testing sample coils on generator 2

[electricpete]

For rewind of a 25kv 1000+MW generator using local vpi epoxy resin insulation system, I am interested in learning what strategy is used for testing sample coils:

1 - Assuming we request supplier to provide 2 or 4 extra bars, how do you select bars to test? Random? Lowest power factor?

2 - What type of test would you recommend to be performed on the sample coils.

3 - A manager asked me if I agree with the following tests recommended by a vendor: voltage endurance, TVA probe, partial discharge, and "CNDF"....what the heck is CNDF? I assume maybe it has something to do with Dissipation Factor (power factor).

4 - I know for motors we typically do dc hi-pot to failure and if that doesn't fail it then surge test to failure. Then we dissect the coil at strategic locations including near the location of the failure where we are more likely to find voids / weaknesses. I know surge test is not applicable to bars which have one (half) turn. What type of destructive electrical test would you recommend to find weak spots to dissect on a Roebel bar ?







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[electricpete]

5 - Is it practical/customary to specify an acceptance criteria for visual inspection of coils cut open? (maybe no voids? or no voids in groundwall? or no voids in groundwall larger than a certain size?)

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[petronila]

Hello Pete

Taking account the machine power and voltage I think the test most go far, starting for one material analysis: Copper, Resins (Wich is my orderly turn for the resin?, how is made the size control for the coils, are the machines on shop new or too old, computer aidded? (Spreaders-Tappering machines, etc), the work shop expertice: thy most provide a complete list of works and client contact in order to verify that works.

About the coils test,usually the coil manufacturers perform own test like Surge, Hipot and Tandelta test, but this one most be in agree with customer, due to some standards are different in europe and U.S

The usual is once the coil set is manufactured you can make a random choice in order to made H.V destruction test or if you are following the tan delta standard test then you take a sample in percent of all population and once you get the results then you can accept or reject the complete coil set.

Regards

Carlos

 

[byrdj]

have you searched EPRI ?

 

[wolf39]

Hi Pete:

Why didn't you ask the manager what a CNDF is?

Is this thread (and the other one depicting the cross section of a 1000MW+ 25kV Roebel bar) having a professional background?

Wolf

 

[electricpete]

have you searched EPRI ?

Good idea. Have not yet, but will. Any particular document?

Why didn't you ask the manager what a CNDF is?

I did ask and he didn't know... he is no technical expert but was relaying to me info from a 3rd party. I was not able to talk to the original source of the information. I also googled and found nothing. There is upcoming a multi-party phone call where I'll have a chance to ask, but if I can figure it out ahead of time that would save everyone's time on the phone.

Is this thread (and the other one depicting the cross section of a 1000MW+ 25kV Roebel bar) having a professional background?

Yes, absolutely.

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[byrdj]

I only had a few minutes to search EPRI and did not find any document that looked promising except for 1, and it just happen to be one that was not free to view.

are you familir with

http://www.generatortechnicalforum.org?

 

[electricpete]

I looked in EPRI also, and could not find anything specifically addressing this.

The closest thing I could find from EPRI was: EPRI 5036, Volume 16 “Handbook to Assess the Insulation Condition of Large Rotating Machines”

It’s a good reference, but doesn’t talk much about sample coils.

A better reference I found: “ELECTRICAL INSULATION FOR ROTATING MACHINES” (by Stone, Culbert etc), Chapter 15 = “Acceptance Testing and Site Testing of New Windings”

Thanks for the link to the site. It looks promising.


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[electricpete]

“CNDF” was actually “C and DF” meaning Capacitance and DF (... and tipup of course).

Since you guys didn’t figure it out, I don’t feel so bad for not figuring it out ;-)


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