Powerline cable count and engineering question (HV power distribution lines)

[girts]

I have seen that typically most 110Kv and 330Kv lines around my place have not 4 wires on them (3 phases and one neutral) but rather 7 wires on them, which I suppose is 3 phases x2 which probably means two cables for each phase and a common neutral wire at the top of each post.

Now is my reasoning correct and if so then is this done to increase the current capacity for a given high voltage line since the voltage doesn't change.
What seems also weird to me is that or example there is a 110Kv line (now turned into a 330Kv one) and there is a substation located some distance away from the line which then supplies some neighboring small towns and villages, now the big 110 (330Kv) line is much like a belt around the part of the country that i live in but at the point where the connection is made and the small substation is tied into the large line it is not simply connected much like you would imagine a wire being connected at a place on a bigger wire but the bigger wire runs itself interrupted, instead the large 110 (330) Kv line is interrupted at that point and goes towards the small substation and then seems like comes back and only then continues further around the country, there is no direct connection in the large line at the point where the wires go to the smaller local substation, why is this?Oh and finally, they now replaced the older 110Kv line with new cables and posts and increased the transmission voltage to 330Kv for larger capacity, but they also run the older 110Kv wires on the same posts at the other side.
What is the benefit of having simultaneously two lines with different voltages, one 110 and one 330Kv?
Is it so that the lower voltage line can be supplied to more sensitive individual households and the larger capacity line runs more or less for large factories and other loads that may have poor power factors and other issues associated with large specific loads?also while on the topic, how can they know at which approximate place has a short circuit in a line happened in the event that a tree falls or another object makes the short circuit somewhere along a line?
is it a bit similar to how train rails are monitored and the operator knows where a train is on a track?thank you.

 

[magoo]

Without seeing a picture, it is hard to guess. However, most 110 and 330 kV lines are transmission lines and don't require a neutral conductor. They may have a shield wire above the phase wires to intercept lightning.

Transmission lines require just the 3 phase conductors. They may have 2 sets of conductors per phase for higher capacity. Regardless, I'd expect to see 3 + 1 or 6 + 1, where the +1 is the shield wire.

 

[anorlunda]

There are two possibilities to what you saw.

1. Bundled conductors. We sometimes use 1, 2, 3 or 4 conductors per phase. That increases current capacity with the same voltage and it reduces corona losses (see picture below).
2. Double circuit towers. We sometimes put two 3-phase circuits on the same transmission tower to save real estate. The capacity of a double circuit, is less than the capacity if they were on separate towers, but the costs are still favorable.

As @magoo said, we also have shield wires which protect from lightning. One or two shield wires per tower. They normally carry no current.

In the picture below you see both bundled conductors with 3 conductors per phase (circled in red) and double circuits (circled in blue).

Which did you see @gerts?

Corona discharge

 

[PhanthomJay]

What seems also weird to me is that or example there is a 110Kv line (now turned into a 330Kv one) and there is a substation located some distance away from the line which then supplies some neighboring small towns and villages, now the big 110 (330Kv) line is much like a belt around the part of the country that i live in but at the point where the connection is made and the small substation is tied into the large line it is not simply connected much like you would imagine a wire being connected at a place on a bigger wire but the bigger wire runs itself interrupted, instead the large 110 (330) Kv line is interrupted at that point and goes towards the small substation and then seems like comes back and only then continues further around the country, there is no direct connection in the large line at the point where the wires go to the smaller local substation, why is this?

If I understand your statement correctly, the 330 Line actually is connected at the substation, passing through a circuit breaker and exiting at the other side, while first supplying the lower voltage lines through transformers. The 330 line can be sourced from either direction depending on load flows, and if one part of the line goes out of service for any reason, the other part on the other side of the breakers supplies the substation.

 

[girts]

Yes that sounds like a good idea that they tie off the large line with an intersection at each substation so in case of short circuit along the line at least some substations would still get power. yes each phase consists of three wires separated by some small distance and held together by spacers which are evenly distributed along the length of the wires.
it's just that the line has 6 such bundled wire cables running on it with the 7th being a single wire sitting on top of the line and not held by isolators which means it has no voltage in it.

for the new 330Kv line I can understand why there are 6 conductors, 3 are for the actual 330Kv line and the other 3 are for the lower 110Kv line, two lines on single post in other words.

so if they don't have the neutral on the lines does that mean that the neutral conductor is only made at the substation at the secondary side of the transformers in order for the smaller distribution network to have a reliable return path, but the distance between the power stations and substations is only covered by the phases since it would't add any benefit to also use an extra wire for the return given the length of it and the fact that it's only necessary at the user end of he lines which is after substation?Also I look forward to the other questions like why still save the 110Kv line alongside the 330Kv line since the 330Kv line has a higher capacity? Is it because each line is used for different loads, factory vs household etc?

 

[anorlunda]

Three phase power does not need a neutral return line. That's why three phase became popular in the first place.

Try the diagram below from By User:J JMesserly modification of original svg by User:SiriusA - File:3-fas-spänningar.svg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5607023

Pick any point in time (the horizontal axis) and add up the three values. They add up to zero always. So no neutral return line is needed.

Also I look forward to the other questions like why still save the 110Kv line alongside the 330Kv line since the 330Kv line has a higher capacity? Is it because each line is used for different loads, factory vs household etc?

There are several possibilities. One of the more likely ones is that the 110 and 330 lines split at some point and go to different geographical locations. Think of two independent lines that just happen to share the same path for a portion of their length.

 

[davenn]

Also I look forward to the other questions like why still save the 110Kv line alongside the 330Kv line

I have never heard of that being the case

do you have an actual reference for that ?

 

[PhanthomJay]

I have never heard of that being the case

do you have an actual reference for that ?

I Think anorlunda last response explains it very well. It is more typical to see lines of different voltages on a separate set of structures, but when space is at a premium...

 

[anorlunda]

I have never heard of that being the case

do you have an actual reference for that ?

Look at that double circuit line in the picture of post #3. There is no strong reason why the two circuits should have the same voltage.

 

[girts]

yes I understand that the three phases system requires no neutral, although can it happen that the loads become unbalances even within the path between power stations and substations if some very large loads become very asymmetric?

also I get that after each local substation and transformer which then directly feeds local shops and homes they introduce the neutral, as I have seen these three phase cables running through cities and rural areas all have 4 wires, while the larger higher voltage lines between substations have only 3 wires.
and no that fourth wire is not ground, that I know for sure, so it must be neutral?
as for the 110 and 330Kv lines on a single post please look at the pictures in the links.
https://www.lsm.lv/raksts/zinas/eko...lektrolinijas-uz-kemeru-apaksstaciju.a243088/
http://foto.delfi.lv/album/134194/?view=blog&page=1

I haven't had the chance to photo these lines myself even though I live close to them but these photos are just as good,
indeed at first I though, hmm it's weird why the poles are asymmetric like that where one side arms extend much further from the pole than the other side and only then while reading info about the new line I understood that they are keeping the older 110Kv line while making a newer 330Kv line also along the same path, because keep in mind this very path had a 110Kv line since like the 1960's so now because of loads increasing they decided to build a stronger 330Kv Line next to it.

That is why I ask in hoping someone here would know whether it is also done like this somewhere else where you keep two different voltage lines in the same path and for what reason?
I mean sure the 330Kv line has greater capacity but the loads are also not that high here why still keep the 110Kv line and not just make the 330 line in it's place?

 

[davenn]

Look at that double circuit line in the picture of post #3. There is no strong reason why the two circuits should have the same voltage.

that doesn't really answer the Q
there isn't any strong reason to suggest they do have different voltages

I was hoping for a specific answer
I will restate
NONE of the main grid transmission, like in the pic you posted, I have seen in Australia or New Zealand were mixed voltages

Do you have any references to the use of different voltages on the same transmission line system
phase at 1 voltage and one at another ?

Dave

 

[davenn]

as for the 110 and 330Kv lines on a single post please look at the pictures in the links.
https://www.lsm.lv/raksts/zinas/eko...lektrolinijas-uz-kemeru-apaksstaciju.a243088/

sorry, can't read that

http://foto.delfi.lv/album/134194/?view=blog&page=1

not sure what you were suggesting with that one ?

 

[anorlunda]

Do you have any references to the use of different voltages on the same transmission line system
phase at 1 voltage and one at another ?

Don't have much time to search today but here's one
https://www.physicsforums.com/threa...tricity-towers-and-poles.794297/#post-4990737

sorry, can't read that

You don't need to read it, just look at the picture. The number of insulators in a string is proportional to the voltage. Clearly in the picture, you can see that the insulator strings on the left circuit are longer than the strings on the right circuit. @girts can be exactly correct 330KV and 110KV

Now the tower itself has height and spacing designed for some maximum voltage, say 330KV in this case. It is possible that the 110KV circuit seen might be replaced with a 330KV circuit in the future without changing the tower design.

 

[Baluncore]

I mean sure the 330Kv line has greater capacity but the loads are also not that high here why still keep the 110Kv line and not just make the 330 line in it's place?

The new line must be built and tested before the old line can be removed.
There is no point in removing the old 3PH line until it is ready to be replaced with a new higher capacity line.
The old 3PH line may be redundant most of the time, but it is an asset when other things go wrong.

 

[girts]

exactly davenn, as anorlunda correctly said I gave the links not for you to read them but to examine the pictures, the links themselves are news articles so from an engineering viewpoint they contain no valuable information.
Now today I made some pics myself while driving by, and just as I read before about the project the two lines will go together on the same poles and will be used,
the old 110Kv line stands on separate poles and is still operational so @Baluncore if they wanted to keep it just as long as the new one is erected then they would not have made the new posts like this in order for them to have two lines simultaneously.

See the pictures, it is clear by looking at them that two lines (110Kv, and 330Kv) will be used in the future side by side. It is not just the isolators that are longer on the 330Kv side it is also the metal extension arms on which the isolators hang that are located further away from the vertical beams, look in the other side where the 110Kv line will be the arms are much shorter and also the isolators are shorter, this can only mean one thing , that the shorter side is for the lower voltage 110Kv line as it doesn't need such a big airgap, there won't be a chance to upgrade to a 330Kv line in the 110Kv side because the very metal pole is made such that there wouldn't be enough clearance between the wires and the grounded metal pole.What I want to understand is why have two different voltage lines running along the same path, what is the benefit?
as I can clearly understand that two different voltage lines means at every substation they need twice as much transformers, one set of traffos for 330Kv and another set for 110,
I guess in the end I will need to contact the energy company that builds these and ask them, because they run three large hydro stations and they also run the whole electrical infrastructure in my country.

 

[girts]

The poled are not yet wires just erected and with isolators attached as can be seen but this doesn't change what i said in my previous post

 

[davenn]

You don't need to read it, just look at the picture. The number of insulators in a string is proportional to the voltage. Clearly in the picture, you can see that the insulator strings on the left circuit are longer than the strings on the right circuit. @girts can be exactly correct 330KV and 110KV

interesting ... have never ever seen anything like that in Oz or NZ ...
Not just the length but also the larger and smaller cross pieces ... maybe it's a "Euro thing " ?

 

[anorlunda]

@davenn
Consider two independent lines at different voltages, such as in the top part of the picture below. Imagine them 100 miles long with towers every few hundred yards. Now imagine making at least part of the distance share the same path and the same towers as in the bottom half of the picture. Wouldn't that save money?

 

[Baluncore]

What I want to understand is why have two different voltage lines running along the same path, what is the benefit?

Lower cost and redundancy. Maybe the grid is not phase synchronised and requires two separated lines. Maybe the power belongs to different corporate bodies, or the 110kV is a backup for an industrial facility such as the pot line of an aluminium refinery or an older power station. It is also possible that the 330kV is national distribution while the 110kV is a supply between two smaller local areas.

See the pictures, it is clear by looking at them ... ... there won't be a chance to upgrade to a 330Kv line in the 110Kv side because the very metal pole is made such that there wouldn't be enough clearance between the wires and the grounded metal pole.

The tower is constructed from galvanised angle iron. It is bolted together. The low voltage side could be upgraded if needed by replacing the arms and insulators.

You can be sure that the design engineer had a good economic reason to have the added complexity of asymmetric voltage and arm design. That reason may not be obvious. You may never find out exactly what particular combination of factors caused the decision.

 

[davenn]

Consider two independent lines at different voltages, such as in the top part of the picture below. Imagine them 100 miles long with towers every few hundred yards. Now imagine making at least part of the distance share the same path and the same towers as in the bottom half of the picture. Wouldn't that save money?

View attachment 226569I don't get what you are saying ??

it doesn't matter

I have now seen a type of layout that I didn't know existed ... thanks @girts

 

[girts]

Your welcome davenn,

@Baluncore is correct , I completely forgot hat these towers are much like lego houses they can be assembled from scratch, sure enough I have been close to them and even climbed one of those while they are off the grid, yes every angle is bolted to another one.Well the 330Kv line is part of the national grid which now got extended to include my region also, it is then tied together with other neighboring countries and extends into Russia and in the west goes to Germany.
Then there are 110Kv lines running across country, it just happened to be that previously in my region there were only 110Kv lines so they were supplying all the load to every town via substations and distribution networks, now they decided to back this all up by the 330Kv line because from what I gather to have a reserve for future industry and also to be able to connect wind power at large capacities because my region is next to the sea and we have strong winds all year round.
Well thank God not hurricane strong but strong enough to blow a bunch of fanblades.

Anyhow I am still a bit curious, sure I understand the economic benefit of having a single tower instead of two separate ones, also less forest needs to be cut when the two lines are run on the same towers.
Even though as for redundancy if the two lines share the same towers they are easier to disrupt both of them simultaneously like for example in a terrorist act or otherwise.

 

[PhanthomJay]

◊ yes you are quite correct that reliability is reduced when placing 2 circuits on a common structure, due to weather related events, etc. , especially lightning storms where the stroke can hit the shielding wire and then travel down the pole and across the support arms and flash across the isolators to the phases of each circuit, taking both circuits out of service with possible major interruptions to customers. Some utilities either try to obtain new land or reconfigure the existing lines to avoid.having to double circuit a new line and the old, and at least one utility I.am aware of is going out of its way at great expense to separate existing double circuit lines by placing one of them onto new single circuit structures where space permits. This is being directed I believe by the regional system operator as a means of lowering the probability of double circuit line outages.

 

[Baluncore]

This is being directed I believe by the regional system operator as a means of lowering the probability of double circuit line outages.

The 330kV international distributor does not service the same area as the national 110kV. There may be one or more parallel lines to this one that also support 330kV and 110kV. That would make this new line a duplication of existing lines. If something knocks out pylons on this line, the other(s) will carry both the 330kV and 110kV circuits while repairs are being made.

 

[anorlunda]

@Baluncore is corrects about redundant oaths.

It is helpful to think of the transmission grid like the metaphor of a copper screen. In order to isolate one patch of the screen, you must cut multiple strands of wire. Of course real grids are not rectangular like a screen, but hey do,have multiple redundant paths.

Distribution grids, such as the power coming down the street to your house, are laid out like a radial metaphor. Think of bicycle spokes with the local power substation at the center. A single tree falling can blackout your whole street.

An ice storm in 1998 in the USA and Canada cut the wires in about 300000 places. Almost all places remained connected to the transmission grid, but it took 4 months to repair all the local distribution lines. Therefore, about 90% of the people in those areas had no blackout or were in the dark for only a few hours, while the unfortunate 10% had to wait weeks or months for restoration. Those are the kinds of numbers reliability engineers use to plan the grids.

 

[Windadct]

I'll just throw a few points:

Reliability of the lines is only part of the reliability of the system, redundancy is more valuable - IMO, so despite the added number of "risks" the overall system reliability is improved.

The 330KV line, having been added, may very likely be "longer haul" - it may be brought to existing substations - and used as 2nd feed in the same subs as the 110KV, line, but then may also continue on further.

If closer to a source - this could be due to the addition new generation.

For Wind - some dedicated, long haul lines are needed to get large amounts of energy from the source to the users. Additionally the instability of wind as a sourse will require additional infrastructure like this.

The original 110KV line - has a system attached to it - to upgrade, transformers, breakers, and all of the protection needs to be re-engineered and replaced.

YOu may be able to google a tensmision grid map for the area - and identify these lines, and see where they originate, tie and terminate.

 

[PhanthomJay]

Redundancy is necessary but the redundant circuit should ideally be on a set of different pylons to improve system reliability for the reasons I explained. These redundant circuit serve 2 purposes at least, when one high voltage line goes out of service, the other in certain cases picks up its load and the current can double on that circuit which is ok for short periods while the first is repaired. The other purpose for certain cases is that for normal conditions, each line shares half the load, reducing resistance heating losses at the receiving end. This is true whether the circuits are on common or separate pylons. Now you don't think that the utility I mentioned would be spending hundreds of millions of dollars to separate designated existing double circuit lines into two separate lines if it wasn't deemed necessary by the regional system operator, do you?

 

[Tom Kunich]

Without seeing a picture, it is hard to guess. However, most 110 and 330 kV lines are transmission lines and don't require a neutral conductor. They may have a shield wire above the phase wires to intercept lightning.

Transmission lines require just the 3 phase conductors. They may have 2 sets of conductors per phase for higher capacity. Regardless, I'd expect to see 3 + 1 or 6 + 1, where the +1 is the shield wire.

Not exactly for higher capacity since a larger wire increases the Ampacity of the wire slower than two wires of smaller diameter for instance: A 1.5 mm copper wire can carry 10A while a 2.5mm copper wire can carry 20A.. This shows that the small increase is wire diameter is gives 2.78 times as much material to achieve the same current carrying capacity as two smaller diameter wires. This is quite counter-intuitive.

It is more for reliability. And it also allows you to use different materials such as aluminum conductors so that if you get hardening from flexing in high speed winds and break a wire you have designed reliability into the system. Power lines are a very highly developed science and nothing is done by chance.

Long distance transmission of three phase does not include a neutral line. This is for short distance transmission normally close to the user's site for instance in and around the step-down transformers.

 

[anorlunda]

Not exactly for higher capacity since a larger wire increases the Ampacity of the wire slower than two wires of smaller diameter for instance: A 1.5 mm copper wire can carry 10A while a 2.5mm copper wire can carry 20A.. This shows that the small increase is wire diameter is gives 2.78 times as much material to achieve the same current carrying capacity as two smaller diameter wires. This is quite counter-intuitive.

It is more for reliability. And it also allows you to use different materials such as aluminum conductors so that if you get hardening from flexing in high speed winds and break a wire you have designed reliability into the system. Power lines are a very highly developed science and nothing is done by chance.

I disagree. See post #3. Also see https://en.wikipedia.org/wiki/Overhead_power_line#Bundle_conductors

 

[krater]

Not exactly for higher capacity since a larger wire increases the Ampacity of the wire slower than two wires of smaller diameter for instance: A 1.5 mm copper wire can carry 10A while a 2.5mm copper wire can carry 20A.. This shows that the small increase is wire diameter is gives 2.78 times as much material to achieve the same current carrying capacity as two smaller diameter wires. This is quite counter-intuitive.

Not the whole story as your 2.78 factor on conductor material comes with an increase in surface area of only around one and one third. Surface area is important for cooling and the increasing size of a conductor further limits its ability to stay at acceptable temperatures as the amps go up. Paralelled conductors are routinely used for economic reasons, as larger conductors become significantly more costly and challenging to handle while the proportion of size to current capacity diminishes.

 

[Baluncore]

You must be clear when you specify conductors. Do you use diameter in millimetres or sectional area in square millimetres. A circle with a diagonal line through it, Φ, is a symbol for diameter and for phase count. For example 3Φ power lines with 25mm Φ conductors.

Power line engineering is a complicated compromise, it is hard to guess what parameters were most important during the design.

Temperature must be kept down to minimise resistance, so cooling is important. But it is often better to not generate the heat in the first place.

For the same power, stepping up the voltage reduces current and so significantly reduces W = I²R thermal losses. Twice the voltage, half the current, gives one quarter the thermal losses.

For DC and low frequencies, the sectional area of a conductor determines the capacity. The current capacity at high frequencies is determined by the circumference of the surface. At 60 Hz, the skin depth in aluminium is 10.6 mm. In copper it is 8.5 mm. Conductors over about 20 mm diameter will have inaccessible unused material near the centre. That central part may be a high tensile steel wire that plays almost no part in the conduction.

Losses due to corona discharge are reduced with larger diameter conductors. By using a cage of several small conductors to create the same radius of curvature and electric field as one large conductor, the mass of the conductors can be greatly reduced. That allows higher voltages with lower currents and greatly reduces mass and windage. Cage conductors may reduce wind induced vibration that might fatigue the conductors if vibration dampers were not used on both sides of every insulator.

If you look along a transmission line in a strong crosswind, you will see that the force on a wire due to the wind can be greater than the force due to gravity. The force on the tower is the vector sum of both windage and the weight of the wire.

When ice forms on conductors it increases both weight and windage, strong winds can then destroy the towers. How much heat do you need to generate in the wires to prevent icing? Who would have thought that there was an advantage gained by encouraging circulating reactive currents in cold weather?

 

[anorlunda]

Power line engineering is a complicated compromise, it is hard to guess what parameters were most important during the design.

Well said, and nearly complete list of design considerations.

 

[Windadct]

I think the word compromise, has been vilified, and particularly to some more idealistic engineering types. One way to see this is an equation, with many possible solutions - and the best one(s) include many variables ( arguments).