Delta tan Delta test versus Megger Test

[BigMotorGuy]

What are the differences between a delta tan delta test versus a traditional megger test insulation integrity for induction motors? I have read that induction motors over about 4kv should have the insulation tested using the delta tan delta method....

 

[electricpete]

I don't think it's a question of one or the other. Megger test (insulation resistance test) is a very simple easy test. Tan delta (and particularly tipup) takes more time, more expensive equipment, a little harder to interpret. It may make more sense to invest this time in larger higher voltage motors. If the motor has stress control coatings including grading, that makes the tipup test less sensitive.

There is some discussion of power factor test here (for all practical purposes the same test as tan delta test);

http://www.usbr.gov/power/data/fist/fist3_1/vol3-1.pdf

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[Zogzog]

One of the main reasons you do a megger test on MV equipment is to be sure it is safe to do an Dielectric strength (Overpotential) test, such as PF/DF, Tan Delta, or hipot.

 

[electricpete]

Tan delta is generally limited to line-to-neutral voltage and below - not a hi-pot.

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[electricpete]

It is fairly routine to do tan-delta test on transformers without doing a megger first. But I agree the megger is so quick and easy you might as well do it before any other test.

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[oldfieldguy]

Zog & electricpete--

The megger was always the first test for the "it was running and then tripped/stopped" scenario. If the megger showed it boad/grounded/shorted, subsequent tests weren't likely to change anything. If the megger test was good, there were still many possibilities that other tests could find failures.

For the "take it down for periodic testing/maintenance" the megger was often one of the tests, but not necessarily the first.



old field guy

 

[Zogzog]

NETA and other standards specify a megger test prior to any overpotential testing.

 

[electricpete]

zogzog - As I stated before, tan delta testing of rotating equipment is conducted at voltages of rated line-to-neutral voltage or below, and as such is not overpotential testing.

See for example IEEE286, section 10.2 which says what I just said - test voltage shall be limited to line to neutral voltage or below.

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[edison123]

pete - I have had motors fail during tan delta test at those L-N voltage levels. I would definitely term it as a potentially destructive test (though not as destructive as ac hipot).

Megger and tan delta are two different tests of the dielectric measuring different quantities. The only thing common to both of them is that they are trending tests.

Muthu

http://www.edison.co.in

 

[BigMotorGuy]

Thanks for the input to all. We currently perform megger tests (along with hi-pot and surge) routinely in production, but not tan delta tests....just wanting opinions of the necessity of adding this test. Our field service guys would not perform this test, so the whole life time tracking thing would not be followed. However, megger tests in the field are routine along with reduced level hi-pot tests.

 

[electricpete]

Edison – thanks for your comments. I can't say I have ever heard of failure during power factor testing but it's good to know it can happen.

I think it should be important to separate standard requirements from opinions – this applies to everyone including myself. Not because standard is better than opinion or vice versa, but just so that people can judge for themselves.

Here are my observations regarding the standards:
1 – The IEEE standards identify ac hi-pot and dc hi-pot as potentially-destructive tests.
2 – The IEEE standards do not identify power factor (tan delta) as a potentially-destructive tests.
3 – The IEEE standards require dc insulation resistance test at IEEE43 levels prior to ac hi-pot or dc hi-pot.
4 – The IEEE standards do not require dc insulation resistance test at IEEE43 levels prior to power factor (tan delta) test.
5 – IEEE standards define overpotential test as one which exceeds rated voltage.

So based on the above, I hope everyone would agree:
Item A – Referring to tan delta as "overpotential test" (as was done 18 Feb 10 8:50) is incorrect. (One could argue that the neutral end sees higher voltage during tan delta than during operation, but the same argument applies to insulation resistance at IEEE43 levels).
Item B– Using item A to imply that the standards require dc insulation resistance test prior to tan delta (as was done 19 Feb 10 12:23) is incorrect.

Now, as far as my personal opinion of what should be done. I am certainly in agreement that IEEE43 winding insulation resistance test is so easy, fast and routine that we should do it every time. That was what I was trying to say in my very first comment of the thread: "I don't think it's a question of one or the other. Megger test (insulation resistance test) is a very simple easy test. " [implied but not stated: you might as well do the megger every time, regardless of whether you decide to do power factor test.].

BigMotorGuy – I can tell you among nuke plants that attend the EPRI Large Electric Motor User's group, there is one large nuke that does power factor testing routinely on large motors. The remaining 15-20 in attendance do not do it routinely. At our plant, we have the power factor test equipment available (for transformer testing), and we have people trained to use it, but we don't go through the trouble to do it on large motors for a routine test. I reserve it for a special troubleshooting test. I also request the shop to do it when in the shop so we can have a baseline to work off of. Note we do periodic insulation resistance test, dc step voltage on selected motors, and we have on-line partial discharge monitoring installed.

It is much more sensitive and provides more diagnostic info than simple megger. It has the benefit over hi-pot that IN GENERAL power factor testing is not considered potentially destructive. But there is a lot of effort to do the test and it is a judgement call whether that effort is needed given the other info available.


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[edison123]

pete - I agree standards do not describe tan delta as a potentially destructive test. It's just my personal experience that makes me wary.

Richard Nailen on effectiveness of tan delta as a maintenance tool.

http://findarticles.com/p/articles/mi_qa3726/is_200508/ai_n14879852/?tag=content;col1

Shorter Nailen - While a bad tan delta value indicates a bad winding, a good tan delta not necessarily indicates a good winding.

Muthu

http://www.edison.co.in

 

[Zogzog]

"So based on the above, I hope everyone would agree:Item A – Referring to tan delta as "overpotential test" (as was done 18 Feb 10 8:50) is incorrect. (One could argue that the neutral end sees higher voltage during tan delta than during operation, but the same argument applies to insulation resistance at IEEE43 levels).Item B– Using item A to imply that the standards require dc insulation resistance test prior to tan delta (as was done 19 Feb 10 12:23) is incorrect."

Read NETA/ANSI test procedures, they require it.

But you are right about my terminology, I and others tend to lump several tests together when using the term overpotential, tan delta should not be lumped in there. I suppose that is because it is used in place of VLF or hipot tests.

 

[electricpete]

This recommended practice is designed to help organizations and individuals
Evaluate the condition of the electrical insulation used in rotating machines
Determine if the electrical insulation of a rotating machine is suitable for return-to-service
Determine if the electrical insulation of a rotating machine is suitable for high-potential testing

Before making a high direct-voltage test, the stator winding should be deemed suitable for high voltage testing. Insulation resistance and polarization index should be at or above the minimum values specified in IEEE Std 43-2000.

5.2 Test conditions
The ambient conditions should be acceptable for insulation testing. Prior to turn-to-turn testing of fully
cured coils, the insulation resistance should be measured in accordance with IEEE Std 43-2000.
The insulation surface should be clean and dry. The coil temperature should be at least a few degrees above the dew point, as a minimum, to avoid condensation on the insulation. The turn-to-turn testing should not proceed until the insulation resistance tests have been conducted successfully. Other tests, such as measurement of dissipation factor, may also be used to determine if the insulation is suitable for turn-to-turn testing.

The insulation resistance test is used to determine the insulation condition prior to application of an overvoltage test.

7.3 Overvoltage Tests
[goes on to discuss ac and dc hi-pot tests]


8.1 Insulation Power-Factor Tests
[goes on to discuss power factor tests.... in a separate section from overvoltage tests, and there is no warning about doing insulation resistance test first]

[Requires limiting test voltage below rated line to ground testing. There is no reference to overpotential test, potential equipment damage, or need to perform IEEE43 insulation resistance test first]

Read NETA/ANSI test procedures, they require it [IEEE 43 insulation resistance test before IEEE 286 power factor test of ac motor].

Based on the secitons I quoted above, I think it is pretty clear that IEEE does not support what you are saying. i.e. they require insulation resisstance testing before hi-pot testing, but not before power factor testing. So I would be very surprised if there was some industry standard requirement (insulation test before power factor), that IEEE somehow "forgot"... even in a standard that specifically addresses power factor testing (IEEE286).

Now you mentioned NETA standards. I looked at NETA MTS-1 (2001). There is nothing that specifically addresses what we have been talking about. For ac rotating machines, what there is is a list of tests and the first one is indeeed IEEE43 is in the list before all others tests. Is that what you referring to? In that case I would suggest you are reading too much into it to infer this constitutes a new requirement which was somewhow overlooked in multiple IEEE documents. We presume that is the recommended order of tests, but that doesn't mean that there are not items within the list whose order can be exchanged provided other more specific industry standard requirements are not violated. For example winding resistance test occurs after IEEE43 insulation resistance test. Are you saying there is a technical reason that the we cannot perform winding resistance test before the IEEE43 insulation resistance test? I hope not.

Can you please provide a specific standard reference, paragraph, and quote to support your contention?

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[Zogzog]

Pete, easy man, I am not disagreeing with you I am just saying what NETA specs say, the new ones not 2001. If there is an error I will get it changed in NETA. Let me double check the newer revisions.

However, you said yourself the megger test should be done before a TD, I assume Edison123 would agree based on his experience, and so do I. So if IEEE and ANSI/NETA disagree on it being required who is in error?

 

[electricpete]

Sorry, I was not trying to hijack the thread. I agree we might as well do the megger. Just wanted to understand what the standards said.

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[edison123]

I do a PI of the winding before TD since moist winding is likely to give erratic TD readings. But for newly made coils/bars in my shop, I do TD directly without IR/PI.



Muthu

http://www.edison.co.in

 

[Zogzog]

The 2007 NETA MTS requires a megger test and a DC hipot is >2300V. THe PF test is optional. The test do not need to be done in the order listed but if the megger test is required dosen't it make sense to do it before the PF test?

 

[electricpete]

Yes, I agree.

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