30+ year old Current Transformers

[gilbertgodin]

I have a project to update power metering in a building. The existing CT's and PT's are 0.3% and would meet project requirements for accuracy but they are over 30 year old. In your opinion do CT's and PT's accuracy degrade over time. Thanks

 

[JG2828]

I don't think so, but you could certainly have them tested to find out.

Here is one test set, but there are plenty out there.

https://us.megger.com/vt-and-ct-analyzer-mvct

 

[cuky2000]

The IEEE Std C57.13 provide guidance to test instrument transformer for performance measurement and accuracy. A dielectric test such as the dissipation factor is available. It provides a diagnostic of an aging process taking place (moisture ingress, partial discharge, etc).

After 30 years in service most likely the Instrument transformer are in the wear-out region of the typical bathtub reliability curve were the risk of failure rate increase continuously. compromising system reliability.

Few strategies have been used to mitigate the risk of failure of re-using an old unit. Often ordering spare unit is only justifiable if the cost and consequences of a power failure can be tolerated. Otherwise, replacing 30 years old equipment may be a reasonable option.

 

[prc]

Just as power transformer losses will not increase with time, accuracy of CTs should not deteriorate with time. Of course if the steel starts rusting, it may due to higher exciting current.

 

[LionelHutz]

I suppose the insulation could degrade, especially the winding insulation of a PT, which could mean it is more susceptible to complete failure. But I wouldn't expect the accuracy to degrade.

 

[oldfieldguy]

I haven't yet seen a failure of a window-type CT that wasn't abused - exposed to weather, high temperatures, open-circuited, etc., in the thirty-odd years I've played with them in metal-clad gear.

Same goes for those mounted internally in transformers and oil circuit breakers.

Of course, getting wet, subjected to high temperatures, proximity to faults (fire, blast debris) negate that 'wasn't abused' caveat.

old field guy

 

[cuky2000]

Below is a sample of a CT failure after testing.

 

[oldrunner]

I would expect that the insulation - both solid and paper has degradation issues. What is the company's standards for when they take these kind of transformers out of service?

 

[Zogzog]

We have found several older ABB/ITE CT's completely break down under normal conditions, summer heat but still normal

 

[Compositepro]

Thermoplastics that melt are rarely used in electrical areas where heat can be high. This is probably a case where some thermoset plastic was not properly catalyzed and therefore never fully hardened. I've come across that a couple times in my life.