Assess Impact of Damaged Earth Conductor

[WhiteyWhitey]

Hi All,

During rectification work on the earth grid of a 275kV Underground Cable project the earth grid was damaged by a Jackhammer removing concrete around the copper horizontal conductors.

The critical info is the following.

The earth grid is designed to take any fault curent from the Cable sheath through 4.5kV Surge Arresters to ground. The system is cross bonded and earthed at both ends. this is a dual circuit system. the Cable sheath is designed for 50kA fault Current.

The earth grid has two separate rings in the JB for each circuit that are connected to the link boxes earth point (bottom of surger arresters). Each circuit has 4 x 2m 19mm earth rods connected with 300mm2 stranded copper.

The damage is:

Location 1: Bird Caged conductor with some broken strands. Centre of conductor (copper Clad steel) deformed from impact.

Location 2: Two Broken Strands of the conductor.

My task is to assess the impact this damage will have on the longevity and operation of the earth grid.

My Question is are there any formula or accepted methods to calculate the derating of the earth grid based on the no of strands in the conductor that are damaged?

What is the magnitude of derating from birdcaging of the conductor?

Are there any quick fixes that I have not though out?

The repair of the problem consists of copper wires that are twisted round the conductor to reduce bird caging.

Thanks in advance for any advice.
Your experience is priceless.

 

[WhiteyWhitey]

Sorry, Forgot the Photos.

 

[TurbineGen]

Why did you not use and exothermic bond such as a Cad-Weld?

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[waross]

Spend a lot of time cleaning the conductors with a wire brush. Erico makes an excellent brush for cleaning copper conductors.
Then use Cad Weld or a compression connector that is accepted by the AHJ.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[busbar]

Suggest that any fixes be with IEEE837-rated materials—be they hydraulic-compressed or exothermic.

 

[WhiteyWhitey]

I gather you are all less than enthusiastic about the current condition.

Thanks for the responses

 

[bacon4life]

I am intrigued whether might actually increase the ampacity of the cable due to larger surface area to transfer heat to the surrounding area. This would be somewhat offset by the current needing flow along the twisted wires instead of strand to strand. In the long term though, it would be more susceptible to mechanical damage and corrosion. Your solution seems to have address both of these.

The simplest formula is just to calculate the area without those strands. Are you looking for something less conservative?

 

[waross]

Generally repairs are made such that the original ampacity of a conductor is maintained. If, in the future, the existing equipment is replaced with equipment with a higher fault current, the engineer evaluating the adequacy of the existing grid will probably not know about the damage and reduced amacity of the existing cable. If the cable is not returned to full ampacity, you may have left a time bomb.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[WhiteyWhitey]

Waross,

Agreed.

Our engineering department has now proposed the following repair.

Bend back conductor to original shape as much as practical.

Fill in any open areas with copper strand

Wrap the damaged area in copper tape to at least the original cross sectional area.

Wrap the area with tape to hold it together.

I think Im satisfied with that. Although I'll be looking for some anticorrosion paste or something to prevent the copper tape from dissappearing.

 

[davidbeach]

Geez, cut out the broken/damaged conductor and cadweld in some replacement conductor. Then forget about it forever.

 

[WhiteyWhitey]

Unfortunately, That would mean we have to cut conrete and reinforcement out of the Joint bay.

Im working within the guidelines of my engineering department (My Boss) and he isnt keen to delay any further on the cable installation.

Its a compromise between the best thing to do and something less elegant but will work I guess.

 

[waross]

Can you put a compression splice over the damage rather than just wrapping it? That should restore the serviceability of the cable now and into the future.

Bill
--------------------
"Why not the best?"
Jimmy Carter