VT primary winding resistance 1

[Chrislatimer]

I have recently tested a 132 kV:110 V, 50(300) VA, Class 0.5M(1M) Trench German GmbH voltage transformer. I obtained a primary winding resistance in the order of 10+ KOhms. A collegue tested an identical unit and came to a value of 40 mOhm.I have tried to locate information on calculations or past test reports to no avail. Could someone inform me of calculation used to determine primary winding resistance of a voltage transformer or supply any past experience/values.

thanks in advance (first time poster so go easy on me
Chris

 

[DTR2011]

I would think this may be due to poor test connections. Have you verified this? Are you using the 4 wire method? Make sure that voltage sensing is "inside" the applied current.

One would expect milli ohms, perhaps a few ohms, depending on design.

 

[Chrislatimer]

Correct the four wire method was used. Omicron CPC 100 was test set. when measured with multi meter, reading was 11 or so KOhm, Omicron unit can only sense to 1000 Ohms, and reading was outside this. Currents on outside, voltage on inside, good connection. Magnetic VT no Capacitive.

My thinking is that due to the number of turns on the primary and low VA rating of the unit that the resistance would be much larger than say for a Power transformer (which has less turns ratio ie 110kV to 22 kV and is larger ie 50 MVA and has winding resistance of roughly 1 ohm)

My thinking is that a VT has longer run of wire and of smaller guage (due to space and size limitations) this would give a much higher resistance than that of a power transformer. is this right train of thought?

 

[edison123]

IMO both the values (kilo and mega ohms) are too high. How are you measuring the resistance ?

Muthu

http://www.edison.co.in

 

[edison123]

You're right that VT will be of smaller conductor (and hence higher gauge) than a power transfomer. When you measure resistances in transformers, you'll have to wait a few minutes for the readings to stabilize.

Muthu

http://www.edison.co.in

 

[Chrislatimer]

just to clarify: milli (not mega)ohms reading vs kilo ohms

measuring using 4 wire method. started with 5 Amp injection, outside measureable range of CPC (primary injection device) lowered current to increase ohm range from 20 Milli ohm-2 ohm to maximum of 1000 ohm. still outside reading.

Some previous test reports indicate reading in K ohms, but looking for calculation/formulae to verify method and answer.

thank you for your quick replies too, was expecting to wait days. this is great. thumbs up for Eng-Tips forum!

 

[DTR2011]

How do the ratio and excitation results look? You may also try a reduced voltage short circuit test. I believe you can view the R-X components and get a better idea. The VERY high readings may indicate some kind of poor connection or a partial open (broken strands). The reason for the 1000 Ohm limit in most test sets is due to the expected readings. I don't recall ever testing a VT with this high of a resistance without a reason. What does the manufacturer have to say?

 

[Chrislatimer]

Hi smallgreek, Ratio values were spot on. didn't do excitation result as was not in scope unfortunately.
Cannot find manufacturers specifications (or previous test report) in relation to expected winding resistance. Trench Germany GmbH SVS 123/3 VT.

 

[scottf]

Primary winding resistance for a 132 kV VT should be in the range of 10 kOhms or so. There is no way to calculate the expected value as a user, as it can vary widely based on the design of the VT.

The milli-ohm measurement was certainly not correct and should be performed again. Sounds like maybe this was a secondary winding resistance?

Did you check the test reports? We normally show the primary winding resistance factory measurements on our test reports.

For those who believe a kOhm reading is very high, remember that a 132 kV VT, probably has in the range of 25,000 to 40,000 turns of very fine wire on the primary.

 

[Latango]

Probably so obvious that it wasn't mentioned, but depending on what the connection point is made out of, oxide buildup on the connection point can cause really high resistance readings. If the points of the test set aren't penetrating the oxide that can cause erronious readings. A light sand paper or wire brush can solve that pretty easily.

As for the 40mohm reading the collegue got, are you sure they didn't test the secondary accidently? Doesn't sound too far off for 110V secondary, but seems too low for primary of a tiny 132kV winding.

 

[Chrislatimer]

Hi Scottf and Latango, yes i believe my collegue measured incorrectly but its hard to inform some people they may have made a mistake

Latango, you raise a good point, thankfully the terminals where in good condition with no evidence of corrosion/foreign debris.

as noted test report for this VT have been lost/misplaced and manufacturers info does not contain factory test reports. (unit is 15-20 years old and due to change over of business from state owned to private some things have been misplaced)

thank you to all who answered this question. I am amazed at the quick response time and would be happy to hear/read more replies/ideas on this.