Earthing Conductor Installation -Transformer Surge Arrester Question 1

[ThePunisher]

In Canada wherein our area is subjected to -40 to -50 Deg C, The insulated single core GREEN earthing conductors from the transformer surge arresters to earth are installed hanging from top bracket to the surge counters with insulator support in between.

I am wondering if this is in violation to CEC 2018 Rule 12-102 and Rule 12-120. However, one may also argue that this is earthing (grounding) conductor and not a "current carrying conductor". If there is conductor movement due to continuous very low temperature variation, will it matter if this was grounding conductor?

If the conductor temperature rating is suitable for -40 deg C, will it matter as well?

See attached typical photo

 

[waross]

From Appendix B, Notes on rules.
Rule 12-102 Notes
Once a conductor is installed in a fixed position, it may operate safely at much lower ambient temperatures. The
lower temperature markings on conductors are based on laboratory tests under controlled conditions and are
provided as guidance from the manufacturer. A conductor suitable for installation down to –10 °C may be
handled (when de-energized) at lower temperatures, with appropriate care.
Appropriate caring includes
(a) minimizing flexing of the conductor;
(b) when flexing the conductor, bending the conductor slowly; and
(c) working with an increased minimum bend radius.
When designing installations intended to operate continuously at a lower ambient temperature, consideration
should be given to installing a conductor suitable for handling at that temperature.

bending the conductor slowly

Bending due to temperature changes will be as slow a bending as we can get.
Rule 12-120 I understand this rule to apply to long vertical runs such as cable chases in multi story buildings.
A look at table 21 supports this understanding.
eg: Per table 21 the maximum support distance for AWG #6 to #0 cable is 30 meters.
I don't think that you have a problem.


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

 

[cuky2000]

Without specific data of the installation is very hard to provide an accurate opinion. However, the first impression does the installation of the SA grounding cable appears to be satisfactory. Here is the best guess that I can offer with the available info:

Q1: I am wondering if this is in violation to CEC 2018 Rule 12-102 and Rule 12-120.
_{However, one may also argue that this is earthing (grounding) conductor and not a "current-carrying conductor".}
A1: Although my limited understanding of the CEC, I do not see major issues that violate Rules 12-102 to 12-120 provide general considerations for the 3 insulated conductors to connect the SA insulated base to the lightning counters to the common ground at the substation grid. _{Generally, the CEC Rule is concerned to the treatment of insulated conductors, flame test coverings, multi- and single conductor cables, conductors in parallel, radii of bends in conductors, conductor joints and splices, treatment of the ends of insulated conductors, termination of conductors, termination, and splicing of aluminum conductors, and supporting of conductors}

Q3: If the conductor temperature rating is suitable for -40 deg C, will it matter as well?
A3: Considering that the linear thermal expansion (assuming a Cu conductor) is 17x10^-6/^oC, for a short run of cable the expansion and contraction appears is not excessive that create a large force for a typical short run on transformer application.

Q2: If there is conductor movement due to continuous very low-temperature variation, will it matter if this was a grounding conductor?.
A2: Judging by the picture, the larger cable section is horizontally from the SA and then 90^o bend (verify R > 8xD) down to the insulator (guessing 15-20 ft). Appears that any contraction or expansion combined with the cable bend will no transmit excessive forces to the base of the SA or the support insulator down in the transformer tank. The short-run to the lightning counter does not see to be a problem impacted by the low temperature.

_{NOTE: The selected cable insulation, should be suitable to operate at sub-zero temperature of the site.}

 

[stevenal]

The arresters are insulated from their supports? If so, you have some very long lead lengths greatly reducing their effectiveness.

 

[waross]

I am a little concerned about the horizontal runs and the fairly tight radius bends having a predisposition to flash-over for lightning surges. 18" radius arcs would be better.
Given the sharp rise time of lightning surges, a sharp bend will add inductive reactance and impedance.


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

 

[cuky2000]

Hi Bill,

Bend on the arrester lead should not be a concern.

 

[waross]

Hi Cuky.
I don't want to contradict your quote, however it may be at odds with the guidelines for the protection of structures.
I understand that these guidelines have been developed in part by observation of the burn paths adjacent to sharp bends of actual lightning induced flash-overs.
I understand that calculations of the self induction of a sharp bend at extremely short rise times supports this experience.
To be fair, your quote states that transformer protection will not be compromised by sharp bends, not that sharp bends will not invite flash-overs.
Quite possibly the voltage to ground from the lower end of the arrester is low enough that the added impedance of a sharp bend will not have an negative effect on transformer protection.
On the one hand I will err on the side of safety and avoid sharp bends as much as possible when grounding arresters.
On the other hand, I am open to changing my methods when I see an acceptable test and demonstration.
All in all, a good point Cuky.

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

 

[jghrist]

The long insulated ground lead will reduce the protective margin of the arresters. There will be a significant L·di/dt voltage drop in the lead which will increase the voltage impressed across the transformer insulation during a surge.

 

[ThePunisher]

Thank you all for your feedbacks.

Waross, the transformer is a 72-13.8kV, delta primary- 13.8 kV wye secondary with neutral connected to a 100 A NGR

I would be getting more photo of the installation and green conductor specs printed on the insulation jacket today.

 

[ThePunisher]

I was also wondering if I we could have just used an extra strength PVC conduit and use an appropriate sized elbow fitting with radius R > (8 x Diameter) at the drop off.

 

[stevenal]

I would be getting more photo of the installation and green conductor specs printed on the insulation jacket today.

It has nothing to do with the insulation specs, it has to do with the lead length voltage drop as jghrist said. What's the purpose of this odd installation, and is it worth sacrificing arrester effectiveness?

 

[cuky2000]

In absence of any data and guessing from the picture originally posted, it appears the primary voltage ~138 kV with a larger lead length assumed <30ft _{(12' Btwn arresters & 18' for down leads)}. A rough estimate from the graph below provide a max. allowable protective distance from arrester to the transformer >20'. Since the arrester is adjacent to the bushings, most likely the arrester effectively will protect the transformer again lightning surge.

Until better data became available, the preliminary conclusion is that the lead length is not excessive and the arrester could provides an acceptable degree of protection.

For reference see also the enclosed thread.

 

[stevenal]

On reread, I see the purpose of the long lead lengths: to enable the use of direct reading surge counters. Have you considered fiber optic connected counters? To see the effect of lead length see IEEE C62.22.