Removing pylons - Page 1
Forum section: electronics / general

author: MikeW
author_rank: Regular Contributor
timestamp: September 15, 2015, 03:40:46 pm
content: The high power side of things fascinates me so when I saw this story about pylon removal...http://www.bbc.co.uk/news/science-environment-34254392...I wondered about the technical challenges involved and the cost per km of doing this. Anyone have any knowledge in this area?

author: nfmax
author_rank: Super Contributor
timestamp: September 15, 2015, 04:14:18 pm
content: One of these lines is local to me - in fact I walked underneath it while hiking last week. Useless fact of the day: the new line will be buried on the brink of Deadman Bottom, only a mile or so from Sandy Balls.

author: IanB
author_rank: Super Contributor
timestamp: September 15, 2015, 04:34:41 pm
content: Quote from: MikeW on September 15, 2015, 03:40:46 pmI saw that story this morning. Burying power cables is expensive, especially the 400 kV National Grid kind. There are issues of land acquisition, heat removal, accessibility for maintenance and so on. From my previous reading on the subject the preferred approach in urban areas is to install the cables in a man-sized tunnel with regular access points along the way. See here for instance:http://www2.nationalgrid.com/UK/In-your-area/Projects/London-Power-Tunnels/What they will do in rural areas I am not sure. Probably dig a deep trench and do something similar.

author: dr_dan
author_rank: Contributor
timestamp: September 15, 2015, 04:59:40 pm
content: Overhead lines are open to the elements, which means they are air cooled quite effectively. Cooling underground cables is more of a challenge. I'm sure there are/were lines that are cooled by pumping oil around them to conduct the heat away. Obviously there is a cost to installing, maintaining, and running that cooling system.Underground cables also behave differently to overhead lines. On a like for like run between two points, the conductors in the buried cable are much closer together, increasing capacitance. This has to be taken into account as well when replacing an existing overhead line. The resulting change in network behaviour may require additional plant in order to maintain sufficient margin of stability - take a look at static var compensators.

author: SeanB
author_rank: Super Contributor
timestamp: September 15, 2015, 05:40:53 pm
content: Underground lines are both a lot more expensive to install and maintain, and have very high losses. You have massive capacitance between the inner core ( which has to be a lot bigger in CSA than the overhead as you are limited on surface temperature, to not melt the polyoleofin insulation used) and the outer shield per core, along with the massive capacitance relative to air, and the dielectric losses are huge. Thus you need to either use a pressurised line with a forced flow for cooling, or it has to be large enough that surface cooling to the surrounding fill keeps the inner core temperature low enough at maximum load.The capacitance and the dielectric loss ( not to mention dielectric absorption) means that the no load current is high, and you need compensation for it. To upgrade you have to dig a parallel trench and lay the same cable set again, while with an overhead line to upgrade you simply hang an extra cable set on the existing insulator set, or upgrade the insulators to heavier and put in the 6 cable bundle in place of the single.To maintain you have to check the cooling if pressurised, and provide maintenance for compressors, cooling units and associated equipment at regular intervals along the line. Overhead is pretty much walk along live and look, and do maintenance on it as needed to replace insulators that are degraded, or to splice in a new section to replace worn or damaged cable.

author: C
author_rank: Super Contributor
timestamp: September 15, 2015, 06:18:56 pm
content: The push for high voltage DC is due to capacitance and the dielectric loss ( not to mention dielectric absorption).With Off shore windmills where the wire is in water for colling, DC can be a better choice.C

author: woodchips
author_rank: Frequent Contributor
timestamp: September 15, 2015, 07:03:08 pm
content: Is there really nothing better to spend a huge sum of money on?Only good news is that since the cable loop area is less then the impedance will be lower. Wow!

author: Delta
author_rank: Super Contributor
timestamp: September 16, 2015, 07:38:09 am
content: Complete waste of money. Replacing an efficient, easily maintained system with a far less efficient, more difficult and more expensive one? Great idea, well done NIMBYs!PS. I genuinely don't mind the look of pylons, they've always been a feature of the landscape to me.

author: Mechanical Menace
author_rank: Super Contributor
timestamp: September 16, 2015, 07:57:53 am
content: Quote from: Delta on September 16, 2015, 07:38:09 amI like the look of wind farms myself. The local NIMBYs are still after decades trying to have the local ones torn down. And all solar installations removed for good measure :/

author: G7PSK
author_rank: Super Contributor
timestamp: September 16, 2015, 08:28:05 am
content: They have just put in an underground 11,000 volt line for a 50Mwatt solar farm near me. The trench width was enormous compared to the amount of land underneath pylons. A swath somewhere around 50 to 80 meters was cut through the local farm land the cables were widely spaced in the trenches and the only time they came close to each other was when tunnelled under the roads even then they were about a meter apart. The trenches had black pipes in them and then the cable was run through afterwards, no cooling other the ground as they only went in at the end of last year and were noy finnished in time for planting this year I do not know how they will affect cropping but as despite the top soil being removed first and put to one side the back filling manage to create soil conditions that do not look as good as the untouched areas either side. The contractors took a lot longer to complete than they told the local farmers it would and a considerable amount of crops have been lost this year, for which no doubt the company building the solar farm will have to pay.

author: dexters_lab
author_rank: Unknown
timestamp: September 16, 2015, 10:44:37 am
content: interesting comments there...anyone know the typical losses over a length of the national grid size cable?

author: Psi
author_rank: Super Contributor
timestamp: September 16, 2015, 10:56:56 am
content: pylon removal?You must construct additional pylons!

author: Andy Watson
author_rank: Super Contributor
timestamp: September 16, 2015, 11:03:58 am
content: All this talk of pylons! Reminds me of my favourite site:http://www.pylonofthemonth.org/

author: SeanB
author_rank: Super Contributor
timestamp: September 16, 2015, 07:26:01 pm
content: Quote from: dexters_lab on September 16, 2015, 10:44:37 amTypical design voltage drop at full load is 100V/km, IIRC, and the design load per cable unit is 1kA. Bundle the cables together and you increase the current rating, but the full load drop is the same. That is why the first thing in the distribution chain at the substation side is an automatic tap changer, to do the regulation for the outgoing 66/33/11kV lines, along with the main power factor correction and the surge arrestors. Utilities use MOV units, just a little bit bigger, like truck size bigger. Even the overhead MV lines have MOV units every so often on the cabling, used to clamp the spikes, though there they just look like rather large insulators and are often associated with a set of line fuses.https://en.wikipedia.org/wiki/High-voltage_cablehttps://en.wikipedia.org/wiki/National_Grid_%28New_Zealand%29https://en.wikipedia.org/wiki/1998_Auckland_power_crisis

author: MikeW
author_rank: Regular Contributor
timestamp: September 17, 2015, 08:28:41 am
content: Quote from: Mechanical Menace on September 16, 2015, 07:57:53 amI was in Clacton on sea last weekend and the wind farm there makes an incredible visual spectacle to me.Yet there are still people who think this totally ruins a bit of flat brown north sea.