BCT polarity testing

[stevenal]

A thread a while back strayed into the subject of testing BCT polarity while installed on a power transformer. I cannot find the thread anymore. Red flagged for some reason? Anyway, I mentioned performing such a test a few years back and claimed I didn't remember shorting the unused power transformer windings. I had occasion to do a similar test this past week, and the instructions were clear that the unused power transformer windings were to be shorted. The test set involved is a Multi-Amp ratio, excitation and polarity tester. I believe Multi-Amp is now Megger. The set applies a voltage to the CT secondary, and displays the primary voltage added to or subtracted from a fraction of the secondary voltage. When switched from add to subtract, reading will drop if polarity is correct and increase if reversed. We were able to quickly correct a REF protection scheme, and increase our confidence in a bus differential scheme.

Just wanted to clear up any misunderstanding I may have caused.

 

[ScottyUK]

Hi Stevenal,

It was my thread - I think it is still there somewhere. Thanks for the update: I've got a generator transformer with a probable reversed neutral CT to test when it comes down for outage in the summer, and this method sounds considerably easier than a primary injection test. It's good to get some feedback from a user of the Megger unit rather than hearing the sales pitch.




-----------------------------------

Start each new day with a smile.

Get it over with.

 

[jbartos]

Suggestion: Visit

http://www.geindustrial.com/products/manuals/GEH-230AF.pdf

for the polarity importance

 

[RalphChristie]

Steve

Is this the instrument you've used?

http://www.advanced.co.th/products/spec/CTER91.pdf

Ralph

 

[jbartos]

Question: Is any industry standard in particular adhered to? Which one?

 

[stevenal]

Ralph.

Yes.

j,

C57.13.1 describes some of the tests this instrument performs, but this particular polarity test is not described.

 

[jbartos]

Suggestion: Reference:
ANSI/IEEE Std C57.13-1978 An American National Standard IEEE Standard Requirements for Instrument Transformers,
Section 8.4 Polarity addresses three methods commonly used for determining CT polarity.

 

[stevenal]

8.4.3 is the basis of the method used.