Power transformer - allowable winding temperatures

[JPetersen]

Dear friends,

We have a transformer, 3000kVA - step-down 6kV/0.4kV - cast resin - insulation class F.

Right now it is running at winding temp 130 deg. cel. (full load - PT100 embedded measurement). Ambient temperature is 20 deg. cel.

Ambient temperature is expected to reach approx. 35.

We expect to see winding temperatures of 130 + (35-20) = 145 deg. cel. when at 35 deg. amb.

Insulation class F allows a max. winding temp. of 155 deg. cel., and calculating with a hot spot temp. of +15 deg. cel. we unfortunately seem to be +5 deg. cel. on the wrong side.

What are your experiences with transformer winding temperatures? - will a 145 deg. cel. winding temp. be critical, or have any impact on lifetime?

At what temperature will the winding insulation break down instantly
Can you recommend any litterature on transformer winding temperatures and lifetime calculations?

Best Regards
J. Petersen

 

[Reactorman]

The transformer at 160 deg C hot spot is only 3- 4% above the Max. limit. This should not cause breakdown. Also since this only occurs at 35 deg C ambient there may not be a significant loss of life. However you might consider adding fans to be on the safe side.

Good Luck

 

[BigWallyB]

Generally agree w/Reactor. None of ours have ever come close to max temp so I cannot speak from experience. There are many variables affecting breakdown- age, history, care level (moisture, cleanliness), design, etc. If this is going to be a continuous problem, it would be best to look into a permanent resolution now rather than later.

Life is what happens while we're making other plans.

Wally