VLF Testing of Generator

[KenAlmon]

We have a 546 MW, 22 kV generator that was factory tested using DC hipot at 45 kV with good results.

The plant I am at presently was wondering whether using a VLF test set as the baseline field measurement would be better than repeating the DC test at site (albeit at a lower "field" value).

I know VLF tests to be superior in many ways for cables, esp HV cables; can the same argument be made for large generators? My review of the IEEE standards do not seem to reflect the growing trend towards using VLF.

Appreciate the comments.

 

[rasevskii]

It is possible that many manufacturers do not permit DC hipot testing of stator windings at all. The company for which I worked years ago did not permit DC hipotting of windings. In fact they had the original patents and developed equipment for 0.1 Hz testing, which I believe later became industry standard. DC hipotting overstresses the potential grading coatings on the coil ends at the point where the coil leaves the stator iron into free air (or H2). So I was told.

What about this: Edison123 and Wolf39?

rasevskii

 

[edison123]

Hi ras & ken

That's one big machine. If it was new machine or newly rewound, 45 KV DC is a very low value for a DC Hipot. For such windings, the AC HV as per standards is 45 KV AC (2U=1) and DC test voltage must be 1.7 x 45 KV.

I use DC hipot for field maintenance tests on large generators and did not see any problem with it regarding the stress grading systems so far. However, I prefer power frequency AC hipot if given a choice (which I rarely get) since it is closer to the real life situation. But it has the undesired effect of being a potentially destructive test. In my repair shop, all windings (whether new or overhauled) will be AC HV tested only.

I haven't used VLF testing till date on generator windings and so don't have much idea about its usefulness except may be the portability of the kit due to low leakage currents at low frequencies.

But none of these hipots are baseline tests for future comparison. For example, DC hipot leakage currents depend on the dryness of the winding and a leakage current today has no correlation with tomorrow's value. As for AC hipot, the leakage current at a particular voltage stays practically the same whether the winding is good or bad. It is more like fail/pass test. I'm sure VLF test would fall in that category.

If you are interested in baseline value for trending, Tan Delta and Partial Discharge tests are better since these values do change as the windings age.

My last $0.02. For such a big machine, it is better to invest in in-house testing facilities like AC Hipot kit, Tan delta kit and PD kit than rely on outside testing agencies.

Muthu

http://www.edison.co.in

 

[wolf39]

Ken:

There must be a misunderstanding. The 45 kV figure is exactly the standard AC test voltage for your 22 kV unit. I've never heard of a OEM using DC for hipot testing at their manufacturing facility for new or newly wound generators. AC is standard and such test equipment must be available at the factory test field. Occasionally DC tests are performed at site because such test equipment is easy to transport and cheap to buy. Nevertheless, to my experience AC is even standard at site. However, high voltage DC can be used advantageously to locate damaged insulation (in cases of transport accidents) in the stator end winding portion. In such case a long plastic rod with a grounded copper mesh at one end is moved on the stator bar surface for possible insulation crack detection. Test duration is no issue here.

As Muthu mentioned: The 1.7 factor still seem to be the standard number to convert AC test voltages to DC figures. I remember a publication in which the 1.7 figure was questioned (according to the author this number should be much higher) and I'll try to find this report.

I have no experience with VLF tests. Never seen it in factories or at site. I therefore very much doubt that such tests are common.

Regards

Wolf

http://www.hydropower-consult.com

 

[wolf39]

The 1.7 factor was introduced at a time when the groundwall insulation was based on shellac mica or asphalt mica. The breakdown voltage ratio of DC versus AC for a modern epoxy mica insulation was found to be about 4 as far as I can remember. I now also remember that it was "Greg Stone" who authored an IEEE paper dealing with HV test standards. I'll try to find out more details with Google.


Regards

Wolf

http://www.hydropower-consult.com

 

[edison123]

wolf

As per IEEE std. 95, that dc voltage to ac voltage ratio is 1.7. I too remember reading some article pushing for higher ratio levels but I am yet to see any standards to that effect.

Muthu

http://www.edison.co.in

 

[KenAlmon]

My mistake folks... the 45 kV test was AC. Sorry for the mis-lead!

 

[wolf39]

Muthu:

You are right, the current issue of standard IEEE 95 still recommends a factor of 1.7, but this may change in the future.

Ken:

IEEE is permitting hipot tests with AC and DC. The 0.1 Hz test is also acceptable but seems to be rarely used.

Now that I know that your unit was properly tested with 45 kV AC, I regard this thread as closed.

Regards

Wolf

http://www.hydropower-consult.com