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Transformer HV Resistance Testsing Trouble

Transformer HV Resistance Testsing Trouble

Transformer HV Resistance Testsing Trouble

(OP)
(This is my first post so be gentle please)
Gentlemen, we have a generator power transformer (875 MVA, 138kv Wye/24kV Delta) that recently experienced a single phase H1 to earth fault external (very near the H1 terminal) to the transformer (4.7 x FLA for 6 cycles or so). The transformer was disconnected and we were called onsite to test the transformer. All capacitance, excitation, turns ratio tests came out fine. We ran into trouble testing the HV winding resistances.

The readings were;

H1-H0 = 67 milliohms
H2-H0 = 20 milliohms
H3-H0 = 38 milliohms
We have 3 years experience testing transformers with the CPC100 (Omicron) and have ruled out connection issues (changed leads, cleaned connection points etcetera).

Historically these windings measure 25 milliohms.

After we finished, a sweep frequency response analysis (20V up to 10k Hz) was performed (all tests pass, no winding deformation was indicated).

Then we retested the high side resistance and the high readings and imbalance went away.

H1-H0 = 20 milliohms
H2-H0 = 19 milliohms
H3-H0 = 19 milliohms
Could the original resistance readings have been adversely affected by residual flux in the transformer core and restored to normal by the high frequency SFRA test?

Or would this indicate an intermittent internal connection issue? So far dissolved gas analysis has not shown a significant increase in methane, ethane or any other fault gas.

Is there a procedure for demagnetizing a transformer core?

Any information you could provide would be greatly appreciated. We are quite perplexed.



 

RE: Transformer HV Resistance Testsing Trouble

Transformers of such large capacity typically need plenty of settling down time to get the final resistance values, as high as 15 minutes. May be the first set of readings was given enough settling down time ?

Muthu
www.edison.co.in

RE: Transformer HV Resistance Testsing Trouble

Omicrondave, I wonder if this trafo has tap changer? I suppose not (as it is step-up transformer), but in case it has may be you have operated several times and contacts in TC have been selfcleaned? I doubt this is the case but it is just a  possible reason.  

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
 

RE: Transformer HV Resistance Testsing Trouble

Dave,

I dont think residual magnetism was the cause of wrong resistance readings.If there was residual magnetism,you would have found high current values during low voltage excitation current test.Residual magnetism will not cause erratic resistance readings.

During resistance measurement of LV windings in large 5 limbed GTs, it takes long time for saturating the core and for getting steady resistance values.It also requires high current source of the order of 50-100A for correct values.
But you are referring to HV winding resistance,even that possibility is ruled out.

Did you operate HV tap changer during SFRA?In case some oxides were there on tap changer contacts, it may get wiped out during tap changing. I had such experience.

Method to demagnetise transformer core- Find out the no-load excitation current of transformer. Send through one of the windings battery DC current = root 2 times AC excitation current.Reverse current flow. Then send DC of 75 % of earlier value. Reverse current flow. Repeat with still lower DC current. By this way you can demagnetise core.Be careful while breaking the circuit .To avoid arc flash, you can put a diode across the winding.
 

RE: Transformer HV Resistance Testsing Trouble

(OP)
We exercised the tap changer full range several times. Normal tap setting is 3. The readings did not change.
Here are the last two excitaion tests (5kV range)
2009 (Jan)cold              2010(June)hot, post-fault
H1-H0 160.43ma, 726.66W     H1-H0 141.95ma, 539.53W
H2-H0 129.77ma, 589.12W     H2-H0 117.79ma, 461.92W
H3-H0 159.19ma, 716.63W     H3-H0 138.89ma, 532.52W

RE: Transformer HV Resistance Testsing Trouble

For that beast, I find that 25 milliohm value is very small for a high voltage winding.

Just make sure you use the same test set with the same operator, if possible.

I'm with Edison, it may take some time to get steady results.

Also measure H1-H2, H2-H3 and H3-H1. Il will give you another set of result to play with.

RE: Transformer HV Resistance Testsing Trouble

Call OMICRON and ask for Chuck

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