Bad transformer resistance readings

[49cubinok]

Good morning

Some techs in our shop recently tested a 480-1100/3811 transformer in our shop. The turns ratios came back just fine but the resistance readings were off on about 2/3 of the taps. Here are some examples.

Tap h1-h2 h2-h3 h3-h1
1-1. 315.8. 159.12. 157.63
1-5. 370.56. 184.95. 186.26
3-2. 196.83. 225.67. 152.30
5-3. 200.79. 197.63. 196.18
So most of the taps 5-1 through 5-5 are close, but most under those taps show very different resistance.
I’m curious if a switch is messed up or corroded badly? Or if it was just tester error. No oil analysis was performed on transformer yet.
Any thoughts?

 

[49cubinok]

Forgot to mention, I did reach out to manufacturer to get original test report but I’m not holding my breath to receive it since they are out of columbia.
Thanks

 

[WasGSOHM]

Can a single short between windings or an open explain these readings? Both at the same time may be unlikely.

Searching through these many combinations and permutations is tedious, but better you have some answer before they call you, as to what's possible and what's not.

 

[MrJLH]

Have you check NETA MTS?

 

[Hv&Lv]

Good morning

Some techs in our shop recently tested a 480-1100/3811 transformer in our shop. The turns ratios came back just fine but the resistance readings were off on about 2/3 of the taps. Here are some examples.

Tap h1-h2 h2-h3 h3-h1
1-1. 315.8. 159.12. 157.63
1-5. 370.56. 184.95. 186.26
3-2. 196.83. 225.67. 152.30
5-3. 200.79. 197.63. 196.18
So most of the taps 5-1 through 5-5 are close, but most under those taps show very different resistance.
I’m curious if a switch is messed up or corroded badly? Or if it was just tester error. No oil analysis was performed on transformer yet.
Any thoughts?

I’m assuming you slid the switch around a couple of times to scrub the contacts before testing?
Those readings do look a little off.

story time...
We were testing a 15MVA transformer and switching around the taps to scrub the contacts.
All the sudden the switch was EXTREMELY easy to move. Sent it off to be repaired. Turns out the cold rolled pin fell out of the rod from the switch handle to the taps

 

[synchro]

Tap h1-h2 h2-h3 h3-h1
1-1. 315.8. 159.12. 157.63
1-5. 370.56. 184.95. 186.26

If the primary is a delta configuration then the two cases above could result from the H1-H2 winding being disconnected. In this case the resistance across the H1-H2 terminals would be the resistance of H2-H3 and H3-H1 in series which is the sum of their individual resistances. This scenario would match the measured results above to within less than 1 ohm.

 

[WasGSOHM]

would match the measured results above to within less than 1 ohm.

You da' man!!!

 

[WasGSOHM]

"Show your work."

So, howdya do it?

Intuition?

Brute force check thru all combos?

Psychic?

Guessed?

Other?

 

[ATSman]

Looks to me like simple arithmetic.
Just draw a triangle and open one of the corner connections to see what synchro is talking about.
This lends new meaning to the term: "Open Delta" where the primaries of two single phase control transformers are
connected in series to get three phase system voltage. The purpose is to eliminate the cost of a 3rd transformer.

 

[jim dungar]

Looks to me like simple arithmetic.
Just draw a triangle and open one of the corner connections to see what synchro is talking about.
This lends new meaning to the term: "Open Delta"....

I recall such a broken delta configuration, used with resistance grounding as a way to create an artifical neutral for MV systems.