Life expectancy of a current transformer 2

[CuriousElectron]

Hello,
For a CT installed indoors in a powerhouse, what is the "typical" or rule-of-thumb life cycle of a CT? What are factors affecting the life expectancy and is it common to see a CT fail in service? Typically, CT's are not maintained, so I was wondering what could be leading up to a failure..
My understanding is that CT's, especially older CT's could last 70 years or more.
Your thoughts are appreciated.
Thanks,
EE

 

[wcaseyharman]

I’ve not seen a CT fail due to age yet. We have CTs that are 64 years old and still in service.

 

[thermionic1]

I just finished a project where one of the buildings had a line up of 15kV metal clad switchgear that was installed in 1954. The breakers had been retrofitted as well as the relaying. It was a damp, poorly heated building. I spoke with the relay tech that was involved in the upgrade a few years ago. They evaluated the CT's with fairly standard tests including, ratio, polarity, excitation and megger. All of the original CT's were deemed ok for reuse. There were 26 breaker cubicle with line and bus dif CT's.

Outside in the switchyard, slip over CT's had been installed 20 years ago for a metering application that never happened. The CT's were shorted at the terminal block. I noticed the CT's were cracked and some were dripping rusty water. We had those CT's removed. This is a northern climate that gets snow and ice.

In my experience, CT's don't fail all that often. I've tested thousands and I can distinctly remember the 2 that were real fails, as they were installed inside large transformers. More often it's wiring and connections. Leave a CT open and it will fail.

I suppose some things that could lead to premature failure would include caustic environment, high temperatures, constant vibration and thermal cycling. If the CT is in metal clad switchgear, it's probably pretty safe. Same for inside a breaker or transformer. I suppose a bar type not rated for and subjected to multiple through faults could fail.