High Excitation and No LV Winding Resistance Problems

Testers, Engineers, and Enthusiasts. Lend me your ears.

Currently trying to troubleshoot a couple of older dry type transformers. I am getting my high voltage side(HV) winding resistance(WR) on the two(1.6 and 0.9% variation averages) but the low side (LV) is not building the current needed, sitting at about 130uA, to achieve any sort of measurable data. I have DEMAG’ed the units and able to get good TTR numbers(.03% and .02% Avg) but still no LV WR. I have checked my connections and cleaned both the alligator clips from the equipment(TRAX 220 w/TSX 303 Switchbox) as well as the connection points on the core. The Excitation current(EXC) on one is approx. 750mOhms and the other EXC is around 380mOhms.

Using the handheld TTR unit(AEMC 8500) I am reading continuity on both units but when the unit runs TTR I am met with an error stating (EXC TOO HIGH).

Any help would move this along as well as broaden my troubleshooting prowess.

Thank you.

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What’s the story on the transformers? Secondary connections, in service prior to testing? What you are describing almost sounds like an open or short in the winding.

What current level are you testing WR? Might need to raise your test current.

Both units are throwing the same symptoms.

Unit 1: In service until unneeded before sitting in storage for 20 years, no secondary connections, and wouldn’t an open/short throw off the TTR (Tested at 125V after discovering the LV fault needed) results? WR was tested at 1A first since its a small XFMR and then 8A with no change.

Unit 2: Built in 2019, never in service and placed in storage until now. Also 208LV. Also tested the same. The 2 ABB units tested that morning were along the same hisitory profile as unit 2 but passed with flying colors. Now I have a New unit and a 20yr old unit exhibiting identical issues but the EXC for the 500kVa is around 780mA and the EXC for the 300kVa is around 280-300mA.

Unit 2 Testing sheet from this morning “Round 2” only difference is the EXC for Unit 1 and possibly ordering a unit to replace Unit 1 but Unit 2 is “new” and needs troubleshooting:

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LV dry types are a pain. I think your test current is really low. It may be “small” but with a 208 secondary the current is much larger than comparable mv unit.
500kva 208 is almost 1400 amps.
snip below is from the TRAX manual.


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Hi, i would agree with RVAsparky on the test currents. Do you have a photo of the name plate? And have you had any luck with the higher currents?

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