Using VLF for m/g tan delta testing 2

[VLFit]

Anyone out there with experience researching and/or using VLF high voltage testers to not only perform an over voltage AC withstand test, but also to measure Tan Delta, Power Factor, and PD of motor and generator windings? thanks.

 

[edison123]

IEEE abstract

The capacitance bridge, first described in its present form by Dakin and Malinaric,1 has proved to be a very useful tool for measuring insulation quality, and is regularly used in our laboratory (along with a 60 Hz Hartmann and Braun transformer ratio arm bridge) for periodic diagnostic measurements of insulation samples undergoing life test. Since we accelerate these life tests by applying a high-frequency voltage at 420 or 1440 Hz rather than 60 Hz, it is essential to know whether the total charge transfer per cycle, Q, due to partial discharges varies significantly with frequency; for only if Q is frequency-independent can the ratio of test frequency to 60 Hz be taken as the acceleration factor. Using the capacitance bridge and a high-power audio-frequency oscillator, we have measured Q over the frequency range from 60 to 1600 Hz on a variety of types of insulation, including various specimens of resin-impregnated-mica generator insulation and solid resin insulation. Where one's interest is in the discharges in internal voids in the insulation, it is necessary that the electrodes should make intimate and void-free contact with the insulation surface and that guard rings should be applied to exclude from the measurement surface discharges at the electrode edges. With these precautions, it is found that, in all samples measured, Q is frequency-independent. A typical example of the charge versus voltage parallelogram display is shown in Fig. 1, for frequencies of 60, 420, and 1440 Hz; no variation is evident.

http://ieeexplore.ieee.org/document/7735899/

No idea if the same holds good for VLF measurements too.

Muthu

http://www.edison.co.in

 

[jfpe]

I'm not sure about motors, but we do VLF and tan-delta testing on cables. NEETRAC (National Electric Energy Testing, Research & Applications Center) gives several figures of merit when reviewing tan-delta data (stability, tip-up, tip-up-tip-up). I've found some of the test set manufacturers provide a lot of useful information as well. I'd try searching NEETRAC's website and asking your test set manufacturer.

-JFPE

 

[electricpete]

If you are an EPRI member, check out these documents

https://www.epri.com/#/pages/product/000000003002008021/

https://www.epri.com/#/pages/product/000000003002010687/

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(2B)+(2B)' ?

 

[edison123]

hey epete, you don't happen to have 40K lying around, do you?

Muthu

http://www.edison.co.in

 

[electricpete]

$40k

Haha, sorry I left that on my dresser this morning.

Some general comments for op:

My understanding is that if you are only interested in testing motors directly from the terminals, vlf offers very little benefit over power-frequency power-factor/tan-delta/doble testing.

The only advantage comes when testing through a cable:
The new-age cable engineers won't allow dc testing for many cable types due to concerns about aging the cable.
Most ac power-frequency testers would struggle to provide charging current for a long run of cable.
The vlf is similar to the power frequency ac tan delta test, except with lower frequency that charging current requirement is significantly reduced.

I skimmed the first EPRI document, not enough to fully understand it or make a judgement on the technology. There are a variety of tests done for eleven medium voltage motors.
They compared results from dc testing to vlf testing, and also compared testing at motor terminals to testing through shielded cable or unshielded cables, depending on the power system configuration for each motor.
The vlf test in addition to giving tan delta at various voltage steps (and tipup) gives something called "variability" or "% standard deviation" which I gather measures the behavior within a 2-minute constant-voltage step.




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(2B)+(2B)' ?

 

[VLFit]

Hey Pete, thanks for the reply. I am very familiar with VLF technology, especially for cable testing. The only other IEEE sanctioned use of VLF (ieee433-1974/2009) is for testing motor and generator coils. However it is not frequently used for this application. Correcting one of your comments, VLF offers a big economic savings and convenience for generator/motor testing due to its far smaller size, weight, cost, and power consumption than power frequency testing, even if using resonant systems. Testing a large generator coil rated for 13.8 kV, even higher, with a winding capacitance of maybe 0.20 - 0.30 uF would be nearly prohibitive in many cases with 50/60 Hz power.

VLF is used but mostly for withstand of coils, when it can also be easily used for TD and PD testing. It would be extremely beneficial to the motor world if it would use VLF for its overvoltage, off-line Withstand and Diagnostic testing. To get there, it would help to start building a library of User experiences and data. The Physics of it is obvious and sound, but many like the comfort of following others that can show results.

Thanks again. I still havent been able to get the two EPRI papers. I would love to at least read the conclusions. And remember, high voltage makes you feel good!