Insulation Resistance test - from switchgear or motor terminals? 10

[electricpete]

An IR test from the switchgear through the power cables is much easier and does not require determing and reterming field cables (as well as reinstalling Raychem insulation for high voltage leads). But the cable IR can be an order of magnitude lower than the motor IR, masking possible problems in the motor until they become very pronounced.

Maintenance Dept argues that if they can achieve the minimum required insulation resistance with cables connected(100 Megaohms temp-corrected for higher-voltage form-wound machines)

I argue Since motor failures are much more frequent than cable failures and therefore end up costing more, it seems that extra effort to disconnect the cables is worth the time to get an earlier warning which may enable us to avert a problem.

How do you approach this?
Are there any standards that provide a basis for one approach over ther other?

 

[buzzp]

We always disconnected the cable to test the motor. But I can sort of see why testing with the switchgear attached might be just as good since you are really testing both directions. If the reading with everything connected is above that required by the motor alone, then there should be no reason to disconnect and test. But if the readings are lower than that required then the cabling could be disconnected and verification if the problem was with the motor or the other way. I am not aware of any standards on the preferred method or required method.

 

[jbartos]

Suggestion. It depends on the motor overall installation and associated equipment. If there are common mode currents, then the motor should be tested with cables. Could you provide more information about the motor overall feeder, branch and control circuitry?

 

[Electrifier]

Preferred way is to check IR from Switchgear end, as it verifies the integrity of complete loop. Obtained reading is actually almost same as motor IR with out leads because the cables always have a high insulation and IR as well.

 

[electricpete]

Thanks Electrifier. My experience indicates long cables on 4160 and 13.8kv systems are often a factor of 10 less than the associated motor. Likewise the limits for acceptable cable and motor differ by a similar order of magnitude.

4kv cable has minimum resistance of 5 Megaohms per 1000 foot per IEEE (50Meg per 100 feet). Modern 4KV form wound motor has minimum resistance of 100 Megaohms, and is often is measured the order of 1000 Megaohms. I have very little ability to identify downward trend in the 1000Megaohm number when connected to a cable that is often 50 Megaohms.

 

[busbar]

I agree with buzzp's comments, but most of my experience is with low-voltage motors, and I never given the differences between winding and cable IR much thought. Maybe a compromise might work in this case—when a test shows a questionable reading, peel apart the Raychem stuff and then look both ways. If you have a motor that needs babysitting, reterminate the motor pigtails with an arrangement that is readily removed and reinstalled like T&B 5kV motor splice kits for the 4160 stuff. They wouldn't necessarily have to be installed en masse, but as the readings warrant.

Sorry for the hideous link, but something like:
5KV Motor Pigtail Connector Supplied with Boot, Pin, Silicon Gel (Two Female Connectors Required)!

 

[rhatcher]

Pete,

It sounds like you already know the answer... I can understand the maintenance department's argument and their desire not to have to break the motor terminations. But, as you pointed out in your last post, they are ignoring the fact that practice has shown that trending of motor insulation is a much more reliable indicator of motor condition than a spot check against a minimum value.

Specifically, the question is: 'does a reading of 100M-ohms mean that the motor insulation is in good condition'. The answer, as you know, is 'no'.

A motor that has historically had insulation resistance of 100M-ohms and continues to read 100M-ohms is good. I would even say that a motor that consistantly reads 50 M-ohms is suitable for service at 4160V (but should be frequently monitored). However, a motor that previously read 1000M-ohms but which now reads 250M-ohms is at risk of failure and should be removed, cleaned, and re-varnished despite the fact it is well above the minimum value.

When you check from the switchgear you often cannot see this since, as you pointed out, long cable runs will mask the higher resistance values of the motor. As well, the idea that you are checking the cable at the same time as the motor when working from the switchgear ignores the fact that the insulation of the motor cables has two components, phase-ground and phase-phase. It is of course not possible to check the phase-phase values with the motor connected. As an aside, I find it ironic that most maintenance departments do not include phase-phase checks on medium voltage cables, contactors, breakers, etc. since the phase-phase voltage values are 1.73 times higher than the phase-ground values.

Anyway, my recommendation would be to individually test both the motor and the cables by breaking the circuit at the motor terminal box. For the cable testing, a set of jumpers can be used to ground the phases not under test. A single test can then be performed for each phase simultaneously checking phase-phase and phase-ground values. If a low value is detected then individual tests will have to be perfomed if you want to determine whether the fault is phase-phase or phase-ground and which phases are affected.

 

[electricpete]

Ray - Thanks for the info. You're right.... I already know the "right" answer from my own perspective.... looking for more ideas of how to convince others.

By the way, we have missed your useful comments on the forum. I assume you've been too busy with real-world stuff to come to the forum much. I know how that goes. Good to hear from you.

 

[jbartos]

Suggestion: Check
IEEE Std 252-1995 (Revision of IEEE Std 252-1977) IEEE Standard Test Procedure for ...
Instructions for conducting and reporting the more generally applicable and acceptable tests to determine the performance characteristics of polyphase...

 

[brupp]

I haven't heard any mention of AC versus DC on this discussion. As far I am aware, DC testing is generally done for motors, while DC Hi-pots have generally been discontinued for cable due to the overwhelming evidence that DC hi-potting destroys MV cable. My two cents worth is that if DC is being used for the motor test, don't consider keeping the cable connected. If AC is being used for the test that argument doesn't hold.

One last comment is that consistency of method is probably more important than the actual method. As others have mentioned above, the readings should be compared to previous readings, which can only be done if a consistent method is being employed.

 

[electricpete]

brupp - thanks. I am only considering dc testing. I don't believe there is any adverse effect of applying the IEEE43-2000 insulation resistance test (megger) DC voltage levels through MV cables. (2.5KV test voltage on 4160v system, 10kv test voltage on 13.8kv system). Please correct me if I am mistaken, since we do this routinely.

You're right that if I were doing a hi-pot, I would be concerned about not energizing the cables, particularly XLPE type and particularly those that may have been exposed to moisture.

 

[jbartos]

Suggestion: Regarding AC and DC Hi-pot tests, there is an interesting posting in this forum, namely
wowski (Electrical) 8/17/01 (posted 1/30) 12 responses
HV cable pressure testing
Electrical/Electronic Engineers Area - Electric power engineering Forum
which can be accessed via Advanced Search and keyword
HV Cable Pressure Testing

 

[kamalpanda]

Dear Mr. Pete
This is really a good issue for discussion and I appreciate the detailed explanation by Mr.Ray.

The practical problems with maintenance Dept. is the safety permit procedures.As to carryout the IR test at Motor terminals it is required to Hot wark permits in many of the industries.To get rid of the headache they prefer to measure from Switchgear end.

More ever it is a standard practice during commissioning of the equipments to Take the IR value measuremet for

1)Motor alone
2)Power cables alone (individual if no of runs more than one)
3)POwer cables along with the motor.

So during periodic maintenance checking in my opinion it is sufficient to check the 3rd reading and compare it with the previous one(Commissioning report).

If there is a variation large variation in the reading then one should go for all the thre checks in details.

More ever if you are relly serious about monitoring the health of the equipment you can rather go for Surge Comparision Taste.That gives you a fair idea about any deviation or leakage in the windings of the motor. This taste can also be done with cables connected and from switch gear end.I found it helpful for both LV(415V ,50 Hz) and MV(6.6KV,50Hz) motors of capacity upto 6900KW.

Hope you understand the problems with Maintenance Dept for not carrying out the IR test separately.

 

[buzzp]

You convinced me with "Motor Trending". This wouldn't be very easy with additional equipment connected. For a
go/no-go type of test it would suffice.