Using overhead distribution lines to carry fibre optic cables
By Jim Rowe, AFL
Every village and small community in the UK is connected to the electricity distribution network, and in most cases this connection is carried on wooden poles stretching out across fields to the nearest electricity substation. Usually these poles carry two or three electricity conductors and would potentially make an unobtrusive and convenient way to install fibre-optic cables to carry broadband connectivity – providing power and Internet over the same poles.
[[wysiwyg_imageupload:17:height=331,width=220]]Electricity is best carried over long distances at high voltages, and then the voltage is reduced in steps as the power is brought closer to houses and businesses. The National Grid operates at 400,000V with conductors carried on massive steel pylons from power stations to primary substations where it is converted to 132,000V for regional distribution. The next set of sub-stations converts the power to 66,000 or 33,000V. Voltages of 66kV and above are usually carried on steel lattice towers and voltages of 33kV and below are normally carried on lines supported by wooden poles. As the voltages come down, the sizes of the support structures get smaller, the number of conductors gets less and the height above ground level decreases.
The next step is to convert the electricity into 11,000V to make the connection to villages and then finally there is a step down to mains voltage for connection to individual properties. Often, all of these final links in the electricity distribution chain are carried on poles above ground with connections to houses at roof-top height. Properties built in the last 25 years will have all of their services connected underground because this has been planning policy since the 1970s, but even so few properties are more than 100m from an overhead electricity distribution line.
Power utility companies are big users of communications to operate and control their networks, and many of the bigger transmission lines already carry fibre-optic communications cables to provide connections between major sub-stations and control centres. Distribution lines radiating outwards from substations towards towns, villages and consumers have, for the most part, not been equipped to carry communications cables because there has been no requirement for the power companies to do so in these parts of their networks.
The same technologies that have been used since the 1980s to add fibre-optic cables to large transmission lines can also be used on medium voltage and low voltage distribution lines, and this has attracted the attention of organisations that are planning to build broadband networks. These lines reach right into the target communities, providing both the means of connecting to the broadband service providers’ infrastructure and also the means of distributing connectivity to individual consumers within the community.
The key advantages of using overhead electricity distribution lines to carry cables providing broadband connectivity can be summarised in three distinct areas: speed, security and cost
- Speed: It is always much, much quicker to install fibre-optic cable by attaching it to poles than it is to dig trenches to bury it underground. Directional drilling or ploughing are alternative ways of installing underground cable, but these are also slow and expensive compared to installation on overhead lines. Circumstances will vary according to the time of year with factors including weather conditions, whether or not crops or animals are in the fields and what the ground conditions are like underfoot; however it is generally possible to install at least 1km of fibre optic cable a day on overhead power lines and up to 5km per day is possible in favourable circumstances.
- Security: is a key concern in any fibre-optic cable installation. Cables have been installed on overhead power lines since the very early 1980s and have developed an excellent reputation for security and reliability over that time. Power utilities use these cables to carry critical communications for control of the electricity network. Fibre-optic cables installed above ground are not subject to ‘dig-ups’ which is the biggest cause of cable damage in the UK. Cables that are installed as part of the electricity infrastructure are protected by the proximity of power conductors, which provide protection against theft and vandalism.
- Cost: The higher unit cost of aerial cables compared to underground cables is more than offset by the much lower cost of installation and therefore aerial cables have the lowest total cost. Aerial cables have much higher installation rates and so networks are built much more quickly, begin to provide services earlier and so have quicker returns on investment. Put another way, with reduced initial costs and earlier in-service dates, aerial cables have shorter pay back times than underground networks.
Several technologies are available to add fibre-optic cables to overhead power lines: ADSS, OPPC and AccessWrap. The choice of which to use will depend upon the type of overhead line.
ADSS (All-Dielectric Self-Supporting) is the simplest concept for aerial fibre-optic cable: it is an underground fibre optic cable made stronger to allow it to be installed by attaching it to a series of poles. The cable needs to be physically strong because it will be supported only at each pole along the route and will have to support its own weight across the half-span on each side of the pole. This is in contrast to an underground cable which is fully supported inside a duct or in a back-filled trench along its whole length.
In addition to its own weight, ADSS cable must support the extra loads imposed by wind pressure and by the build up of ice when this is problem in exposed locations. These extra loads can be significant and require carefully designed clamps to spread the mechanical strain over several metres of cable at each pole to prevent any risk of damage.
ADSS cables have the advantage that they are completely independent of the electricity supply network, even though they are installed on the same poles. Potentially the two networks can be owned, managed and maintained by different organisations, although there are safety issues when people carrying out installation and maintenance activities are working in close proximity to live electricity conductors. This will inevitably mean that communications technicians working on the fibre-optic cables will have to be trained and certified by the electricity industry to work on energised power lines.
The main concerns regarding the use of ADSS are related to the amount of load exerted on the supporting poles and the clearance between the ADSS cable and objects around it, be they trees close to the line, traffic or farm vehicles passing underneath or the electricity conductors on the line itself. Since the local landscape changes from line to line and since there are many different designs of poles in use, in some cases it will simply not be possible to install ADSS into a suitable location to provide a secure and reliable installation.
OPPC (Optical Phase Conductor) is a replacement electrical conductor that has optical fibres built into it as part of the manufacturing process. The fibres are inside the conductor, usually contained within a stainless steel tube. OPPC is installed on an overhead electricity line in place of one of the normal conductors.
OPPC replaces one of the normal conductors and therefore it adds nothing to the appearance of an overhead line and it does not affect the mechanical or electrical rating of the line. From this point of view, OPPC is the least obvious and most secure of all of the cable types. However it is also the technology that is most intimately associated with the electricity supply network as it physically forms part of this network. Any maintenance activity on either the communications or power network involving OPPC will have an impact on the operations of both networks.
OPPC is normally only installed as part of the construction of a new line or during the complete refurbishment of an existing line and so it is unlikely that OPPC will be specified by any organisation other than a power utility company.
AccessWrap is a technique that installs a fibre-optic cable onto an overhead electricity distribution line by wrapping it securely onto one of the power conductors. This is a scaled down version of the SkyWrap process that has been used since 1982 to install fibre-optic cables onto power transmission lines; the smaller, lighter AccessWrap machine is designed to work on power lines supported by wood or concrete poles and with conductors spaced only 0.5m apart.
The optical cable is supported by its host conductor and so it does not need to carry any of its own weight. Therefore it can be very small and this means it has little effect on the mechanical and electrical performance of the overhead line; it also has little impact on the appearance of the line. Installation is carried out using a special device which travels along the host conductor carrying a drum of optical fibre cable. The device rotates as it moves and wraps the cable under carefully controlled tension onto the host conductor at a pitch length of about three quarters of a metre. Clamps are used on each side of each pole to hold the cable in place on the conductor. The machine moves at about walking pace with about 15 minutes required at each pole to lift the machine onto the next span and put the clamps in place.
AccessWrap does not place extra load on the poles supporting the power line nor does it reduce the ground clearance under the line and these are major advantages over ADSS in some circumstances. However, AccessWrap is wrapped onto one of the power conductors and therefore it is much more closely associated with the power network than ADSS. Even so, evaluations carried out in the early part of 2011 by several utilities in UK and Ireland have shown that routine maintenance tasks on overhead distribution lines such as replacing insulators and transformers can be carried out without disturbing AccessWrap.
These products create an opportunity for power utilities to roll out communications networks on to their electricity distribution infrastructure, potentially connecting all the way through to the users’ premises and linking these to headends at major substations or regional control centres. This type of infrastructure may be required to provide the communications networks to support Smart Grids, and utilities may build these networks for this purpose only. However, once built, such networks would support other applications and could generate revenue opportunities in providing carrier services to third parties such as broadband service providers and mobile operators. The combination of Smart Grids that enable utilities to meet Green Agenda targets and access to additional revenue streams from existing assets may provide sufficient encouragement to utility companies for them to begin building these networks. If and when this happens, broadband connectivity would be extended to many isolated communities right across the country.