Baker-Hi Pot testing from motor junction box or circuit breaker? 1

[brucecounty09]

Our Motor testing program is in it's infancy- we have the Baker AWA and want to conduct testing on the 4160V and larger motors.
We have hit a roadblock as the system engineers- electrical are concerned that conducting the Baker DC- Hipot test from the circuit breaker will age or prematurely fail the power cable from the circuit breaker to the motor junction box. Baker AWA can conduct a stepped DC Hipot that can shut down before damaging the cables.

Baker suggested voltage ( 4160 VAC)
Twice voltage + 1000 V =9320VDC

Are you testing your 4160V motors fronm the circuit breaker or the motor junction box?
Justification/Risks?

 

[Zogzog]

DC hipot testing of service aged MV cables is considered a destructive test by ANSI, NETA, IEEE,ICEA and just about every other standard you can find. You may want to reconsider.

Read this

http://www.eng-tips.com/viewthread.cfm?qid=248906

 

[electricpete]

I appreciate zogzog taking the time to list those standards in the last thread. Imo, it should be a FAQ and there should be a more extensive excerpts (anyone who wants to take the time to do it will be voted a 10 by me). The reason is that there is a lot of subtleties to be found. For example when we asked for the actual wording in the last thread did we learn that what NETA does is not list a test voltage value beyond 5 years.... that is quite a distance from saying that NETA calls dc testing destructive imo.

Another subtlety is the reason the standards are dropping the test.... a lot of them have to do with effectiveness of the test. Effectiveness and destructiveness are 2 different things as we know and for example in the current context of motor testing we don't care about the effectiveness of the dc for detecting cable defects, we just want to know if we can easily test or motor through the cables.

And as was pointed out, this all started with concern about wet XLPE cables. It's not clear how it has gotten extended to all types of cable material and all conditions wet and dry. Would love someone to explain the basis for that to me.

Now getting more to the idea of surge testing. I don't know a lot of plants that do it in place. I think the cables will interfere with your results. Also the rotor can interfere with your results. Best results come in the shop tested directly at the motor leads with rotor removed.

Judging from your comments (system engineer) and handle, I have to ask, are you from a CANDU plant?

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(2B)+(2B)' ?

 

[electricpete]

Actually I jumped in line behind ZogZog in grouping surge testing in the same category as dc testing with respect to destructiveness, but in retrospect, I'm not sure if that's the case. The waveform is not dc. I don't think it would promote treeing. Do any of these cable standards address surge testing?

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(2B)+(2B)' ?

 

[brucecounty09]

1)Zogzog, I read that thread before I posted and passed the info to my fellow component (in particular benlenz comments)it still does not get around my original question- of what is the common practice for those with established motor testing programmes? What are their compensatory actions if they chose to test from the circuit breaker knowing that there is the body of knowledge to suggest that damage can occur on the 20+ year cables.
2)electricpete, CANDU- yes

My fellow component engineer was at previous EPRI-LEMUG conferences and asked the same question and a rough count by a show of hands indicated that about 75% of the group who conducted testing on 4160V motors test from the circuit breaker. I was hoping some of the forum members have an established motor testing programme and how they infuse some practicality to it?

 

[Zogzog]

It is often done at the breaker because it is easier, but the cables will effect your test results so to be more accurate you shoulod do it at the motor.

EP, I was in the middle of the NETA board discussions on this issue, lots of political reasoning behind the wording selected.

 

[electricpete]

Sorry, a lot of my comments in this thread applied to surge testing because that is what I associate with "Baker". Disregard my discussion of surge testing.

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(2B)+(2B)' ?

 

[rhatcher]

In order to effectively use the Baker AWA for a motor testing program you need to perform the testing at the motor terminals.

The tester will perform an automatic test sequence to include (not in this order): hi-pot (insulation resistance), polarization index, surge comparison test, and winding resistance.

The results are stored and can be compared to later tests of the same motor to allow for trending.

The reason that this testing must be performed at the terminals is that all of the tests are rendered ineffective if the motor lead cables are in the circuit.

For the hi-pot/insulation resistance test, the maximum voltage you would probably want to use with the (old) cables in the circuit is 4160 x 1.4 = 5825VDC. This is much less than the recommended value for the motor alone of 9320VDC.

More importantly, the reading that you get for insulation resistance will be for the parallel combination of motor and cables.

For the polarization index test, surge comparison test, and winding resistance test, the presence of the cables will also skew the results.