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Differences between electric cables designed for High, medium and low voltage

by Charles123
Tags: cables, designed, differences, electric, voltage
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Charles123
#1
Mar2-12, 08:38 AM
P: 140
I am trying to find the differences between electric cables designed for High voltage, medium voltage and low voltage electric transport, but I am failing to find an answer. I was wondering if you could help me with this.
I know that aluminum is almost always the metal of choice, thicker is good but "Thicker wires would lead to a relatively small increase in capacity due to the skin effect, that causes most of the current to flow close to the surface of the wire." (from http://en.wikipedia.org/wiki/Electri...r_transmission). So what are the differences? Also are the high voltage cables always insulated?
I am referring to what usually the power distribution companies use.
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yungman
#2
Mar2-12, 11:19 AM
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I did a lot of HV stuffs using cable, we go up to 20KV. The only thing I am aware of is the dielectric thickness that govern the standoff voltage. Most of the dielectric material are about 400V per mil. So people design with max of say 100V to no more than 200V per mil. thickness. Usually they are more conservative. but that is the first hand rule in designing insulation thickness.

I am not an expert on the material use for dielectric, so I cannot answer on the physics end.
psparky
#3
Mar2-12, 11:21 AM
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Copper is a better conductor than aluminum.....at least according to the code book.

High voltage cables are often insulated......except in the case of overhead wiring or power lines. They are connected to the poles by heavy duty insulators but otherwise are wide open to the air. Excellent for heat dissapation and max current.

Charles123
#4
Mar2-12, 11:30 AM
P: 140
Differences between electric cables designed for High, medium and low voltage

Is overhead wiring not insulated for all kinds of voltages?
psparky
#5
Mar2-12, 11:39 AM
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Quote Quote by Charles123 View Post
Is overhead wiring not insulated for all kinds of voltages?
Yes.....why would you waste money insulating it if you don't have to.

The higher the voltage.....the bigger the distance between the wires to keep them from arcing or shorting.

When you drive down the road....take a good look at all the overhead wiring. Very educational.
Averagesupernova
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Mar2-12, 05:19 PM
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Quote Quote by psparky View Post
When you drive down the road....take a good look at all the overhead wiring. Very educational.
I thought I was the only one who did this. Nice to know I'm not alone in the world.
davenn
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Mar2-12, 10:13 PM
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Quote Quote by Averagesupernova View Post
I thought I was the only one who did this. Nice to know I'm not alone in the world.
hahaha no you are not, But it is a dangerous activity, I wrote my company vehicle off whilst following overhead lines looking for a short cct and not keep one eye on the road ahead

Dave
Charles123
#8
Mar3-12, 02:52 AM
P: 140
Thank you all for your feedbacks. But the differences/characteristics of the different voltage cables would be what?
Regards
psparky
#9
Mar3-12, 06:55 AM
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Quote Quote by Charles123 View Post
Thank you all for your feedbacks. But the differences/characteristics of the different voltage cables would be what?
Regards
Amps are amps so that's not gonna be a factor other than thickness of the wire. Bigger wires do carry bigger amps as you know....but once you reach 1" in diameter or so....the skin effect you suggest does come in to play. So bigger wires than 1" don't make much sense....and I think it's actually closer to 3/4".

The only difference between high voltage and low voltage insulated cables is that the insulation gets thicker on the higher voltage cables for obvious reasons.

So in other words......you can use a copper cable for a 9 volt application....or a copper cable for a 12,500 volt application. Copper is copper. Or you can use aluminum as you suggested....same situation. This certainly applies for the non-insulated cables as well.

Again, look down the road as you drive.....preferably at a stop sign to avoid crashing!!!
If you can sneak a peak while driving down the freeway...you will get a super view of some beautiful 3 phase high voltage lines that are bare. The insulators keep the phases from shorting out to eachother or even ground. The insulators are the funny looking spring things that look like shock absorbers.

Look at the structural engineering involved with overhead lines. Look at the gigantic footers on the big poles on the freeway. When an engineer figures the loads put on these poles....he has to assume worst case.....a half inch of ice on the wire and high winds. He also needs to figure out the sag effect with these factors going on. Ironically, the wires sag less in the winter...even with the ice....and sag more in the summer!!! The final weight determined from the wire calculation then needs to be translated into the proper insulator choice. The insulator chosen needs to be picked by its strength rating....and it's voltage rating. The poles and footers chosen will also be determined by the weight of the cables. Also notice the "guy wires" placed at specific spots to keep the overhead lines....well static....or to keep them in place. Remember your statics class? Look at the angles of the guy wires. Look at some of the gigantic footers just holding the guy wires. And you better believe those guy wires have some insulator on them....they typically look 5" white thimbles....for lack of better term. Take a look...you will see them. Exciting stuff I know! Enjoy!!!!
SirAskalot
#10
Mar3-12, 09:49 AM
P: 141
Different dielectric effects come to play when the nominal voltage increase.(dielectric heating, partial discharges, electric treeing, water treeing etc.) Thus different insulating materials are used for different voltage levels and type of application. A overhead cable is different from a ground cable who again is different from a subsea cable. Especially different types of shielding (semiconducting screen) change from application to application, length of cable etc.

And a comment on the question:
"Is overhead wiring not insulated for all kinds of voltages?"

Some overhead lines are insulated. Why? Safety concern for people and animals. Residential intake by overhead line are insulated cables. Some areas with endangered birds (some owls have large wingspan > 1.6 m) use insulated overhead lines to prevent flashover. Also the size of structure can be drastically decreased with insulating lines.

A comment on the statement:
"So bigger wires than 1" don't make much sense"

Yes it makes much sense. Most HV lines > 132 kV/66 kV use larger wires than 1", both for current carry capacity and strength. The nomenclature FeAl widely used with overhead lines refer to a wire with steel core for strength and aluminum strands twisted on top for current carrying capacity. Thus most of the current travel trough the aluminium. (skin effect)
psparky
#11
Mar3-12, 10:05 AM
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Some overhead lines are insulated. Why? Safety concern for people and animals. Residential intake by overhead line are insulated cables. Some areas with endangered birds (some owls have large wingspan > 1.6 m) use insulated overhead lines to prevent flashover. Also the size of structure can be drastically decreased with insulating lines.
Interesting. I was thinking more along the big power lines...but what you say is an exception that makes perfect sense.


A comment on the statement:
"So bigger wires than 1" don't make much sense"

Yes it makes much sense. Most HV lines > 132 kV/66 kV use larger wires than 1", both for current carry capacity and strength. The nomenclature FeAl widely used with overhead lines refer to a wire with steel core for strength and aluminum strands twisted on top for current carrying capacity. Thus most of the current travel trough the aluminium. (skin effect)
Interesting again. I now remember the "Curlew" wire I was looking at to carry 1,000 amps. It did have the steel core with the aluminum strands. I had to find the tension of the steel core...then the tension on the aluminum separately.
Windadct
#12
Mar3-12, 10:17 AM
P: 575
The conductor is not the difference between LV (<600-1000V), MV (1000 to 69KV) and HV (70K +) Cable, and by cable I refer to insulated conductors. For overhead conductors there are some insulated varieties and there are types with steel strands - for example ACSR (http://en.wikipedia.org/wiki/Alumini...eel_reinforced ) but this does not really affect the LV, MV or HV application,

It is all about the insulation - for LV Cable the insulation is pretty basic, but the ruggedness is a major factor as " non-qualified" people may come in contact with it and handle / manipulate the cabling. Also - in LV many conductors can be pulled in the same conduit, raceway - etc. so they can be physically stressed many times through it's life.

MV & HV - cable the WAY the insulation is used changes - the key factor is to try to get the Voltage Gradient ( E Field ) to reduce gradually through the insulation. A high E field will damage the molecular structure of the insulation - and it will break down over time - in Cable and in Air this is referred to as Corona Discharge. Most MV cables are shielded - and the if you look at a cross section (http://en.wikipedia.org/wiki/High_voltage_cable) you can see there are a number of layers in the construciton. All of this design is really about getting a uniform distribution of the E field between the energized conductor and the outer layer - the grounded shield.

HV Cable - is even rigorously designed - but really trying to do the same thing as the MV.

Pretty much ALL MV and HV insulation systems are about uniform E Field Gradient.

Another thing to keep in mind - this is the same reason sharp corners are avoided in MV and particularly HV systems - the large round tubes you see in HV junctions(in substations and transmission lines) are there to give a smooth gradient - otherwise the Air breaks down in the area if the High E Fields Stress - the E filed literally Ionizes the air, or any other insulation if the E field strength is too high. If you are near a large substation or transmission line you will often hear a hissing sound - this is actually due to two things : Tracking, or current leaking across the surface of the insulators, and Corona Discharge.


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