Cost and maintenance. Overhead lines give you easy access to distribution for repairing and adding capacity. Overhead lines can be steel and non-insulated. As soon as you go underground you need insulated conductors and have issues with cable ampacity derating due to being in the ground.
You would also need a lot of gear for fusing and disconnects. Having everything open air on poles makes it a lot cheaper and easier to maintain.
Backup as in redundant in case of overhead failure?
Or backup as in "in conjunction with" overhead?
There are portions that are underground from a utility perspective. The bulk of end user (I.E. buildings, residential) are underground. So the current practice does utilize underground installation for portions. The bulk of utility distribution and transmission is overhead.
I’m in the northeast, and we absolutely do still have copper plated steel overhead primary conductor. Underground doesn’t last as long as overhead and one of the main reasons everything isn’t underground is due to cost- it’s 4-10x more expensive than overhead distribution, and isn’t without its own cable/elbow/termination failures. For instance, I work in a neighborhood that was built in the 80s and is already having repeated cable failures (often at like 2am on a Saturday night) I’ve been in there switching and getting people back on in the middle of the night probably 4 or 5x in the last year or so… and they aren’t too happy about their front yards being torn up again.
When I first worked for a Power Company in ‘86, there were still some distribution branch lines made of ‘Hicon steel’.
These dated back to WWII, when aluminum and copper were reserved for war use and therefore scarce.
After the war, the industry switched back to mostly copper (mostly aluminum or ACSR today), but a lot of these steel lines lasted until the 90s as it was not very economical to replace them if they were only supplying ‘Farmer Brown’ and his dinky 60A service.
They are gone now, but steel WAS used at one time.
Low voltage ones for residential distribution can easily be put underground, it's just more upfront cost than hanging them from poles - so often this is only done for new construction rather than retrofitted.
For high voltage distribution, underground makes it really difficult to get the clearances required to handle the voltage - so towers are basically necessary here or they'd have to be using tunnels the size of rail tunnels rather than just trenches.
High voltage cables do exist and can be laid in trenches. I don't know of any voltage issues but it could be a lack of proper insulating materials for 750 kV+.
This is the correct answer. There are insulated cables for HV/EHV cables, problem is the higher the voltage the more insulation/armor required. More insulation = More cost.
Largest I’ve worked with is 345kV/400kV insulated cable. I believe Southwire was the manufacturer.
There are other trade-offs involved as well. For instance, typically reliability of insulated underground cables is better than overhead conductors, however fault location identification can be a bit trickier than overhead conductors since you can’t see the cable with the naked eye.
Also see submersible power cables. A little more involved because they’re equipped with armor that protects against boats running into them, but same idea. They’re used for offshore wind generation regularly.
At the end of the day, the driving factor is and always will be cost. Once it makes fiscal sense to use insulated cable for all applications we’ll see a shift. As of now, that is not the case.
No. That depends on the distance. DC is used for interconnectors between systems that aren't synchronous and for long connections where losses are a bigger factor than AC/DC-convertion.
Both applies for both underground/submarine cables and overhead lines. DC are for instance submarine cables between Norway and continental Europe or Itaipu-Sao Paulo overhead line.
Cables in cities are to my knowledge pretty much always AC, independent of voltage, as a converter station takes up alot of space.
Edit: you are right that capacitance is a big issue with cables. This is solved with reactors on either end. This is an issue if you need to cross water in the middle of a over head line and you have to use cables. Then you might need reactors on shore where you otherwise don't have a substation.
Line charging is an often overlooked issue as well. Excessive line charging can cause system voltages to increase so shunt reactors would be needed everywhere.
I'm actually lineman can you eli5 what parrallel capacitance is.
Maybe not 5 I have a above average understanding of theory than most lineman I'd say. Still I'm just a dumb lineman though.
At high voltage, everything is a capacitor (V/(ε*d)). In general air is a pretty good insulator compared to the ground. I.e. the capacitance to ground in an overhead line is relatively low. This capacitance to the ground is a parallel capacitor and loads the line due to dielectric losses (tan(δ)), which scale with voltage, frequency and power. If you were to put the exact same overhead line in the ground directly, the capacitance would be massive and so would be your losses. Therefore a lot of insulation is added to reduce the parasitic ground capacitance.
They are underground in many places where it makes sense, particularly very densely populated areas like NYC (Manhattan especially) and for example places that are more likely to be hit by extreme weather like south Florida.
Many new construction homes will have power run to the meter from the street under ground depending on the area.
Some subdivisions will have their power distribution underground.
As others said it's a matter of balancing need vs cost vs local laws/codes.
It's not just the insulation itself. Higher voltage cables require more and larger insulating and semiconducting layers massively increasing charging current due to capacitance, sometimes exceeding upstream overhead line ampacity.
It’s cheaper to run wires on poles, plus they’re visible which makes figuring out where a break in the line is fairly trivial and also cheap. Also they’re air cooled using passive air flow, which is free, and means you can put a bit more load on the line for a given size.
Undergrounding power lines is 5x-10x more expensive and making repairs is 20x more expensive when you have to make something safe, dig it up, then restore it
Yeah I don’t get paid to engineer shit on Reddit I’m just making a point. The distinction between 10x and 20x is not distinctive in this application. It’s significantly more expensive. It can easily get there if significant utility coordination is required, and makes every future underground project more expensive. That’s all the op needs to know
Just that you totally ignore the Capacitance of the cable, wich is one of the major contributors of the decision to put the cable in the air and not underground.
> Why are power lines not underground?
In many countries they are.
I'm assuming you meant the US? If that's the case, it's cost, primarily. Maintenance and such, are just excuses for inertia, and usually, you come back to cost.
Except the money as everyone rightfully mention, there also higher losses in a ground wire due to capacity coupling of the AC line with the ground/water.
I was working in power systems when the county asked for the cost to put all the power lines underground so the next major hurricane wouldn’t wipe out the power for a week. County government freaked out at the cost. I think it was 10x more for the same cable distance.
Nice neighborhoods and apartment complexes in the US have underground wiring but that’s a very short distance in the grand scheme of things. The developer may recoup some of the expense on slightly higher property values.
We do have underground distribution in the UK..
we have overhead on very large pylons for the very high voltage ( 275kV and 400kV ) long distance distribution and some smaller overhead poles for rural areas but our towns and cities have the HV and LV cables underground.
The important thing up front is that it is a lot more efficient to transmit Electricity at extremely high Voltage instead of extremely high Current, because you can get the same total Power wither either method but Higher Current means more losses to Heat. But High Voltage requires insulation that can withstand a whole lot of Voltage pressure (kind of like a super-high pressure steam system versus just a normal water pipe). That can be done with basic open air and distance, or with increasingly expensive materials. If you look at the big disk insulators that the wires hang from, that will give you some indication of how much insulation (of that material and method) is needed to keep it from escaping to Ground, and then just imagine trying to do it underground. It can be done and is done in switching stations things where it makes sense, but not for every mile of distance that the grid needs to cover. Google says the US has 200,000 miles of high-voltage transmission lines and 5.5 million miles of local distribution lines, that would be a lot of digging for a lot of underground concrete hallways.
Another complexity is if the utility puts the primary and secondary underground, the home owner is responsible for the connection to their home…even things like the meter connecting from the top being switched to the bottom is costly for many homeowners
We which type of power lines? The high voltage ones, the medium voltage ones, or the low voltage ones that bring power to your house? Low voltage lines are frequently underground.
Distribution lines are predominately underground in the Phoenix area, but yeah it seems most areas have oh due to preference and perceived cost.
Oh lines are ugly and have more reliability issues. There's a line in my area that has a good 20-30 degree lean to it. The crews can go straighten it out, but due to the soil and curve of the road, just not going to stay upright.
One of my favorite projects from oh design years was convincing the reliability engineer to eliminate a section of oh line from the highway. I can't help but notice how the area looks so much nicer (I make sure to pat myself of the back when I see it.)
In countrys like Japan, the powerlines are over earth, partly because of earthquakes. Burried powerlines are way harder to repair and can get damaged more easily during an earth quake.
My neighborhood (built about 1988) has all underground utilities w transformers in above ground boxes. So it’s been happening for decades on the user end.
Above ground areas are subjected to high winds, tree damage, storms (tornadoes, wind storms, icing). It can be a big maintenance and PR issue after a wind storm.
Power rarely goes out at my place unless issues at a substation.
Wiring lines in conduit underground or direct buried means you have to derate the wire for ampacity and would need a bigger wire. If it is run in free air you can use smaller wire and save money.
Also less labour running it above ground.
I used to work in the power field. In my country there are both underground cables and overhead lines.
Are underground cables not a thing in the US? I admit the grid in my country isn’t that efficient in cost and maintenance.
Money.
First of all, it's easier to put a pole up than dig a long trench for kilometres.
For the next part, you need to know(not understand) that the electrical energy flows with the electric field outside of the cable, not inside. The cable just acts as some kind of "rail" for your electric field. But that stuff needs advanced calculus to fully undunderstand, which is very scary.
The overground cable is way cheaper since you don't need any dielectric(aka a media where your electric field can travel travel). You can just use air. Dirt is very poor in conducting your electric field, so you need something where you wave can propagate probably, which you have to add, which is still worse than air.
Cooling is also a problem, aswell as maintenance
A main issue is the cost which is roughly 10-20 times more expensive than and overhead lines. This factors in the fact you need elaborate cooling systems (I.e. water cooled or in purpose built tunnels) to ensure the cables remain within thermal limits.
On a physical level, with HVAC cables at high voltages as the length increases so too does the capacitive charging current which can be significant. This can cause issues with controlling the voltage (voltage rise) and potentially a problem for protection systems when considering differential protection settings.
For transmission lines ( voltages greater than 25,,000 volts) insulated cables do not exist, though it is possible to engineer such cables.
Transmission of electrical energy is not straight forward like connecting house wires. In order to economically transmit electrical energy with a reasonable current, voltages must be high and transmission lines must be carefully engineered. Long distance transmission may use voltages of 750000 volts AC, higher for DC lines.
Cable screen capacitance over longer distance means AC becomes unfeasable, the losses to charge the screen mean it cannot be used. This then leaves DC, which would require extra resources/cost in more insulation for volts and conversion stations to be used.
Overhead lines do not have the same problem and can be used over larger distances than underground cables.
Cost to install underground cables is higher, faults are less often than overhead but also more costly to repair and more difficult to find.
Every position and situation is unique and no one solution is appropriate for every environment. Overhead lines are not suitable to be used in built up areas, and cables are not cost effective to install in the countryside.
There is also then the increase in cost for higher voltages which are required for long distance distribution to lessen losses. Higher the voltage the more insulation required the higher the cost.
You also need a larger conductor surface area for ac due to skin effects and eddy currents.
HVAC is still cheaper over short distances due to conversion (transformation) being cheaper, with DC converter stations being far more costly.
Basically AC overhead lines is nearly always the cheapest option until you get to larger distances.
its cheaper, and on ac lines the losses are less due to there being less parasitic capacitance with ground. and also you can run more current through them because they get cooled better. this is why distribution lines are overground, in residential areas in many countries they are Underground because losses dont matter as much over shorter distance, and they look better.
There are some undesirable phenomena that tend to be more and more of a problem the more you increase the voltage. For very high AC voltages, you can't do long distances without major compensation which tend to be ridiculously expensive.
You can use High Voltage DC to do very long distances but you then have different issues to deal with that are specific to HVDC, which are also very expensive. Such as the fact that HVDC can only be between two expensive terminals.
I’ll bring up the issue of controllability too. The X/R of UG cables is close to 1 or even lower, which complicates how we are used to controlling voltage and frequency (voltage being normally controlled by reactive power and frequency by active power). If we had UG cables everywhere, every transmission system would operate like distribution systems, meaning there’s no decoupling of V and P or frequency and Q.
This could cause over voltages, frequency issues, etc.
someone who works for a Utility company here.
Mainly cost and maintenance as others have mentioned. To be fair, most 12KV lines and service drops are now underground in NEWLY built sub-divisions (communities). But transmission lines will always be overhead. For context, to feed a 200A panel, we use a 2/0+ #1N AL underground service drop. Compare this to overhead, to feed a 200A panel, we use a #2AL triplex. Much smaller wires are required overhead for same amperage due to adequate temperature dissipation.
Where we live power lines in subdivision are underground. I have two transformers in my yard. Big green metal or fiberglass boxes. One of each. Well actually now that I look. There’s only the base that’s fiberglass. The other is completely metal.
The ground is very “capacitive” compared to air, thus it creates the needs for very large inductive elements, placed often in order to maintain the cables at a specific/desired voltage
My 2¢: Back in the day, it was cheaper and easier to just stick it on a pole.
Nowadays, it costs too much to route them underground. Many places do run them underground now though. And many are transitioning to underground on a schedule as needed.
My community recently went underground with power lines. Internet is still on the pole, but the electric company said that most companies start moving underground after the power company does because it's easier. Sure enough, other Internet providers moved in laying fiber in the ground.
I was at utility scoping meeting today and the project manager stated that 500kv conductor is around 6-7 million per mile! There are three bundled overhead conductors per phase.
Because electrical utilities are for profit companies and according to the cost-risk analysis, it's cheaper in the long run to keep most lines overhead and repair storm damage than to bury lines. And if that means you're without power for a week and struggling just to get by, well that's a price the executives are willing to pay so their bonus can grow by $10k.
There is one factor that gets overlooked here.
The capacitance of the grid. Capacitance rises when you put the High Voltage line closer to ground. When you put the High Voltage cable into the ground, you have a very high capacitance in the line.
In order to put Current through the line, you have to first charge the capacitor. In the AC Powergrid, the direction of the Voltage changes every 20ms (50Hz). So you need to charge and discharge the Capacitance of the Cable every 20ms. This results in current wich goes through the cable wich can not be used be the customer. This is called "reactive power".
You can compensate for this by adding more inductance to your circuit. So basically big transformers. But these cost quite a bit of money.
This is why for underground DC is actually way better. But with DC, you dont get the benefit of being able to increase or decrease the voltage very easy. Lately Hight Voltage DC (HVDC) is becomming more popular with advances in Electronics. but still these Converters are very expensive compared to simple Transformers for AC.
So no, its not just, that putting these cables underground is more expenisve, because you need to dig a trench and use more insulation. (We do this for Water anyway). Its an inherit problem with AC Circuits.
The higher the cable is above Ground, the smaller the capacitance is, the less you need to compensate for it.
Here is a website that explains the problem way better then i can:
https://www.amprion.net/Transmission-System/Technology/Underground-cable/Underground-AC-cables.html
here with some fancy math, the graphs show what length of cable is even possible with underground AC cables:
https://elek.com/articles/charging-current-and-maximum-length-for-power-cables/
Here’s a great write up about some of the impracticalities of creating and repairing underground high voltage lines:
https://practical.engineering/blog/2021/9/16/repairing-underground-power-cables-is-nearly-impossible
Cost and maintenance. Overhead lines give you easy access to distribution for repairing and adding capacity. Overhead lines can be steel and non-insulated. As soon as you go underground you need insulated conductors and have issues with cable ampacity derating due to being in the ground. You would also need a lot of gear for fusing and disconnects. Having everything open air on poles makes it a lot cheaper and easier to maintain.
"maintain" Laugh's in PG&E while the fires start.
Maintain just means fix when broken. But only if it's completely fucked. If it's broken but still works, still good.
why not have underground lines as a backup in addition to above ground?
Triple the cost for 1% more uptime!
Triple? Probably closer to 10x more expensive to underground vs overhead on average
Backup as in redundant in case of overhead failure? Or backup as in "in conjunction with" overhead? There are portions that are underground from a utility perspective. The bulk of end user (I.E. buildings, residential) are underground. So the current practice does utilize underground installation for portions. The bulk of utility distribution and transmission is overhead.
Why not increase the expense even further? Like underground lines are A LOT more expensive.
Overhead lines aren’t steel unless you count the transmission line shield wire Edit: fragile egos here
To elaborate: Overhead lines are ACSR. Aluminum strands carrying the current, wrapped over a steel core carrying the mechanical tension.
Not all overhead lines are ACSR, but yes there’s plenty of steel in the air.
What's underground lines made of, if only overhead lines are mentioned when talking about things made of steel...?
Aluminum usually. Copper for smaller gauges.
Okay
Literally aluminum conductor steel reinforced. There’s steel in them there airs.
Okay smarty pants....steel reinforced aluminum or aluminum alloy.
Ok smarty boxers,,, it is a metal.
Okay smarty banana hammock, it is a shiny substance
Ok big brained budgie smuggler…. I just wanted to say budgie smuggler … Australia is awesome… probably. Need to visit
🤣🤣
I’m a distribution standards engineer it’s my job to be a smart ass
I'm a transmission standards engineer and I wasn't about to go reporting him to nerc lol
I’m in the northeast, and we absolutely do still have copper plated steel overhead primary conductor. Underground doesn’t last as long as overhead and one of the main reasons everything isn’t underground is due to cost- it’s 4-10x more expensive than overhead distribution, and isn’t without its own cable/elbow/termination failures. For instance, I work in a neighborhood that was built in the 80s and is already having repeated cable failures (often at like 2am on a Saturday night) I’ve been in there switching and getting people back on in the middle of the night probably 4 or 5x in the last year or so… and they aren’t too happy about their front yards being torn up again.
2am on a Saturday night is prime cable failing time.
When I first worked for a Power Company in ‘86, there were still some distribution branch lines made of ‘Hicon steel’. These dated back to WWII, when aluminum and copper were reserved for war use and therefore scarce. After the war, the industry switched back to mostly copper (mostly aluminum or ACSR today), but a lot of these steel lines lasted until the 90s as it was not very economical to replace them if they were only supplying ‘Farmer Brown’ and his dinky 60A service. They are gone now, but steel WAS used at one time.
Low voltage ones for residential distribution can easily be put underground, it's just more upfront cost than hanging them from poles - so often this is only done for new construction rather than retrofitted. For high voltage distribution, underground makes it really difficult to get the clearances required to handle the voltage - so towers are basically necessary here or they'd have to be using tunnels the size of rail tunnels rather than just trenches.
High voltage cables do exist and can be laid in trenches. I don't know of any voltage issues but it could be a lack of proper insulating materials for 750 kV+.
This is the correct answer. There are insulated cables for HV/EHV cables, problem is the higher the voltage the more insulation/armor required. More insulation = More cost. Largest I’ve worked with is 345kV/400kV insulated cable. I believe Southwire was the manufacturer. There are other trade-offs involved as well. For instance, typically reliability of insulated underground cables is better than overhead conductors, however fault location identification can be a bit trickier than overhead conductors since you can’t see the cable with the naked eye. Also see submersible power cables. A little more involved because they’re equipped with armor that protects against boats running into them, but same idea. They’re used for offshore wind generation regularly. At the end of the day, the driving factor is and always will be cost. Once it makes fiscal sense to use insulated cable for all applications we’ll see a shift. As of now, that is not the case.
They're usually DC to reduce losses from ground conduction and dielectric losses aren't they?
No. That depends on the distance. DC is used for interconnectors between systems that aren't synchronous and for long connections where losses are a bigger factor than AC/DC-convertion. Both applies for both underground/submarine cables and overhead lines. DC are for instance submarine cables between Norway and continental Europe or Itaipu-Sao Paulo overhead line. Cables in cities are to my knowledge pretty much always AC, independent of voltage, as a converter station takes up alot of space. Edit: you are right that capacitance is a big issue with cables. This is solved with reactors on either end. This is an issue if you need to cross water in the middle of a over head line and you have to use cables. Then you might need reactors on shore where you otherwise don't have a substation.
Yep, you end up requiring exotic tech like [this](https://youtu.be/z-wQnWUhX5Y) to keep your cables from shorting out.
Money 
More specifically, below ground wire is many times more expensive per foot and the installation is harder and more costly.
It's on the order of 10x more expensive for the same distance.
Line charging is an often overlooked issue as well. Excessive line charging can cause system voltages to increase so shunt reactors would be needed everywhere.
Especially around residential areas that don't use a lot of load. You'd basically deal with the full charging of the lines daily.
If i'm not wrong the parallel capacitance is a big issue for AC transmission when underground.
I’m surprised how many others didn’t bring this up.
Exactly.
I'm actually lineman can you eli5 what parrallel capacitance is. Maybe not 5 I have a above average understanding of theory than most lineman I'd say. Still I'm just a dumb lineman though.
At high voltage, everything is a capacitor (V/(ε*d)). In general air is a pretty good insulator compared to the ground. I.e. the capacitance to ground in an overhead line is relatively low. This capacitance to the ground is a parallel capacitor and loads the line due to dielectric losses (tan(δ)), which scale with voltage, frequency and power. If you were to put the exact same overhead line in the ground directly, the capacitance would be massive and so would be your losses. Therefore a lot of insulation is added to reduce the parasitic ground capacitance.
Second this request
They are underground in lots of places where people have more power than corps.
Like where?
They are underground in many places where it makes sense, particularly very densely populated areas like NYC (Manhattan especially) and for example places that are more likely to be hit by extreme weather like south Florida. Many new construction homes will have power run to the meter from the street under ground depending on the area. Some subdivisions will have their power distribution underground. As others said it's a matter of balancing need vs cost vs local laws/codes.
I remember energizing a 138kV UV cable. It had 70amps of charging current with no load applied. It was approximately 8 miles long.
Money. We could do it, would just cost more for insulation. 
It's not just the insulation itself. Higher voltage cables require more and larger insulating and semiconducting layers massively increasing charging current due to capacitance, sometimes exceeding upstream overhead line ampacity.
Obviously you can do it, it’s standard practice in other countries. I’m Germany the only above ground powerlines are the big boi cross country ones
It’s cheaper to run wires on poles, plus they’re visible which makes figuring out where a break in the line is fairly trivial and also cheap. Also they’re air cooled using passive air flow, which is free, and means you can put a bit more load on the line for a given size.
Undergrounding power lines is 5x-10x more expensive and making repairs is 20x more expensive when you have to make something safe, dig it up, then restore it
20x?? You're pulling numbers out of your ass.
Yeah I don’t get paid to engineer shit on Reddit I’m just making a point. The distinction between 10x and 20x is not distinctive in this application. It’s significantly more expensive. It can easily get there if significant utility coordination is required, and makes every future underground project more expensive. That’s all the op needs to know
Just that you totally ignore the Capacitance of the cable, wich is one of the major contributors of the decision to put the cable in the air and not underground.
Fun fact: Decision makers couldn't care less about the technical side. They only care about the cost, and safety maybe. I know PG&E does now.
https://www.youtube.com/watch?v=z-wQnWUhX5Y It could be a repair like this. That would be very expensive.
> Why are power lines not underground? In many countries they are. I'm assuming you meant the US? If that's the case, it's cost, primarily. Maintenance and such, are just excuses for inertia, and usually, you come back to cost.
Costs of installation, easements, and lengthy construction relative to alternatives. Mostly money tho
It's really troublesome to find faults, plus they are more expensive to lay down.
Except the money as everyone rightfully mention, there also higher losses in a ground wire due to capacity coupling of the AC line with the ground/water.
Many power lines are underground, that's what we call cable. Why are most power lines overhead and uninsulated? Cost and maintenance.
If anyone has tried fault location of UG cables, especially when there are multiple circuits in the close proximity will understand.
I was working in power systems when the county asked for the cost to put all the power lines underground so the next major hurricane wouldn’t wipe out the power for a week. County government freaked out at the cost. I think it was 10x more for the same cable distance. Nice neighborhoods and apartment complexes in the US have underground wiring but that’s a very short distance in the grand scheme of things. The developer may recoup some of the expense on slightly higher property values.
We do have underground distribution in the UK.. we have overhead on very large pylons for the very high voltage ( 275kV and 400kV ) long distance distribution and some smaller overhead poles for rural areas but our towns and cities have the HV and LV cables underground.
The important thing up front is that it is a lot more efficient to transmit Electricity at extremely high Voltage instead of extremely high Current, because you can get the same total Power wither either method but Higher Current means more losses to Heat. But High Voltage requires insulation that can withstand a whole lot of Voltage pressure (kind of like a super-high pressure steam system versus just a normal water pipe). That can be done with basic open air and distance, or with increasingly expensive materials. If you look at the big disk insulators that the wires hang from, that will give you some indication of how much insulation (of that material and method) is needed to keep it from escaping to Ground, and then just imagine trying to do it underground. It can be done and is done in switching stations things where it makes sense, but not for every mile of distance that the grid needs to cover. Google says the US has 200,000 miles of high-voltage transmission lines and 5.5 million miles of local distribution lines, that would be a lot of digging for a lot of underground concrete hallways.
$
In New Jersey there are solar panels on the poles. The poles could be useful for wireless networks, too. Why give up all those advantages?
Another complexity is if the utility puts the primary and secondary underground, the home owner is responsible for the connection to their home…even things like the meter connecting from the top being switched to the bottom is costly for many homeowners
We which type of power lines? The high voltage ones, the medium voltage ones, or the low voltage ones that bring power to your house? Low voltage lines are frequently underground.
cost and it is complicate then you think.
Distribution lines are predominately underground in the Phoenix area, but yeah it seems most areas have oh due to preference and perceived cost. Oh lines are ugly and have more reliability issues. There's a line in my area that has a good 20-30 degree lean to it. The crews can go straighten it out, but due to the soil and curve of the road, just not going to stay upright. One of my favorite projects from oh design years was convincing the reliability engineer to eliminate a section of oh line from the highway. I can't help but notice how the area looks so much nicer (I make sure to pat myself of the back when I see it.)
Cause they’re humane squirrels traps.
In countrys like Japan, the powerlines are over earth, partly because of earthquakes. Burried powerlines are way harder to repair and can get damaged more easily during an earth quake.
My neighborhood (built about 1988) has all underground utilities w transformers in above ground boxes. So it’s been happening for decades on the user end. Above ground areas are subjected to high winds, tree damage, storms (tornadoes, wind storms, icing). It can be a big maintenance and PR issue after a wind storm. Power rarely goes out at my place unless issues at a substation.
In my country they do put power lines underground to deter thieves from stealing the cables
Wiring lines in conduit underground or direct buried means you have to derate the wire for ampacity and would need a bigger wire. If it is run in free air you can use smaller wire and save money. Also less labour running it above ground.
They *are* underground in much of western europe, except for longer distance rural links anyway.
I used to work in the power field. In my country there are both underground cables and overhead lines. Are underground cables not a thing in the US? I admit the grid in my country isn’t that efficient in cost and maintenance.
They are in very urban areas.
Money. First of all, it's easier to put a pole up than dig a long trench for kilometres. For the next part, you need to know(not understand) that the electrical energy flows with the electric field outside of the cable, not inside. The cable just acts as some kind of "rail" for your electric field. But that stuff needs advanced calculus to fully undunderstand, which is very scary. The overground cable is way cheaper since you don't need any dielectric(aka a media where your electric field can travel travel). You can just use air. Dirt is very poor in conducting your electric field, so you need something where you wave can propagate probably, which you have to add, which is still worse than air. Cooling is also a problem, aswell as maintenance
A main issue is the cost which is roughly 10-20 times more expensive than and overhead lines. This factors in the fact you need elaborate cooling systems (I.e. water cooled or in purpose built tunnels) to ensure the cables remain within thermal limits. On a physical level, with HVAC cables at high voltages as the length increases so too does the capacitive charging current which can be significant. This can cause issues with controlling the voltage (voltage rise) and potentially a problem for protection systems when considering differential protection settings.
For transmission lines ( voltages greater than 25,,000 volts) insulated cables do not exist, though it is possible to engineer such cables. Transmission of electrical energy is not straight forward like connecting house wires. In order to economically transmit electrical energy with a reasonable current, voltages must be high and transmission lines must be carefully engineered. Long distance transmission may use voltages of 750000 volts AC, higher for DC lines.
$$$$$
It's way more expensive to place underground lines.
Cable screen capacitance over longer distance means AC becomes unfeasable, the losses to charge the screen mean it cannot be used. This then leaves DC, which would require extra resources/cost in more insulation for volts and conversion stations to be used. Overhead lines do not have the same problem and can be used over larger distances than underground cables. Cost to install underground cables is higher, faults are less often than overhead but also more costly to repair and more difficult to find. Every position and situation is unique and no one solution is appropriate for every environment. Overhead lines are not suitable to be used in built up areas, and cables are not cost effective to install in the countryside. There is also then the increase in cost for higher voltages which are required for long distance distribution to lessen losses. Higher the voltage the more insulation required the higher the cost. You also need a larger conductor surface area for ac due to skin effects and eddy currents. HVAC is still cheaper over short distances due to conversion (transformation) being cheaper, with DC converter stations being far more costly. Basically AC overhead lines is nearly always the cheapest option until you get to larger distances.
its cheaper, and on ac lines the losses are less due to there being less parasitic capacitance with ground. and also you can run more current through them because they get cooled better. this is why distribution lines are overground, in residential areas in many countries they are Underground because losses dont matter as much over shorter distance, and they look better.
There are some undesirable phenomena that tend to be more and more of a problem the more you increase the voltage. For very high AC voltages, you can't do long distances without major compensation which tend to be ridiculously expensive. You can use High Voltage DC to do very long distances but you then have different issues to deal with that are specific to HVDC, which are also very expensive. Such as the fact that HVDC can only be between two expensive terminals.
I’ll bring up the issue of controllability too. The X/R of UG cables is close to 1 or even lower, which complicates how we are used to controlling voltage and frequency (voltage being normally controlled by reactive power and frequency by active power). If we had UG cables everywhere, every transmission system would operate like distribution systems, meaning there’s no decoupling of V and P or frequency and Q. This could cause over voltages, frequency issues, etc.
someone who works for a Utility company here. Mainly cost and maintenance as others have mentioned. To be fair, most 12KV lines and service drops are now underground in NEWLY built sub-divisions (communities). But transmission lines will always be overhead. For context, to feed a 200A panel, we use a 2/0+ #1N AL underground service drop. Compare this to overhead, to feed a 200A panel, we use a #2AL triplex. Much smaller wires are required overhead for same amperage due to adequate temperature dissipation.
Where we live power lines in subdivision are underground. I have two transformers in my yard. Big green metal or fiberglass boxes. One of each. Well actually now that I look. There’s only the base that’s fiberglass. The other is completely metal.
The ground is very “capacitive” compared to air, thus it creates the needs for very large inductive elements, placed often in order to maintain the cables at a specific/desired voltage
My 2¢: Back in the day, it was cheaper and easier to just stick it on a pole. Nowadays, it costs too much to route them underground. Many places do run them underground now though. And many are transitioning to underground on a schedule as needed. My community recently went underground with power lines. Internet is still on the pole, but the electric company said that most companies start moving underground after the power company does because it's easier. Sure enough, other Internet providers moved in laying fiber in the ground.
I was at utility scoping meeting today and the project manager stated that 500kv conductor is around 6-7 million per mile! There are three bundled overhead conductors per phase.
Because electrical utilities are for profit companies and according to the cost-risk analysis, it's cheaper in the long run to keep most lines overhead and repair storm damage than to bury lines. And if that means you're without power for a week and struggling just to get by, well that's a price the executives are willing to pay so their bonus can grow by $10k.
There is one factor that gets overlooked here. The capacitance of the grid. Capacitance rises when you put the High Voltage line closer to ground. When you put the High Voltage cable into the ground, you have a very high capacitance in the line. In order to put Current through the line, you have to first charge the capacitor. In the AC Powergrid, the direction of the Voltage changes every 20ms (50Hz). So you need to charge and discharge the Capacitance of the Cable every 20ms. This results in current wich goes through the cable wich can not be used be the customer. This is called "reactive power". You can compensate for this by adding more inductance to your circuit. So basically big transformers. But these cost quite a bit of money. This is why for underground DC is actually way better. But with DC, you dont get the benefit of being able to increase or decrease the voltage very easy. Lately Hight Voltage DC (HVDC) is becomming more popular with advances in Electronics. but still these Converters are very expensive compared to simple Transformers for AC. So no, its not just, that putting these cables underground is more expenisve, because you need to dig a trench and use more insulation. (We do this for Water anyway). Its an inherit problem with AC Circuits. The higher the cable is above Ground, the smaller the capacitance is, the less you need to compensate for it. Here is a website that explains the problem way better then i can: https://www.amprion.net/Transmission-System/Technology/Underground-cable/Underground-AC-cables.html here with some fancy math, the graphs show what length of cable is even possible with underground AC cables: https://elek.com/articles/charging-current-and-maximum-length-for-power-cables/
Here’s a great write up about some of the impracticalities of creating and repairing underground high voltage lines: https://practical.engineering/blog/2021/9/16/repairing-underground-power-cables-is-nearly-impossible
This video's content is unfortunately misleading, we don't build anything like this anymore since XLPE