DC Resistance difference between phases

[kobavr]

Hi everybody,

I tested 180 MVA, YNd1 transformer secondary winding DC resistance as follows,
a-b = 2,5 ohm
b-c = 3,5 ohm
c-a = 2,5 ohm

What can be the reason of this test result? Why "b-c" resistance is different?

Thanks in advance..

 

[7anoter4]

It is very difficult to say all these could happen at the same time simultaneously:
No one individually could produce such a difference.
As a general factor we'll take 3.5/2.5=1.4
The result it will be the product of these factors:
1)differ temperature as b-c coil is in midlle of the tank 1.1572[90-55oC]
2)a slight difference in copper diameter4% factor=1.083
3)a slight difference in number of turns 1.04
4) a slight difference in coil diameter 1.05
5) a loose contact resistance 0.02 ohm[could be dangerous!] 1.02
So, it could be something more serious.

 

[DTR2011]

#1 I would expect milli or micro ohms on 180 MVA LV winding. - Check test connections / test equipment. DO you have factory test reports to compare field test results with?

#2 Testing resistance on Delta windings requires special considerations to test connections and / or further calculations. Check with you test equipment instruction manual on proper connections.

 

[unclebob]

Are these values consistent with the previous ones if any?

 

[sibeen]

I must admit I'm a tad bemused by the opening post. You got a resistance measure of a few ohms on the output of a large transformer, and you're worried about the difference in ohms?

Check your test equipment!

 

[ScottyUK]

What are you using to make the measurements? The winding inductance can cause results to take a long time to stabilise.

If you are using a high current test set then be careful when you disconnect the leads: wind the test current down slowly otherwise you will get a large inductive back EMF which may harm your instrument or you.

 

[PHovnanian]

What is the voltage of the LV winding? I have the same concern that smallgreek has: these are pretty high readings for low or even medium voltage winding resistances.

As to the unbalances, how were these measurements made? Was the delta opened up for each measurement? Or perhaps left connected inadvertently for one or two? Leaving 2R in parallel with R would give you a (false) 2R/3 reading, which is close to what you are seeing.

 

[khanengr]

if as I assume, all reading taken with correct measurement procedure and apparatus, can we say corrosion of winding wires is occuring so that the resistance is increased over all and more corrosion or oxidation between phase b and c???

what are the possibilities of it???

 

[ScottyUK]

Corrosion due to what? A transformer that size must contain about 100,000 litres of oil - is the oil contaminated?

 

[DTR2011]

Sanity check...
180 MVA GSU Transformer - LV winding in ANSI world would be ~13.8 kV to ~23 kV. I would guess in IEC world ~22kV. Thats a lot of
copper and milli or micro ohms would be expected. The I2R losses at Ohms would burn things (or severely heat) up ~ full load.

This is either a misplaced or mislabeled reading or as PHovnanian mentioned, the 2R/3 calculation was not made, or the test connections are not correct.