metal label-tags on medium voltage motor T-leads

[electricpete]

Does anyone have opinions / recommendations / references regarding the use of metal label-tags (T1, T2, T3 etc) on mediu voltage (4kv, 13.2kv) motor T-leads?

Apparently they are sometimes used because they are more durable. However I wonder whether they might pose a concern for voltage stress concentration... considering leads are unshielded and the metal tag from one phase is not prevented from touching the metal tag from another phase.

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[7anoter4]

Hi electricpete,
If I well understood your question it is about Type I terminal boxes as per NEMA MG1.
According to NEMA MG1 part 20 there are two types of terminal boxes: Type I and Type II.
For up to 8 kV you may use Type I- no support insulator included and reduced dimensions. This is the type suitable for hazardous location where the reduced volume corresponds to an increasing resistance to explosion.
The connection is usually done using screw bolts in a MCK-5-8 Raychem [or equivalent] termination.
In a usual environmental atmosphere and for more elevated voltage you will need type II.
There are many advantages for Type II as you could visit the inner live parts in order to fit a temporary ground connection [for instance].
But some time the box could be bigger than the motor itself!
See, for instance:
ABB HV_NEMA_catalogue

http://www05.abb.com/global/scot/sc.../$file/hv_nema_motors_catalogue_en_022009.pdf

 

[electricpete]

Hi 7anoter4.
Thanks for your response, but unfortunately I was just talking about metal labels placed on T-leads for purposes of distinguishing T1, T2, T3, T4, T5, T6. Some use a plastic kind and some use a metal kind because the plastic don't make survive a bake cycle very well (like bake to dry after steam cleaning). I'll see if I can find a picture to show it.

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[electricpete]

Sorry, I may not have described the problems with the plastic well (after all insulation has temperature limits as well). I don't really know why the metal are used so much but they are.

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[edison123]

I see them all the time and they have not been an issue. After all, don't these leads pass through grounded stator terminal box ?

Muthu

http://www.edison.co.in

 

[electricpete]

I'll see if I can find a picture to show it.

Attached is a photo to identify T2 lead. A metal band that extends all the way around the lead.

Muthu - I agree T-leads are run along the grounded stator frame. The stress distribution in the insulation depends not only on the potential but the geometry... so sharp edges tend to cause much more stress intensification than for example lead running adjacent to a flat metal plane. At locations where the T-lead crosses a sharp corner (such as going through the throat of motor frame into the term box), softening materials are tpyically applied as good practice. One obvious purpose is to prevent mechanical chafing, but they also have the benefit of moving the cable a short distance away from the sharp corner which would reduce the max stress within the insulation. Without such softening I would suggest you would not only see possible mechanical chafing but also partial discharge (for example 15kv cables used on 13.8kv system).

I have seen three times where we saw partial discharge at T-leads in terminal box. Will post about that next.

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[electricpete]

Attached are photo's from one very recent example freshest in my mind. Following an outage we saw an increase in measured partial discharge on a 13.2kv motor (from maybe 200 millivolts to 500 millivolts... alert level is 400 millivolts).

We operated for 18 months. During the next outage by coincidence an electrician was working in the term box and observed what he considered improper lead spacing and resulting partial discharge (the electrician was not aware we had identified high partial discharge through our testing). Specifically leads of different potential were laying against each other at an angle. Photo's of this condition and the partial discharge is attached. I was not 100% sure at the time whether this was partial discharge or not.

We repositioned the leads, didn't do anything else to the motor (other than megger testing) and upon restarting the motor the partial discharge has returned to normal 200 millivolts. I conclude the visual indications shown on the photos really are partial discharge and separating the leads reduced the partial discharge.

Some other points of discussion:
1- 2 leads of different potential touching each other have higher stress concentration if they crossing at an angle then if they run parallel (as they do inside the motor).
2 - Looking at the photos I see there are 6 leads coming through from motor to term box. If you asked me to guess I would have said the top 3 holes are line side leads and bottom 3 leads are neutral. But the pd is in fact occuring at location of contact of 2 leads among the bottom 3. I suspect for some reason we have line side leads coming through on bottom... I will ask about that Monday.


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[electricpete]

Stress concentration can be associated with a ground plane which is a "corner" in either the circumferential or axial direction.

The worst case stress concentration would be a ground shaped like the point of a needle held next to the insulation, because it is a stress concentration in all directions.

I guess one factor working in the favor of the metal label (if grounded) is that it would be a stress concentration only in the axial direction... since the ring is uniform in the circumferential direction. The fact they are widely used would seem to be an important factor to suggest they are ok, but they still make me nervous.

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[electricpete]

In the attachment posted 9 Jan 11 15:14, the 1st photo is the as-found position of the leads. The 2nd two the leads are moved to get a better view of the white suspected partial discharge area between the leads.

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