Transformer Power Factorr Test 3

[rhatcher]

I am going to be perfoming some maintenance and testing on 9 single phase, low voltage transformers set up as 3, three phase banks. The transformers are dry type, 450V-120V, 250kva. Among the specifications is the requirement to perform power factor testing.

My experience with transformer testing is limited to the various types of high potential tests (DA, PI), impedance test, TTR test, and surge comparison test.

I understand in principle what the power factor test is and what the purpose of the test is but I have never performed it.

What voltage should be used? My understanding is that this test could be performed at low voltage (ie 200V) and then the results could be interpolated to any higher voltage because the power factor of the insulation will not change with increasing voltage as long as the insulation does not break down. Is this correct?? If not, what is the recommended test voltage for the 450V primary, 120V secondary transformers?

My other question is how do you interpret the result?

 

[oldfieldguy]

Mr. Hatcher-

If you employ a qualified electrical testing firm to do this, you leave these questions in their hands. Further, if they use equipment and are clients of Doble Engineering, then your testing agency will have access to a huge database of previous test results of many types of transformers, making the interpretation of results mych easier.

old field guy

 

[rhatcher]

oldfieldguy,

Thanks for the response. My post described my experience and the performance of this test is not beyond the abilities that my previous experience would require. This is a test that I understand in principle but have never done before. I was hoping for a response from someone with experience who is willing to help.

If you have performed this test, or any of the tests that I listed in my experience, then your help would be appreciated.

Otherwise, thanks anyway.

 

[rhatcher]

I would add to my original post that I understand that this type of test is not normally performed on this class/type of transformer. This is another reason that I am seeking advice from someone with experience in performing this type of test.

 

[oldfieldguy]

Mr. Hatcher--
Forgive me if I sounded unsympathetic. Many users do not perform power factor testing on transformers of this size and much power factor test equipment is ill-equipped to handle voltages this low.

Were you my client I would have advised thusly. However, if you must do the tests, then take care of test voltages and if you're using older, manual equipment, heed your scaling and multipliers. However, if your replicate the test regimen on all units, you should be able to note any that test much differently from the rest. I would expect all results to plot within 10% of one another in respect to charging currents and watts loss.

Power factors on dry-type transformers ranges widely, unlike most oil-filled transformers, so interpretation will likely be relegated to comparison between like units. It may also be useful to compare questionable power factor results with your DC (PI, DA) tests to corroborate questionable findings.

It is common practice with low voltage units to just ground the low voltage winding and energize the high voltage winding for testing.

Good luck.

old field guy

 

[rhatcher]

oldfieldguy,

Thanks, your advice is greatly appreciated. I have already questioned the test as being unusual for this equipment and have been told that the specification must be followed.

For the question of test voltage, I have identified a PF tester that is rated for 200V that I would like to use. The type of testers that are more common are rated at 10kV. Although they have a variable output that could theoretically be set for 450V, I question how reliably or safely (for the transformer) that you could operate such a device at a fraction of the output voltage. I am thinking that 200V will be adequate since I am under the idea that the insulation PF is independent of voltage until the voltage begins to break down the insulation. Does this idea sound correct to you?

Finally, your answer on interpreting the results is thoughtful. Like many other interpretive tests, there does not seem to be one 'correct' answer. What I am seeing is that lower power factor indicates good insulation and higher power factor indicates poor insulation. Since I do not have previous test results that could be used to indicate a trend, you are right that the best result that I could offer is a comparative analysis of the various transformers against each other. Since the transformers in question are 40 years old, I suppose that if all transformers indicated a high power factor (>50%) that it would be safe to say that they are all equally aged...

Thanks again for your help.

 

[electricpete]

"IEEE Standard Test Code for Dry-Type Distribution and Power Transformers" (IEEE Std C57.12.91-2001) states:

10.8.3 Applied voltage
The voltage to be applied for measuring insulation power factor shall not exceed operating voltage to ground
for any part of the winding or 1000 V, whichever is lower.

=====================================
(2B)+(2B)' ?

 

[electricpete]

"IEEE Standard Test Code for Dry-Type Distribution and Power Transformers" (IEEE Std C57.12.91-2001) states:

10.8.3 Applied voltage
The voltage to be applied for measuring insulation power factor shall not exceed operating voltage to ground
for any part of the winding or 1000 V, whichever is lower.

=====================================
(2B)+(2B)' ?

 

[electricpete]

Sorry, some problems with my posting.

Also some general comments about power factor testing dry type transformers, which may be an overlap with the comments above:

IEEE57.94-1982:

NOTE — The insulation system power factor of a dry-type transformer is a function of the type of winding design and the type of
insulating materials used in the windings plus other possible design variables. As a result, it is not possible to specify a
single satisfactory power-factor value. However, data on changes in power factor over a period of time may prove useful,
particularly when considered in conjunction with other maintenance test data such as insulation resistance. Care must be
exercised in recording atmospheric conditions (humidity, temperature), insulation system temperature, time since being
energized and when the transformer was last cleaned, all of which influence the power factor. See ANSI/IEEE
C57.12.91-1979 [11] for further information on how this test should be conducted.

Also, the bushing type and surface conditions can make a big difference.

=====================================
(2B)+(2B)' ?

 

[electricpete]

NETA MTS-1 (2001) section, regarding "large" dry type transformers states:

CH and CL power-factor or dissipation-factor values will vary due to support insulators and bus work utilized on dry transformers. The following should be expected on CHL power factors:
Power transformers: 2.0 percent or less
Distribution transformers: 5.0 percent or less
Consult transformer manufacturer's or test equipment manufacturer's data for additional information. Tip-up test watts loss values should indicate no significant increase in power factor.

=====================================
(2B)+(2B)' ?

 

[DTR2011]

Both Omicron and Megger make units that operate at 200V P-P that can safely measure the power factor (tan delta / DF). I believe there are several capacitance bridges that operate at lower levels as well.

 

[rhatcher]

Thanks (lps) to old field guy, electricpete, and smallgreek for the much needed advice. I will be performing the power factor test using a Meggar IDAX-300 (200V P-P). I'll let you guys know how it goes.