Interior 13.8 KV Busway Life Expectance

[hewl35]

I work for a large midwest medical university we have almost all of our high voltage inside in a some what climate controlled enviroment (basement) with no sun exposure supported by cable trays. The cable is all copper three conductor, 15KV (running at 13.8 KV)IAC amoured cable, 133% insulation, shielded cable that is anywhere from 1 year to 30 years old. The wire sizing for the most part is mixture of mcm350 and mcm500. We have eight feeders running less than 100 amps each. Seeing as how we are hardly straining the cable during normal use when we hy-pot the cabling after working it we only go to 18 KV to prevent a blowout. As the facility gets older we are seeing more and more blips, dips, etc and are wondering how much of this is a cabling problem. Certain parts of the facility is heavy into research and need to have a zero probablity of failure. The question came up is what exactly is the life span of the cabling? Where is your comfort factor? Thanks

 

[jbartos]

Suggestion: Try to find out the cable manufacturer and obtain it from the manufacturer. Some projects, e.g. nuclear power plants had their equipment specified for forty one years. Now, the nuclear power plants are applying for a twenty year extensions. The time expectancy then would to amount to 61 years. However, the full extent of these extensions success is still remained to be seen.

 

[DanDel]

It has been my experience that any 15kV cable over 20 years in age is living on borrowed time, especially if the insulation is XLPE. EPR may last longer than XLPE, and PILC can last the longest(maybe up to 40-50 years). 3-conductor cable is usually more fragile than individual single cables, also.
Every maintenance test you perform above the voltage rating of the cable will degrade the insulation and take some life away, though the voltage you're using(I'm assuming that's 18kV on a DC hipot) is below the peak voltage at 13.8kVrms.
There are other tests which are not destructive. The 5kV or 10kV PI test, or using a Doble PF set(you will probably need a resonating inductor for cable testing unless the cable is extremely short) can be used, though the data derived is typically less useful than a hipot test.
I don't think any of your 'blips and dips' are a problem caused by the cable; if you had any kind of cable problem, it would quickly fault to ground. Rather, voltage transient problems may cause damage to the cable insulation.
Forget about 'zero probability of failure; it doesn't exist in the real world. I don't have access to your cable testing results, but for ELP and PILC, cable degradation may be visible over time. Don't trust XLPE, it is prone to sudden failure.
Since you do apparently have some scheduled shutdowns for maintenance testing, for maximum reliability, my advice would be to plan on:
1. Changing out the older cable with new 133% EPR single conductor, and
2. Installing what you need for a parallel feed in the event of a failure in each of your primary circuits.

 

[peebee]

"Heavy into research" really does not sound that critical to me, depending on your exposure to loss in the event of a cable failure. In a nice climate-controlled environment, it wouldn't surprise me if you got 30 to 50 years out of most of your cables (although some would be expected to fail prior to that).

If you are talking life safety or BIG money operations, I'd recommend replacement after 20 or 30 years. Not only that, but in a truely critical operation you should be looking at redundant MV cable systems and redundant UPS systems and generator systems for power backup.