes (less than 1,000mm2) and without joints and in the mid 1990’s for long cable circuits with large conductors (1,200mm2 and greater) and with joints. The number of SCFF cable systems selected by utilities for new applications fell worldwide to minimal
Although deceptively simple in construction, modern EHV XLPE cables have only been made possible by major developments in dielectrics science[20] [21], polymer chemistry, clean materials, manufacturing equipment and accessories[22].
Figure 50. 500 kV XLPE cable
XLPE has a combination of properties that have led to its development for high voltage, high power cable applications. The base compound is LDPE (low density polyethylene). Polyethylene is a thermo-plastic which upon heating starts to soften at 60oC to 70oC and which at 107oC changes from a white, semi-crystalline solid to a low viscosity transparent liquid. . When in liquid state a tube of it can be
cooled to return to its original solid, white, semi-crystalline state. The cross-linking process permits the safe operating temperature of the cable insulation to be raised to 90oC, so that it can competitively carry similar levels of power to the SCFF cable. Additionally, some of the electrical properties of XLPE are superior to those of SCFF insulation, such as low power loss, which further improves its power carrying efficiency.
The main thrust of the development of EHV XLPE cable has been to increase the design stress of such that thickness of insulation, overall cable diameter, cable span length, and most importantly, service life reliability are competitive with those of the highly evolved paper and LPP cables. The development of high stress XLPE cable systems is described by Attwood et al in 1998[23]. Figure 51 shows how the cable design stress at the conductor shield (screen) and insulation shield have been increased at higher system voltages. A key design parameter for the accessories is the stress at the cable insulation shield.
Different manufacturers select different design stresses, the value in Figure 51 for a 500 kV cable, being approximately 7.5 kV/mm. This is similar to the average of the design stress values proposed by the manufacturers who participated in the 500 kV Study Project.
Figure 51. Increase of cable shield stresses at higher transmission voltages
XLPE cable has now supplanted the SCFF type at all system voltages up to and including 500 kV.
Numerically, there are now many service applications of XLPE cable at 400 kV, although individual times in service are comparatively short compared to SCFF cable.
2000.
2000
500 kV Shinkeiyo-Toyosu Project in Tokyo [27] [28] [29]
, comprising a 40 km long double circuit length with a conductor size of 2,500 mm2. The circuits were installed in a tunnel and commissioned in 2000. This is the first 500 kV XLPE circuit with joints and is still the longest EHV cable circuit in the world.
2004
400 kV Barajas Airport[30] in Madrid, comprising a 12.8 km double circuit length with a conductor size of 2,500 mm2. The circuits were installed in a tunnel.
2005
400 kV Elstree-St Johns Wood[31] [32] [33]
, London comprising a 20 km single circuit length with a conductor size of 2,500 mm2. The circuit was installed in a tunnel.
Progress in the adoption of 400 kV XLPE cable projects in Europe is recorded in Jicable technical papers in 2003[34]and 2007[35].
4.4 Advantages of extruded cross-linked polyethylene cables