Covered conductors used in medium voltages are covered with a 2.3 mm layer of black weather resistant XLPE. At higher voltages a single layer covering is not sufficient due to the higher electrical stresses. This means that when operating above medium voltage levels the partial discharge phenomena inside the covering has to be consid- ered, especially in situations where fallen trees lie on the line or phase conductors are touching each other. Therefore, a new covered conductor type was developed which has a thin layer of extruded semi-conducting XLPE compound controlling the electric field at conductor surface.
The main part of the HVCC covering consists of extruded water–tree retardant XLPE insulation compound. Unfortunately its capability to sustain ultra-violet light, usually from solar radiation, is not very good, which is why there has to be an extra layer applied that guards the main covering against UV light. This outermost layer consists of weather and track resistant black XLPE insulation compound.
All three layers, conductor screen, main covering and outer weather resistant coating, are extruded simultaneously during a highly controlled completely dry cur- ing and cooling process. Thus the triple extruded XLPE covering forms one-piece completeness with various functions.
HVCC SAX consists of watertight, stranded and compacted all aluminium alloy conductor covered by a triple extruded XLPE layer.
A schematic picture of HVCC SAX for voltages from 66 to 132 kV is shown in Figure 9.4.
Covered conductors can have one, two or three sheath layers at medium voltage – at 132 kV the conductor may have five layers. The presence of the sheath (a thin layer of XLPE or HDPE covering – typically 2.3–3.3 mm) allows:
• improved contact outage resistance compared with bare wire • no clashing problems
• reduced phase spacing • reduced wildlife problems.
Figures 9.5 and 9.6 show that the presence of the sheath allows the very much closer phase spacing and compact construction typical of CC lines.
As the sheath is insulating (but not insulated) there will be a low-level charging current flowing along its surface. This arises because the sheath forms an insulating
1 2 3 4
Figure 9.4 HVCC SAX conductor made in Finland
1 watertight, stranded and compacted aluminium alloy conductor,
cross-sectional areas available from 120 to 355 mm2
2 extruded semi-conducting XLPE screen
3 extruded water–tree retardant XLPE insulation compound, thick-
ness 2.5…5.5 mm
4 extruded layer of weather and track resistant black XLPE insulation
Bare, insulated and covered conductors 153
air +PE
V1 V2
Figure 9.5 The CC sheath allows closer phase spacing
Figure 9.6 The typical close spacing and compact construction of Norwegian CC lines which prompted the first use of CC in the UK
layer between the high-voltage conductor (metal) and the pin or post insulator to earth. This current will normally be less than 0.3 mA. Its characteristics are: • current inevitably flows phase–phase or phase–ground
• must be low to reduce tracking and erosion, especially under polluted conditions • metal helical ties form an intermediate electrode and can cause discharge problems
(corona) at the ends (if bare).
In cases of surface damage or local pollution, this current can increase sufficiently to cause surface tracking and eventual sheath and subsequent conductor failure. The most common CC in the UK is the single sheath version from Finland shown in Figure 9.7.
The characteristics are: • single layer
• typically low-density polyethylene
Figure 9.7 Single sheath compacted covered conductor
A B
A –2 mm PE
B –2 mm HDPE
Figure 9.8 Double layer sheathed CC
A B C
Figure 9.9 Triple layer sheathed CC
• lower impulse strength than two- and three-layer designs
• provides some resistance to outages caused by tree and wildlife contact. The impulse strength of a single layer of XLPE sheathed CC is around 115 kV. Also, the electrical stresses caused by trees on the line or conductors on the cross-arm can erode the sheath in periods from months to minutes depending on the system voltage. Surface voltage stresses are increased if porcelain rather than polymeric insulators are used due to the difference in the dielectric constant of the porcelain (three times that of polymeric insulators). The use of floating helical fittings can also cause surface tracking of an XLPE sheathed conductor in coastal environments especially if the carbon content is around three per cent (which is a common practice). This effect can be reduced by the use of polymeric insulators or switching to an HDPE sheathed conductor that contains substantially less carbon black.
Figure 9.8 shows a double layer CC that is used in the USA but not currently in the UK. This type has higher impulse strength than the single layer.
Figure 9.9 shows a common CC used in Sweden and in some areas in the UK. It is a triple layer with:
• semi-conducting layer • PE layer
Bare, insulated and covered conductors 155
ground ground
Figure 9.10 Reduction of voltage stress in an uncompacted conductor by the use of a semi-conductive layer waterblocking compound semi-conductive layer pure XLPE XLPE with CB
Figure 9.11 Triple-layer uncompacted CC
The semi-conducting layer is intended to reduce voltage stresses. The metal strands may be compacted or uncompacted.
The voltage stress is inversely proportional to the strand radius and so a lower voltage stress will occur because of larger effective radius of the whole conductor with the semi-conductive layer as shown in Figure 9.10.
A cross-section of the triple-layer uncompacted CC is shown in Figure 9.11. This conductor had an internal mastic layer to restrict moisture travel along the con- ductor. Uncompacted conductors have a higher susceptibility to moisture travel than the compacted versions that often use a powder or water-swellable tape to restrict moisture travel. One version of this conductor uses a totally re-cyclable green HDPE outside layer.
9.5
Spacer cable
Spacer cable systems are essentially three CC phases in a polymeric support cradle supported by a messenger cable. Figure 9.12 shows the support system at a pole and Figure 9.13 shows the cable strung at a test site in the UK. Basically, the system has: • a messenger supported three-layer cable construction in a close triangular
configuration
spacer three-layer cable messenger ring tie tangent bracket
Figure 9.12 Triple sheathed version of the spacer cable (courtesy Hendrix Inc, USA)
Figure 9.13 Spacer cable strung at a UK test site along with other CC and bare wire systems
Bare, insulated and covered conductors 157
• the electrical strength to prevent faults due to phase-to-phase or phase-to-ground contact, tree contact or animal contact
• a complete co-ordinated system including cable, messenger, spacers, insulators and hardware.
The conductor may be of the double or triple sheathed CC versions discussed in the previous section. The system is used widely in the USA and parts of Canada and is being marketed in Europe. It has been tested at experimental sites in the UK and some network trials are planned for 2005.
9.6
Aerial cable systems
Aerial cable systems are basically cables that can be strung overhead and run under- ground and underwater. Such systems obviate the need for OHL/cable junctions and have a very low susceptibility to lightning. They:
• have fully shielded three-core power cables • have excellent contact resistance
• go overhead, underground, underwater • have no cross-arms
• have no OHL/underground cable junction • are made in USA, Sweden and Finland.
Figure 9.14 shows the structure of the Swedish version that is now being used in many parts of the UK. The cable does not use a support wire or messenger, as it is totally self-supporting. It has a high impulse strength of 400 kV and uses an earthed screen. Figure 9.14 shows the aluminium conductor version and Figure 9.15 shows a smaller copper conductor version. The conductor is held up by hooks attached to wood poles in a similar fashion to LV ABC.
Possible uses of aerial cable are: • replacement of CC overhead lines • distribution cable in the countryside
dead end spiral sheath
screen XLPE insulation
conductors
Figure 9.14 The Swedish aerial cable system (courtesy Ericsson Technology Networks Ltd)
Figure 9.15 The copper conductor version of the aerial cable system
• areas with safety implications with overhead lines near buildings • temporary installations
• underwater cable
• supply lines that have demand sensitivity • areas with climatic problems.
Cable systems are more complicated to joint than single-phase OHL conductors and so aerial cable is more suitable for long runs where there are few transformers and spurs lines. It is commonly strung together with LV cable on the same poles.
9.7
PVC conductors
Before 1994 the only covered conductors in widespread use in the UK above 1000 V AC were PVC covered. PVC is one of a family of sheath materials known as thermoplastics. The characteristics of these materials are:
• no chemical bonding of molecules
• many materials available (PVC, polyolefins, polyethylene) • PE makes up the vast majority of the volume
• a properly designed PE overhead cable will provide long life and excellent outage resistance
• thermoplastic PE cable design is available to provide the same operating temperature rating as XLPE
Bare, insulated and covered conductors 159
Figure 9.16 Surface tracking on an XLPE covered conductor used with porcelain insulators
The XLPE material is a typical thermoset material. Early PVC covered conductors were susceptible to UV degradation and also responsible for turning water ingress into an acid that then dissolved the metal conductor. PVC sheathed conductors are not normally used now in the UK except for specific local purposes, the developments in thermosets having superseded their use.
9.8
Tracking
Tracking has been mentioned as a problem with covered conductors with high carbon black contents in the sheath material. The carbon black is there as a very effective and cheap UV inhibitor. HDPE materials tend to use the more expensive titanium dioxide as an inhibitor. This does not have the same tendency towards tracking. Tracking results from surface voltage stresses typically between insulation piercing connectors (IPCs) and helical fittings that are not at network voltage. In a highly salt polluted environment, such as within 10 km of the UK coastline, tracking can reduce conductor lifetimes to a few years (Figure 9.16). There are several ways to reduce tracking problems:
1 reducing voltage stress (using polymeric instead of porcelain insulators 2 connecting helical ties with any insulating piercing connectors (IPCs)) 3 using crimp connectors instead of electrically floating helical ties 4 using reduced or zero carbon content sheath materials.
9.9
Keeping the power on
Figures 9.17 and 9.18 show real examples of how the power has stayed on when sheathed conductors had been used. Figure 9.17 shows aerial cable where a tree has fallen on the line. The power stayed on and no repairs were required after the tree was removed. Figure 9.18 shows a covered conductor line where a snow-laden branch is
Figure 9.17 A storm in Sweden brings a tree down on an aerial cable line (courtesy Ericsson Technology Networks Ltd)
Figure 9.18 A snow-laden branch resting on a double circuit three-phase CC line in Finland (courtesy Pirelli Cables & Systems Oy)
Bare, insulated and covered conductors 161
resting on a three-phase line. If this had been a bare wire line there is no doubt that the power would not have stayed on.
9.10
Novel conductors
9.10.1
Definitions
This section introduces some novel concepts in conductor design [1]. Although ini- tially intended only for transmission lines, they are increasingly being investigated for use at 132 kV and could be used in the future on 66 and 132 kV wood pole lines. The abbreviations used in this section are defined below:
ACSS – aluminium conductor coated steel supported
GTACSR – gap-type TAL aluminium alloy conductor, steel reinforced HS steel – high strength steel core wires for ACSR
Invar steel – a steel core wire made with high nickel content to reduce the thermal elongation coefficient
Knee-point temperature – the conductor temperature above which the aluminium strands of an ACSR conductor have no tension or go into compression
TACIR – TAL aluminium alloy conductor reinforced with an invar steel core TACSR – TAL aluminium alloy conductor reinforced by a conventional stranded
steel core
TAL – an aluminium zirconium alloy that has stable mechanical and electrical properties after continuous operation at temperatures of up to 150◦C
TW conductor – a bare overhead stranded conductor wherein the aluminium strands are trapezoidal in cross-section
ZTAL – an aluminium zirconium alloy that has stable mechanical and electrical properties after continuous operation at temperatures of up to 210◦C
ZTACIR – ZTAL aluminium alloy conductor reinforced by an invar steel core.
9.10.2
Shaped-strand conductors
Overhead line conductors are normally constructed from helically wound strands of circular cross-section. This gives a conductor cross-section containing large inter- strand voids, with∼20 per cent of the total cross-sectional area of the conductor being air. By using strands with a trapezoidal shape, conductors can be constructed with a higher proportion of metal within their cross-section. Compacted conductors can be homogenous with all strands except the centre strand being of trapezoidal shape, or non-homogenous with a round-stranded, steel core surrounded by trap- ezoidal aluminium strands. However, the strands may also be ‘Z’ shaped and so effectively lock together.
Shaped-strand conductors have a larger aluminium area and thus lower resist- ance than a normal round strand conductor with the same outside diameter. When re-conductoring an existing line with shaped-strand conductor, the increased weight of the conductor will result in slightly higher support structure loads, but climatic loads due to wind and/or ice will not be increased, as these are a function of diameter.
For wind-only loading conditions, loads may actually be lower, due to their lower drag coefficients at high wind speeds. One example of a shaped-strand conductor that achieves a low drag coefficient is one that has an oval cross-section, the orientation of which varies along its length, giving a spiral–elliptic shape [2].
Shaped-strand conductors have also been shown to possess slightly better vibra- tion characteristics due to the higher surface area of the contacts between strands of adjacent layers, which results in lower inter-strand contact stresses [3, 4].