Overhead Ground Conductor Wire ACSR or Copper? 1

[majesus]

We shall install an overhead line running about ~7km west as well as ~5 km to the North East. The O/H line shall be 13.8kV, high resistance, 3 wire, 60Hz, and we shall use Partridge 266.8 MCM ACSR.

We shall also use an overhead ground conductor wire and I am wondering if we should go with bare copper or ACSR for the ground conductor. I've read that:

Many utilities use copper for the entire length of the grounding conductor. Some utilities use aluminum or ACSR. Typically, the aluminum or ACSR is used above grade, spliced to copper, which then runs below grade. Some utilities utilize copper-coated steel. Copper substitutes have become popular due to copper theft.

From McGraw-Hill's National Electrical Safety Code (NESC) handbook


My question is why would Copper be used? ACSR is much more economical. Is there an advantage of Copper over ACSR for the grounding conductor? Can someone please elaborate?

Thanks,
Majesus

 

[Marmite]

It wouldn't. On an ACSR overhead line the ground wire would be the same type of conductor as the phase conductor. I think your quote is referring to grounding generally, rather than specifically to overhead lines.
Regards
Marmite

 

[7anoter4]

I agree with Marmite the static wire [or Grounding wire] has to be the same material as the phase conductors.
Close to the sea salt atmosphere the aluminum conductor will corrode fast then Copper is more indicated.
From the following publication [for instance]:
Ergon Energy Corporation Limited
NETWORK LINES STANDARD -GUIDELINES FOR OVERHEAD LINE DESIGN ch. 2.3 Corrosion Performance
From Table no.2.1 it results AAC is not corrosion resistant in alkaline atmosphere. So, along the ocean or seashore HDCu is indicated. See[for instance]:

http://www.ucic-cables.com/itmImgs/HDCU.pdf

 

[majesus]

Thanks for the help, but my quote in the original post was specifically for Overhead lines.


Can you please elaborate why the overhead ground wire has to be the same material as the phase conductors? Why can't ACSR phase conductors be used and a hard drawn bare copper static wire above?

 

[isquaredr]

Conductor sag will be different between the two types of conductor. There is an increased risk of the ACSR corroding with the copper conductor supported above it.

 

[Marmite]

Majesus, your quote talks about acsr above grade and copper below grade. How many overhead lines run below grade? Copper coated steel is used for earth rods, not overhead line conductor.
I don't think that quote is referring to overhead lines at all.
Regards
Marmite

 

[wareagle]

Majesus
Why do you think you need an overhead conductor? Are you concerned with lightning? Most utilities I know do not have an overhead grounded conductor. Its call a neutral. The guys are giving it to you correctly. Ditch the book.

 

[stevenal]

Did you look at Fig. 093-8? This shows the grounding conductor which is quite distinct from the overhead ground wire (the definition of which points you to shield wire in the code). The handbook cannot substitute for the code itself. The handbook does not need to be ditched, it just need to be used along side the code as intended.

 

[cuky2000]

Most distribution systems do not use shield wire. However, if a shielding protection is required, the shield wire do not have to be of the same material as the phase conductor.

The most popular and cost effective shield wires in the US utilities are:
• Galvanized steel wires and zinc coatings
• Cu or Al steel clad

Surge arresters may be used as alternative to the ground wire to achieve similar degree of shielding protection.

 

[majesus]

I read the comments in this post and see the confusion... So I went and looked at the source: the National Electrical Safety Code NESC).

The rule I quoted is for grounding and not overhead conductors. It was my misinterpretation. The quote I provided via the handbook is confusing because I was looking for cooper vs ACSR conductors and reading the paragraph on its own from the Handbook I assumed it was for overhead lines:

Many utilities use copper for the entire length of the grounding conductor. Some utilities use aluminum or ACSR

Anyways, many help on explaining why we use ACSR vs Copper, I appreciate the feedback and info.

Finally, in regards to why we plan on installing an Overhead grounding conductor. We are installing a transmission line in the arctic, where there exists deep permafrost. An overhead grounding conductor provides an additional grounding path to assist with the problematic grounding into frozen soil. Along with various other techniques: high resistance grounded systems, bonding all our pipelines, using ground enhancement material with 6m ground copper-clad rods and UFER systems in the foundations, we have be able create a large grounding plane over a wide area. I sees this as a bit overkill under normal conditions, but in such a difficult environment to work with, it has been effective.

 

[stevenal]

I'll agree with isquaredr regarding sag, but don't see it as a reason to use the same wire. The higher shield wire carries no load so it will be lightweight and stay near ambient temperature while the much heavier and warmer load conductors will have significant sag. If spacing is correct initially, there is no risk of shield wire sag causing a clearance problem. I also agree that copper should not be above aluminum. The copper salts from the copper greatly accelerate the rate of aluminum corrosion.

 

[davidbeach]

Not that I've had ( or ever will ) to deal with artic conditions, but in that case I'd use the same as the phase conductors. If you go smaller, you need to seriously consider the highest fault resistance you want to clear and see if you can detect that fault under all conditions.  Otherwise you're quickly looking at something other than an effectively grounded system.

 

[majesus]

The static line shall be the same size as the phase conductors.

The resistance of the ground line has occured to me, but it may still be lower than frozen ground. In addition it is also not our only grounding method but applied with other techniques to establish an effective grounding system.

 

[stevenal]

The original post says it's high resistance grounded. Will the Resistance of the shield wire really make a difference? I think you'll be needing a voltage rather than a current detection method to detect ground faults anyway. Never heard of a shield wire sized to match the load conductors.

 

[cuky2000]

Below are a few remaks for your consideration:

1) The O/H line shall be 13.8kV, high resistance, 3 wires, 60Hz. High Resistance Grounded (HRG) is not recommended for 13.8 kV. Known references suggest that there are no successful 13.8-kV HRG systems because have considerably higher capacitive charging currents which cause the ground fault currents to easily exceed the 8 A level.

2) … We are installing a transmission line in the arctic. Since the keraunic level in artic region is negligible, shield overhead ground wire is not required.

3)"Overhead ground wire" & "static wire" This terminology used in your post mislead and bring confusion to the discussion because many of us interpreted as a "shield overhead ground wire".

4)The intend of use HRS imply the fourth conductor is most likely be a metallic ground return. Any concern with corrosion or issues with different materials? It is possible to install the neutral below the phase conductors?

5) The static line shall be the same size as the phase conductors. Why? What percentage of single phase load it is expected to be carried in the neutral conductor?

 

[majesus]

Cuky2000, thanks for info

Here is my response:

1) HRG shall be 50A, (maybe this should be worded low resistance grounded, anyways the resistor shall limit the ground fault to 50A).

2) Understood the keraunic levels are low, but we shall use the 4th wire as a grounding conductor and not a neutral.

3) Understood, I shall call it ground wire.

4) It shall be ACSR, why not keep it above the phase conductors in case there is a lighting storm? I see this as an added benefit. Is installation costs with mounting above the phase vs below the phases dramatic? To me it seems neglible.

5) Same comment as 3) Not a static line, but a ground conductor.

 

[stevenal]

And around and around we go. Are we back to the pole ground?

Per the NESC "overhead ground wire" = "static wire" = "shield wire". "Shield wire" is preferred. There was some confusion originally, but we now understand that this is what majesus is talking about.

"Grounding conductor" is not in the definitions, but by its usage it is used to connect equipment to a grounding electrode.

"ground wire" is not used in the NESC without the word "overhead" in front.

In sizing this shield, I would first look at the minimum size needed to carry 50A. I would then see how the full length of the wire affected the 50A to see if it were still detectable. If not, adjust upward until it becomes detectable.

 

[stevenal]

I need to correct my 1st Dec 14 post. The case where the shield wire is covered with ice and the load conductors are bare should be considered. Good information at

http://www.rurdev.usda.gov/SupportDocuments/UEP_Bulletin_1724E-200.pdf.

I know this is not transmission, but the OHGW stuff is applicable.