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AC Hipot of Large Generator Coils - uF ratings?

AC Hipot of Large Generator Coils - uF ratings?

The problem with using power frequency AC test sets to perform AC Withstand testing and Partial Discharge and Tan Delta/Power Factor testing is the high kVA capacity required if your test winding is high in capacitance. VLF can be used to help minimize this power. Question: is there a source somewhere that shows the typical uF ratings of stator windings, and others, of typical size motor and generator coils? Maybe a few for windings with a kVac input voltage of 2.4, 4.16, 13.2, 25, etc, I know there are a million possibilities but if perhaps any model listings of vendors that might show the capacitance along with all the other specs. Or, how about just for hydro generators. thank you.

RE: AC Hipot of Large Generator Coils - uF ratings?

I apologize if I'm telling you something you already know, but it's fairly easy to estimate
1 - Estimate total surface area A of coil insulation in contact with slot iron. It would be something like number of slots times slot length times (slot width plus twice slot depth)
2 - Estimate groundwall insulation thickness T (use something like 0.001" thickness per 50 to 100 volts if you don't have anything better)
3 - Estimate relative constant e_r dielectric constant of groundwall insulation (3 - 4 once impregnated...not sure about green coils)
4 - Calculate C ~ e_r*e_0 * A / T
This would be estimate capacitance to ground when installed and I assume (?) you will simulate this configuration during test. Once again sorry if I've missed your mark.

(2B)+(2B)' ?

RE: AC Hipot of Large Generator Coils - uF ratings?

Thank you electricpete,

I am familiar with calculating the capacitance like you illustrate. This assumes that one has the coil in possession or the data for a specific coil. I am more looking for any data that shows the range of capacitance's for several voltage classes of generators for the purpose of predicting the kVA needed to test the object. I know there are too many possibilities to what I ask, but if even a dozen samples can be had it would help. For instance: does GE or some other producer have a chart of the test voltage and power requirements, or the actual uF ratings of the coils, for a commonly used list of generators with a 6 kV or 13.2 kV input and up to maybe 100 mW. Maybe it doesn't exist and the AC power requirements from a hipot must be calculated each time for each specific motor. Thanks anyway for your reply.

RE: AC Hipot of Large Generator Coils - uF ratings?

From your title I thought you were asking about coils testing (which to me implies individual coil testing). That's what I was responding.

Now I understand you are more interested in the winding of whole machine.

Knowing capacitance of machine to ground is important in electrical protection. I'm sure I've seen some graph summaries of that and posted some here on eng-tips. Let me see if I can find that.

(2B)+(2B)' ?

RE: AC Hipot of Large Generator Coils - uF ratings?

Here's one reference that gives capacitance to ground for 4kv induction motors over a narrow range of ratings and wide range of speeds.


"Industrial Power Systems" by Shoaib Khan, Sheeba Khan, Ghariani Ahmed.
ISBN 978‑0‑8247‑2443‑6.....

4 System Neutral Grounding...

4.9 System Capacitance Data...

4.9.9 Induction Motors

Zero-sequence capacitance (C0) in uf/phase is given in table 4.8.

Table 4.8
Zero-Sequence Capacitance Data for Induction Motors (uf/phase)
kW kV 1800 rpm 1200 rpm 900 rpm 600 rpm
225 4.0 0.009 0.012 0.014 0.016
300 4.0 0.01 0.013 0.015 0.017
350 4.0 0.012 0.014 0.016 0.018
400 4.0 0.014 0.016 0.017 0.021
Source: Westinghouse, "System Neutral Grounding and Ground Fault Protection," publication PRSC- 4B-1979, Westinghouse, 1979.

(2B)+(2B)' ?

RE: AC Hipot of Large Generator Coils - uF ratings?

Thanks for the data. I found some other data when searching using some of the keywords from your source. To your remark about individual coils versus entire machines, I need to size the AC test equipment for both situations. Sometimes a standard design of AC hipot will be used, even if rated 30 or 40 kVA, and sometimes series or parallel resonance systems will be used. Thanks again.

RE: AC Hipot of Large Generator Coils - uF ratings?

Large hydro and thermal generators I have tested usually had a winding capacitance from 0.20uF to 0.36uF per phase. A tan-delta kit is a good way to know the winding capacitance.


RE: AC Hipot of Large Generator Coils - uF ratings?

Thank you Muthu. That helps me.

RE: AC Hipot of Large Generator Coils - uF ratings?

Just an update to OP.

Did a AC HV test on a 250 MW, 11 KV hydro machine stator today. 23 KV, 12.5 A at 50 Hz = 1.74uF per phase.


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