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Underground and overhead power – what’s the story

By planeguy - 5 January 2010 35

Okay, so around my way, all the houses are fed from overhead power, and the poles have TransACT strung a few metres below. However, the surprising thing for me at least, is that the streets have street lighting, on quaint little poles in the nature strip. These are NOT connected to the overhead cabling, and so I must conclude that there is a second power network running through the suburb that is underground.

Is this common in Canberra?

Is the underground power 240V too? If so, why would the power company go to the trouble of running underground power, without removing costly to maintain (but cheap to install) overhead cables, when the main cost of underground is the excavation?

Or is this just Actew/Govt incompetence again?

What’s Your opinion?


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35 Responses to
Underground and overhead power – what’s the story
Jack Kirby 9:06 pm 05 Jan 10

The Institute of Engineering produced a book (now available online) called Canberra’s Engineering Heritage. It was written in 1983, with an update written in 1990

You may want to investigate:
Street Lighting and Electricity

JC 3:36 pm 05 Jan 10

Braddon boy you are half right. Power comes from stations at very high voltage. This all goes to the main station out on Parkwood road in West Belconnen plus a secondary station just off the Monaro highway. These sub stations are owned by the national grid people.

From here it is stepped down and sent to suburban sub stations of which there is a good dozen or so in the ACT all owned by ACTEW. These step it down to 11KV which is feed to the suburbs. It is all 3 phase in a delta configuration, meaning it has 3 wires. All this is underground.

In the burbs you will find 11KV delta to 230v star transformers. In older suburbs these are on poles and in new suburbs in largish street cabinets, normally in parks or road side reserves. With the pole ones you will see a large cable run from underground up to the trannie and from the trannie 4 wires running along the pole. These are the 3 phases plus a neutral. These run along poles in back yards where the drop to the house comes off. In underground installations the 240 runs to small house boxes, this is were the house is connected to.

All new suburbs are fully underground, most of the older ones, say from the mid 80’s use above ground. The only thing unusal about the ACT compared to say the norm in NSW is our house distribution poles are in back yards wheras elsewhere they are out the front and often also carry street lights. Though a lot also have separate street light distribution.

Now getting back to street light, not 100% sure of the exact voltage for street lights is, but I would have though it would be higher than 240, not lower as CAF has suggested. Regardless of them being underground or above ground they are different systems than household power for a number of reasons. One such being that they only come on when the sensor or timer say’s its time to come on.

Aubergine 3:26 pm 05 Jan 10

#10 said “The little streetlight poles on the nature strip are not 240V – they’re on a much lower voltage supply (which, among other things, removes the incentive for people to steal the globes).”
If it’s not common knowledge that the streetlights aren’t 240V, how is that supposed to remove the incentive to steal bulbs from them?

bd84 2:54 pm 05 Jan 10

youami said :

From reading this, #4 (‘phil m’) has been the only post to attempt to reply to the OP. Braddon Boy, thanks for your sparky insight but I don’t think planeguy was after an electricity lesson. After all, he can search the internet like anybody else can on the matter. The OP questioned, if I may para-phrase, why there are two diametrically opposed ways to power things in streets in the older inner suburbs. In other suburbs in Canberra (and Queanbeyan etc.) and most other cities, power is distributed to properties and streetlights either all overhead or all undergound but not a mixture of both. I don’t think planeguy cares if the power was 240v or whatever or on a reticulation network because that was not the question being sought to have answered.

I am no expert on Canberra planning so I am only making a presumption here. I do like your thinking ‘phil m’ but I don’t think it relates to having the infrastructure installed at different times because logic would suggest why not use the existing method of powerline distribution (ie. overhead or underground whatever was there first as one would presume it would be cheaper to tap into existing infrastructure than reinventing the wheel and building brand new parallel infrastructure). It just doesn’t make sense.

I think the reason for parallel infrastructure relates to the reasons why back boundary overhead lines were installed in the first place, ie. to remove the ‘ugly’ power poles in the ‘Garden city’ streetscape. So the dilemma for the planners was how to get power to the streetlights out front from the overhead poles out back without adding a row of unsightly overhead poles? So the logic would be to disguise the poles or go underground but just for streetlight power. This leaves the only poles in the streetscape, the streelight poles themselves. (I call them streelights loosely as they do nothing to light up the street, generraly only the footpath). So the answer IMHO is aesthetics and nothing else.

Happy to be corrected. 🙂

The issue regarding underground infrastructure was discussed publically last year by ACTEW, with them floating the idea of householders contributing to the cost of converting the services to their house underground.

Ongoing costs would be their number one reason.

Wires on powerpoles have always been the cheapest option for ACTEW when creating the infrastructure as it costs more to dig a hole, howerver they have now realised the cheap cost is pretty much restricted to the establishment costs and it’s now cheaper and easier for them to maintain underground infrastructure. Underground lines are less effected by the elements (falling trees etc) which lowers maintenance costs and it gives them easier access instead of requiring access to backyards when they do need to fix something.

youami 1:25 pm 05 Jan 10

From reading this, #4 (‘phil m’) has been the only post to attempt to reply to the OP. Braddon Boy, thanks for your sparky insight but I don’t think planeguy was after an electricity lesson. After all, he can search the internet like anybody else can on the matter. The OP questioned, if I may para-phrase, why there are two diametrically opposed ways to power things in streets in the older inner suburbs. In other suburbs in Canberra (and Queanbeyan etc.) and most other cities, power is distributed to properties and streetlights either all overhead or all undergound but not a mixture of both. I don’t think planeguy cares if the power was 240v or whatever or on a reticulation network because that was not the question being sought to have answered.

I am no expert on Canberra planning so I am only making a presumption here. I do like your thinking ‘phil m’ but I don’t think it relates to having the infrastructure installed at different times because logic would suggest why not use the existing method of powerline distribution (ie. overhead or underground whatever was there first as one would presume it would be cheaper to tap into existing infrastructure than reinventing the wheel and building brand new parallel infrastructure). It just doesn’t make sense.

I think the reason for parallel infrastructure relates to the reasons why back boundary overhead lines were installed in the first place, ie. to remove the ‘ugly’ power poles in the ‘Garden city’ streetscape. So the dilemma for the planners was how to get power to the streetlights out front from the overhead poles out back without adding a row of unsightly overhead poles? So the logic would be to disguise the poles or go underground but just for streetlight power. This leaves the only poles in the streetscape, the streelight poles themselves. (I call them streelights loosely as they do nothing to light up the street, generraly only the footpath). So the answer IMHO is aesthetics and nothing else.

Happy to be corrected. 🙂

caf 1:20 pm 05 Jan 10

The little streetlight poles on the nature strip are not 240V – they’re on a much lower voltage supply (which, among other things, removes the incentive for people to steal the globes).

As for the rest, the interstate supply at 330kV arrives at Canberra at the Canberra Zone Sub, which is out west of Holt. Here it’s stepped down to 132kV, and distributed out over a ring to the network of Zone Substations (of which there are about 15 around the Territory). A typical example of these is the Wanniassa Zone Sub which you can see at the corner of Athllon Dr & Sulwood Dr. At the Zone Subs it is stepped down to 11.5kV and distributed out to the Distribution Substations in the suburbs. The Distribution Subs are just the transformers that you see on poles or in the large grey boxes (about 1.5m high and a few metres long) on the side of the street – here its stepped down to 415V and distributed out to the houses. (Note that all of these voltages are the three-phase voltages – if you take a single phase of 415V three-phase, you get a 240V single phase).

captainwhorebags is correct – the little green link pillars (about 80cm high and 30cm square) are just 415V distribution links, they don’t contain transformers.

niftydog 1:18 pm 05 Jan 10

115kV would be a big transmission lines – I would think the highest you’d find within the suburb is a few kV up to 11kV or so. But precise info is very hard to find these days, thanks to some bloke called Osama.

The street light network is a fixed, known load – they don’t change – this makes it easy to design and requires little or no updating for decades. Households are a different box of fish. They’re somewhat unpredictable, their demand increases over time (I assume) and varies significantly from hour to hour and over the seasons.

If you could get a look at a schematic for two suburbs, say like Holt (old) and Crace (new), you’d find massive differences in the topology and technology of the network. For eg; the two poles in my backyard feed 7 houses in my section in a star arrangement, but there’s no way they would use the same topology if the cables were underground. Instead, perhaps the main distributor cable would feed a street instead of a section which would make it easier and cheaper to trench.

troll-sniffer 11:03 am 05 Jan 10

planeguy

I think you’ll find that there are a whole raft of considerations that are responsible, such as responsibility for the infrastructure, (streetlighting is public, your power is a private matter), access, need to keep paths, roads and driveways clear, tampering, unauthorised access to ‘free’ power, and so on.

Call me an anti-cynic in this case but I suspect the powers that be have a pretty good handle on what they’re doing.

marcothepolopony 10:57 am 05 Jan 10

The overhead power lines certainly are an eyesore in the Tuggeranong Valley, spoiling the view of the Brindabella Ranges for just about everyone. I am unfortunate enough to have a ACTEW Sub-station on my front lawn. It all adds to the ambience.

Recently, however, I have made a unusual discovery in my back yard!
Doing some weeding along the fence prior to the weeds coming into seed, I found a thick black cord running along the entire fenceline, hidden in the long grasses.
It came from the rear neighbours electricity pole, ran along the ground on my fenceline, through some bushes and up the ACTEW pole in the next door neighbours back yard!
It crossed my mind, briefly of course, to sever this menace with my mower or whipper snipper.
Instead, I collected it and tossed it over my fence into the neighbours place, she has not tossed it back – yet. Does one contact TRANSACT to have this dangerous eyesore removed?
Poite suggestions appreciated.

captainwhorebags 10:52 am 05 Jan 10

planeguy: The above ground electricity distribution network runs along the back boundary of houseblocks, not in the streets. Canberra is unusual in this regard, as most cities supply above ground power from the front of the block. This was considered unsightly in the “Garden City”.

Powerlines you may see in the streets will be a much higher voltage (if I recall correctly, 115kV) and aren’t connected to houses.

Street lamps would only require a single phase, relatively low current supply. No doubt there’s only a dozen or so connected underground before it rejoins the above ground network anyway.

Newer suburbs use underground power running along one side of the street in the nature strip. The green mini pillars don’t have transformers – just fuse blocks. I think they supply six houses each.

Braddon Boy 10:50 am 05 Jan 10

Planeguy, that’s where you are wrong, the reticulation network in the streets is not 240v, it is much higher. The little green boxes on the street and grey boxes on the poles are small substations, there are about 1 of these for every 6 or so houses. The only part of that network that is 240v is the streetlighting cable and the cable that comes from these mini substations to your house.

Now for a crash course in electrical distribution…

Electricity comes from the power stations located at Lithgow, Snowy Mountains, Hunter Valley etc. at massively high voltages. It arrives in Canberra along those huge ugly high voltage lines with steel trusses holding them up. It goes through a big substation (the type that are fenced off) to supply a region (say half a dozen to a dozen suburbs) at this point it is still very high voltage. From there it goes to what is called a “link Pillar” which is a smaller substation servicing about half a suburb, a couple of hundred houses. For an underground network these are the largish grey cabinets you see every now and then. They are about as tall as a man and maybe 1.5-2m long. On a quiet day you can hear them hum. From here the electricity is still quite high voltage; I don’t know the numbers but a couple of thousand volts. This is now the reticulation that runs down every street in front of every house. Every hundred or so meters (6 or so houses) it connects to a minipillar (cute green boxes for underground, grey boxes for above) where it is converted to 240v. From there you have your own private cable running all the way to your house (or streetlight, or string of streetlights). I’m not sure why this needs to be an individual cable for each customer/streetlight string, but it is.

The reason that it is so complicated is because of the properties of electricity. Total energy equals voltage times amps. But, and this is a big but, the amount of energy loss through transmission is only dependent on amps and distance. So, increase the voltage, reduce the amps (same total power) means less transmission loss. So for as long as possible you want the voltage to be as high as possible. That’s why it is only converted to 240v right at the last minute.

I’m not trying to claim that the whole system isn’t confusing. Electrical engineering is widely accepted to be the most technically challenging branch of engineering. The people who design these networks go to uni for four years to learn how to do so. And they don’t just do the technical side of it, cost/benefit scenarios are also rigorously analysed.

phil m 10:30 am 05 Jan 10

Street lights will ultimately be fed from the overhead power lines at some point, however they will connect to each other via a rather small power cable between themselves underground.

The reason why the main power network is overhead and the street lights connected to each other underground is that the street lights were installed (or upgraded) at a different time to the existing overhead power lines.

Overhad power lines exists AFAIK only in much older suburbs. Since a certain point in time going forward all power network is installed underground.

planeguy 9:48 am 05 Jan 10

Braddon Boy,

I’m a bit confused by your post. Are you confirming that the underground street lighting is 240V?

If so, then why is it not fed off the 240V overhead lines. I am aware that the distribution network is at higher voltages, but everything on the residential side of the transformers is 240V. Why would the utility provider duplicate the 240V distribution between the street lights and the houses?

Which transofrmers/substations does the underground power get sourced from?

As for danger, a 240V line with street house power, say a few hundred amps of current is dangerous – but so is a 240V line just for street lights (at a rough guess 50A for a residential street). I can’t believe that there would be two different standards for trench depth for the same voltage supply. Can you or anyone else, post a reference from the Standards?

Braddon Boy 8:52 am 05 Jan 10

Firstly, you have to remember that there isn’t an entire network of underground electricity just to supply the streetlights. The streetlights are fed off the main reticulation network, in the case of an underground network then from the little green boxes, and for over head supply from those big grey boxes on the poles.

Also, the electricity in the main network is rarely 240v, it’s usually much higher. Those little green boxes or big grey ones are mini sub stations/transformers that step the voltage down for us to use in our homes. These high voltage mains, between big substations and the cute little ones in the street, need a much wider clearance to other services (telco, gas, water, sewer, stormwater, fibre optic etc.) and needs to be deeper than the 240v that powers the streetlight. This is in part because they are far more dangerous and also if you dig up a streetlight cable, your street is a little duller than normal, if you dig up a main cable, the whole street could be without electricity.

In short, no, it’s not ACT Government/ActewAGL incompetence; there are a lot of very smart people that have come up with these regulations.

As a side note, it is true that the installation for overhead wires is cheaper; which is why all the older suburbs have that, they were built in a time when the developer didn’t pay for electricity instillation. Now-a-days all new developments have underground electricity, the difference in installation costs between under and above ground is borne by the developer, and consequently the home buyer. The theory is that people are willing to pay more for homes with underground electricity as opposed to above ground. ACTEW have recently put a dollar figure on this value, but I can’t quite remember what it is, about $10,000.

sloppery 8:08 am 05 Jan 10

Go out to Jerra – all power and services underground.

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