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Electricity Pylons
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Why do we have an urban sprawl of these pylons littering the countryside when they could be run in insulated underground tubes? Surely it cannot be the cost as pipes would be far cheaper.
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I can provide an outline of why laying cables underground is often not feasible, but this is a hell of a question to answer in detail and I'll probably miss a few things out.
To begin with, it's not the cost of the underground pipes that's prohibitive - it's the cost of laying them. Some years ago in my home town, a steelworks bought a state-of-the-art electric arc furnace to melt scrap metal. The setup cost just over �30 Million when it was bought. To power the arc furnace, a 275KV supply voltage was required and the local National Grid distribution network could not supply a dedicated supply of that nature.
The steelworks paid the National Grid for the erection of well over a hundred pylons between a power station in the next town, some thirteen miles away, to supply the power required. The cost was phenomenal and came to around a half of the cost of the arc furnace. However, a costing was also made for laying the cables underground, at the suggestion of environmentalists. Due to the nature of the terrain, old marshland etc en route, the predicted amount for laying cabling underground was �62 million.
So cost is the first factor.
(continued)
To begin with, it's not the cost of the underground pipes that's prohibitive - it's the cost of laying them. Some years ago in my home town, a steelworks bought a state-of-the-art electric arc furnace to melt scrap metal. The setup cost just over �30 Million when it was bought. To power the arc furnace, a 275KV supply voltage was required and the local National Grid distribution network could not supply a dedicated supply of that nature.
The steelworks paid the National Grid for the erection of well over a hundred pylons between a power station in the next town, some thirteen miles away, to supply the power required. The cost was phenomenal and came to around a half of the cost of the arc furnace. However, a costing was also made for laying the cables underground, at the suggestion of environmentalists. Due to the nature of the terrain, old marshland etc en route, the predicted amount for laying cabling underground was �62 million.
So cost is the first factor.
(continued)
In situations where HV cables are buried underground, the National Grid lays down strict rules about how they are treated. To start with, although the cables are laid at a minimum depth of 1.1 metres underground, they specify to the landowner that under no circumstances is soil etc to be piled on top because it will hinder maintenance.
They also dictate that piling nearby with a view to building structures is prohibited as this may disturb and harm the cables. No trees/shrubs are allowed to be planted three metres from the position of cable ducting as the roots may disturb or penetrate the ducting. In practice, this means that no new houses may be built in the vicinity of a HV underground cable, which is a concept very unappealing to developers and town hall planners.
If a developer really wants to build on a site near an underground HV cable, they have to bear the cost of rerouting the cable. It seems that the National Grid tend to prefer pylons in these circumstances, often citing safety reasons. There are very few developers with the money to pay for a pylon array, and as you can imagine, it�s not done very often.
(continued)
They also dictate that piling nearby with a view to building structures is prohibited as this may disturb and harm the cables. No trees/shrubs are allowed to be planted three metres from the position of cable ducting as the roots may disturb or penetrate the ducting. In practice, this means that no new houses may be built in the vicinity of a HV underground cable, which is a concept very unappealing to developers and town hall planners.
If a developer really wants to build on a site near an underground HV cable, they have to bear the cost of rerouting the cable. It seems that the National Grid tend to prefer pylons in these circumstances, often citing safety reasons. There are very few developers with the money to pay for a pylon array, and as you can imagine, it�s not done very often.
(continued)
In addition, when HV cables are underground in ducting, special tiles are placed above the ducting, which will withstand a certain amount of impact and will protect the cable concerned. However, there have been cases where a JCB has sliced through such a cable with devastating consequences. This is obviously another reason why pylons are preferred.
Voltage drop is always an issue when electricity travels distances. It�s even important in a household environment when using 6mm or 10mm cable to wire an electric shower as the electricity regulations provide tables showing the drop over given distances. It�s a lot worse in HV cabling. Remember also that enclosed cabling gets hot. Cabling suspended in air remains at a more or less constant temperature and little is lost as heat. There's no doubt that energy lost as heat is inefficient.
I think that covers the gist of it. Just don't start me off on the dangers of living in close proximity to HV Pylons!
Voltage drop is always an issue when electricity travels distances. It�s even important in a household environment when using 6mm or 10mm cable to wire an electric shower as the electricity regulations provide tables showing the drop over given distances. It�s a lot worse in HV cabling. Remember also that enclosed cabling gets hot. Cabling suspended in air remains at a more or less constant temperature and little is lost as heat. There's no doubt that energy lost as heat is inefficient.
I think that covers the gist of it. Just don't start me off on the dangers of living in close proximity to HV Pylons!
As well as theprof's excellent answer I guess the following has to be considered.
Pylons can easily go over roads, motorways, railway lines, rivers, hills and valleys, and housing estates.
Now suppose you had to route the cables UNDER the ground, you would have to dig under roads, railways lines, rivers, hills and valleys, and houses.
Plus you would also need to get permission to rent or buy ALL the land that the cable was going to go under, and probably a fair bit either side.
Plus you would need to make sure you avoided all the existing cables and pipes such as electricity, gas, telephone, sewers, and all the other things that go under ground.
Laying new electricity cables underground is far from an easy option.
Pylons can easily go over roads, motorways, railway lines, rivers, hills and valleys, and housing estates.
Now suppose you had to route the cables UNDER the ground, you would have to dig under roads, railways lines, rivers, hills and valleys, and houses.
Plus you would also need to get permission to rent or buy ALL the land that the cable was going to go under, and probably a fair bit either side.
Plus you would need to make sure you avoided all the existing cables and pipes such as electricity, gas, telephone, sewers, and all the other things that go under ground.
Laying new electricity cables underground is far from an easy option.
Just a thought on the profs heat loss comment, not a criticism.
Surely the heat is generated by the resistance in the cable and would be the same.
Don�t underground cables get hot because the heat cannot get away?
Overhead cables stay relatively cool because the heat is dissipated.
Is it to do with the extra resistance due to the temperature?
Or is it because of the density of the conducting medium?
Good answer Prof.
Surely the heat is generated by the resistance in the cable and would be the same.
Don�t underground cables get hot because the heat cannot get away?
Overhead cables stay relatively cool because the heat is dissipated.
Is it to do with the extra resistance due to the temperature?
Or is it because of the density of the conducting medium?
Good answer Prof.
Thanks all for your kind responses to my answers.
I think I'd better admit that I'm a professor of biochemistry not power engineering. However, my dad was the chief electrical engineer at the steelworks I mentioned and I worked there myself between uni terms (semester is a horrible word!) when the modernisation scheme was underway.
Veryhelpfulguy has underlined the practical difficulties of laying cables underground. In the steelwoks scheme, this would have meant issuing thousands of property compulsory purchase orders, excavating beneath a main railway line a number of times and purchasing part of one of the biggest British Steel (Corus) works in the UK. When you add in the cost of diverting or working around gas pipelines, telephone cables and sewerage piping, it was a non-starter.
(continued)
I think I'd better admit that I'm a professor of biochemistry not power engineering. However, my dad was the chief electrical engineer at the steelworks I mentioned and I worked there myself between uni terms (semester is a horrible word!) when the modernisation scheme was underway.
Veryhelpfulguy has underlined the practical difficulties of laying cables underground. In the steelwoks scheme, this would have meant issuing thousands of property compulsory purchase orders, excavating beneath a main railway line a number of times and purchasing part of one of the biggest British Steel (Corus) works in the UK. When you add in the cost of diverting or working around gas pipelines, telephone cables and sewerage piping, it was a non-starter.
(continued)
Thanks Oldboy913. I don't profess (no pun intended!) to know much about this. Underground cabling does get hot because the heat cannot get away as efficiently and air cooled cables are more efficient at dissipating heat. This is what I meant when I said that cabling suspended in air remains at a more or less constant, cooler temperature. It's the same principle in a domestic situation, where the IEEE wiring regulations show that a surface-mounted TW/E cable can carry more current than a cable enclosed in conduit.
It seems that officially underground cabling is currently costed at anything from ten to twenty times the cost of overhead transmission. One of the reasons for this is that the cable is hugely more expensive to manufacture with its layers of insulation and armouring and also because it generally needs oil-filled ducting within the sheath to dissipate the heat. The oil is cooled at special reservoir joints placed at regular distances along the cable run. I also understand that underground cabling is lossy and therefore inefficient, because of relatively high parasitic capacitance between the closely bundled conductors and the earthed outer sheath. It seems that even a modest cable capacitance can absorb and waste a lot of power at high AC voltages.
(continued)
It seems that officially underground cabling is currently costed at anything from ten to twenty times the cost of overhead transmission. One of the reasons for this is that the cable is hugely more expensive to manufacture with its layers of insulation and armouring and also because it generally needs oil-filled ducting within the sheath to dissipate the heat. The oil is cooled at special reservoir joints placed at regular distances along the cable run. I also understand that underground cabling is lossy and therefore inefficient, because of relatively high parasitic capacitance between the closely bundled conductors and the earthed outer sheath. It seems that even a modest cable capacitance can absorb and waste a lot of power at high AC voltages.
(continued)
-- answer removed --
The cables used in overhead transmission are made of aluminium and are bunched in groups of two or four to reduce corona discharge and the accompanying risk of electromagnetic interference. As aluminium on its own would stretch and break, the cables have steel cores for strength. Overhead cables are not insulated since they are placed out of reach and insulation would only add to the weight. Because of this lack of sheathing , parasitic capacitance does not arise.
It�s acceptable in the industry to lose around 1.5% of the power during transmission via heating of cables etc, but when the transmission voltage is 275KV or 400KV, the current is reduced for a given power and therefore the I2R losses are reduced. (I2R = Heat loss ie cable current squared, multiplied by the cable resistance will give the heat loss in watts). In practice, this means that the overhead cable diameter can be reduced and the overall efficiency of the system is increased.
It�s acceptable in the industry to lose around 1.5% of the power during transmission via heating of cables etc, but when the transmission voltage is 275KV or 400KV, the current is reduced for a given power and therefore the I2R losses are reduced. (I2R = Heat loss ie cable current squared, multiplied by the cable resistance will give the heat loss in watts). In practice, this means that the overhead cable diameter can be reduced and the overall efficiency of the system is increased.
I think we are creating a problem where none exists. No towns or cities have pylons and are laid underground.
Fields would create less of a problem. Just 50 yards from me they have laid part of a national grid of gas pipes. To mark their direction marker posts are laid at intervals. They are put about 6' down.
Fields would create less of a problem. Just 50 yards from me they have laid part of a national grid of gas pipes. To mark their direction marker posts are laid at intervals. They are put about 6' down.
Sorry Kwicky, but virtually every town or city in the UK has pylons within a few miles.The majority of pylons end at grid switching stations which are sited near large centres of population and/or industry then drops the voltage to 33KV for feeding into the primary distribution system. These grid switching stations are on the outskirts of most towns and are normally fed by pylons
Major users and large industrial estates are supplied at 33KV from substations along with the Intercity rail network which has the voltage dropped to to 25KV. The substation also drops the 33KV to 11KV, which is known as the secondary system. It is this 11KV network that is mostly underground in urban areas not the 275KV or 400KV primary system. The cables you have seen carry this 11KV NOT the 275KV or 400KV of the primary network, which require the pylons.
This 11KV system is dropped to the domestic 230V single-phase and the 415V three-phase supply for commercial premises.
The 11KV supply cannot feasibly be carried to every outlying village and remote farm underground. The network is instead carried by wooden poles that are a familiar sight in the country. Each village normally has a pole transformer mounted between a pair of poles to reduce the voltage to 230V and/or 415V as required.
(continued)
Major users and large industrial estates are supplied at 33KV from substations along with the Intercity rail network which has the voltage dropped to to 25KV. The substation also drops the 33KV to 11KV, which is known as the secondary system. It is this 11KV network that is mostly underground in urban areas not the 275KV or 400KV primary system. The cables you have seen carry this 11KV NOT the 275KV or 400KV of the primary network, which require the pylons.
This 11KV system is dropped to the domestic 230V single-phase and the 415V three-phase supply for commercial premises.
The 11KV supply cannot feasibly be carried to every outlying village and remote farm underground. The network is instead carried by wooden poles that are a familiar sight in the country. Each village normally has a pole transformer mounted between a pair of poles to reduce the voltage to 230V and/or 415V as required.
(continued)
Fields are not less of a problem. Every inch of land in the UK is owned by someone and licences are held to bury cables underground. What happens on the surface above these cables cannot be predicted in the long term and I discussed some of these issues above. I live in a village just outside a city and the city is slowly expanding into green-field sites. Nothing's safe nowadays - today's field is tomorrow's housing estate. The National Grid are simply not prepared to take the risk of burying primary network cables in land where the future use cannot be predicted.
Gas pipes are less of a problem as supplies are easier to reroute if absolutely necessary. Gas piping contains complex valves and within limitations, there is usually more than one way of getting gas from A to B by rerouting all the way to the consumer. The National Grid doesn't have this luxury as the secondary network is usually the responsibility of the appointed local electricity company.
Incidentally, on thinking about, I may have misunderstood the first sentence of your original post . Urban means "of or relating to a city or a town" yet you discuss pylons in the context of the countryside. Could I ask you to clarify this for me? Thanks.
Gas pipes are less of a problem as supplies are easier to reroute if absolutely necessary. Gas piping contains complex valves and within limitations, there is usually more than one way of getting gas from A to B by rerouting all the way to the consumer. The National Grid doesn't have this luxury as the secondary network is usually the responsibility of the appointed local electricity company.
Incidentally, on thinking about, I may have misunderstood the first sentence of your original post . Urban means "of or relating to a city or a town" yet you discuss pylons in the context of the countryside. Could I ask you to clarify this for me? Thanks.
If I could just add that cabling buried underground will always be a major problem regardless of whether its a field or otherwise as I said earlier due to the prohibitive cost.
I've just had a phone call from a friend who's a reader in power engineering at my university. To satisfy my own curiosity, I asked him about the relative cost of overhead versus underground HV transmission. He told me that at present it's �3 million per mile underground and �250,000 per mile via pylons.
I think that says it all.
I've just had a phone call from a friend who's a reader in power engineering at my university. To satisfy my own curiosity, I asked him about the relative cost of overhead versus underground HV transmission. He told me that at present it's �3 million per mile underground and �250,000 per mile via pylons.
I think that says it all.
Kwicky, everyone who has replied to your question has done nothing but give you the raw facts about overhead versus underground transmission. On looking through the posts, I can't find a single sentence showing that any poster prefers one over the other or any sign of a NIMBY feeling on this matter. If you think otherwise, could you point it out to me please.
Yes, the reality is that the National Grid prefers overhead to underground transmission because of the cost. In fact, I'd go as far as saying that it's about 90% of the reasoning behind it. The rest is because of the safety aspects of laying the cable underground and the cost of the very expensive underground cable including tunnelling through hillsides and just about everything in between.
There's no "seems" about profits coming first. It's vital and just about the only thing in the minds of the people in charge of the National Grid. They are in business to make a profit after all. The safety features that the National Grid have to include with every pylon erected is just down to them complying with the law. On the other hand if the country was prepared to pay a hell of a lot more for electricity, yes the money could be returned to the National Grid and there would be very few pylons spoiling the view. Would you be prepared to pay a minimum of twelve times the cost per unit of electricity than you do at present to fund such a scheme?
Incidentally, I'm personally not in favour of having pylons on my doorstep and believe me, over my years as a biologist, I've beaten the drum often enough over environmental and ecological issues. But, I'll tell you for nothing that I wouldn't support a transfer to underground cabling if it hit me in my pocket to that extent. That's the reality for us all.
Yes, the reality is that the National Grid prefers overhead to underground transmission because of the cost. In fact, I'd go as far as saying that it's about 90% of the reasoning behind it. The rest is because of the safety aspects of laying the cable underground and the cost of the very expensive underground cable including tunnelling through hillsides and just about everything in between.
There's no "seems" about profits coming first. It's vital and just about the only thing in the minds of the people in charge of the National Grid. They are in business to make a profit after all. The safety features that the National Grid have to include with every pylon erected is just down to them complying with the law. On the other hand if the country was prepared to pay a hell of a lot more for electricity, yes the money could be returned to the National Grid and there would be very few pylons spoiling the view. Would you be prepared to pay a minimum of twelve times the cost per unit of electricity than you do at present to fund such a scheme?
Incidentally, I'm personally not in favour of having pylons on my doorstep and believe me, over my years as a biologist, I've beaten the drum often enough over environmental and ecological issues. But, I'll tell you for nothing that I wouldn't support a transfer to underground cabling if it hit me in my pocket to that extent. That's the reality for us all.
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