Sealed winding conformance test / submersion test 5

[electricpete]

Next week I will have the opportunity to see a sealed winding conformance test (per NEMA MG-1 section 20.18) on one of our newly-rewound 13.2kv motors. The motor stator will be thoroughly wetted with water (including a wetting agent like detergent), and then will have ac voltage of 115% of line-to-line voltage applied line to ground applied. Pretty much an underwater mini hi-pot.

I haven’t ever seen that before… it promises to be an interesting test.

Has anyone ever seen this done before? Any tips or suggestions?


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[motorspert]

Hi Pete
I have been involved a few times (as a vendor). If its VPI and has been well impregnated it should be ok. Main problem area was the cable exits from the coil ends water seeps in the joints and fizzzzzzz.
The otehr problem we had (a long time ago) was rust at back of core - make sure they dry it off after the test
Have fun

 

[edison123]

Post the drama here after it is over.

Muthu

http://www.edison.co.in

 

[electricpete]

I'll be sure to do that, Muthu.

I was reading an article “WINDING IMMERSION TESTING "ARE OUR FEARS ALL WET?" by Costello, Cook, and Heredos, IEEE PCIC-93-18. It says

The purpose of the 500 Volt D.C. test following the [115% VLL wetted] AC hipot is to detect damage that may not have surfaced during the 60Hz hi-pot test. The value of the insulation resistance measured at this stage is closely monitored for a change which may indicate partial damage.

So I have dc insulation resistance/polarization index test before everything begins, and then another dc test after everything is done.

What kind of change from before to after Insulation Resistance / PI test results should I expect… and what change should cause us concern?

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[edison123]

That's strange. What a 500 V dc test could possibly detect that a 115% ac hipot didn't ?

Muthu

http://www.edison.co.in

 

[electricpete]

The one thing I can see is that AC hi-pot is go/no-go, whereas insulation resistance / polarization index give a value which can be compared. On the other hand as you mention it's not terribly sensitive. I imagine a dc step voltage test before/after would be better to check for things like tracking.

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[electricpete]

Test is complete. Attached are some pictures. Notes and questions below.

MOTOR DATA: 13.2kv, 3500hp, 324 rpm (torque equivalent of 30,000hp 2-pole), 50,000 pounds assembled.

CALCULATIONS RELATED TO TUB FILL HEIGHT: It is deisred to fill the tub to the right height so that level would be above connections, but not overflow the tank. Tub is 11.5' diameter and 80" tall. Stator weight unassembled is: 20,000 lbm. Estimated stator volume based on 500 lbm/ft^3 desntity of steel would be 40 ft^3. Since more dense copper is also present, it will be a little less volume. Left only 8" between water level and top of the tank prior to inserting stator. That doesn't look like much, but it equates to something like 50 ft^3 which was enough. Result was acceptable (did not overflow and covered the connections).

WATER TYPE: Pure demin water (non-conducting) was used and a non-ionic (covalent) wetting agent was added to obtain desired surface tension 31 dyn/cm specified in NEMA MG-1. The wetting agent supposedly helps the water get into smaller cracks than it otherwise could. NEMA MG-1 does not say much about the conductivity of the water to be used.

ELECTRICAL TESTS BEFORE PUTTING STATOR IN WATER:
1 - Megger at 10kvdc. 3.8 Gigaohms at 1 minute, 25 gigaohm at 10 minute
2 - Surge test – Sat I forgot the voltage.
3 - AC Hi-pot to 27,500vac for 1 minute for each phase to other two grounded. Sat. Amps 1250. Smell of ozone and sound resembling bath shower. Compute impedance of 22,000 ohms (capacitive)
4 - Winding resistance – balanced within 0.1%
5 - PD 7600vac. Individual phase to others/ground: 260milliamps. Compute impedance of 27,500 ohms (capacitive)

ELECTRICAL TESTS WITH STATOR IN WATER
1 - Performed IR at 1kvdc(*). 4.9Gigaohm at 1 minute. 33 gigaohm at 10 minutes
2 – Performed ac test at 12.5kvac. 2200 millamps (all 3 phases). Compute impedance 5680 ohms. Could not obtain target 15kvac due to ac hi-pot trip setting 2500 milliamps
3 - Repeat IR at 1kvdc(*). Results Gigaohms. (slightly better than before).

* I agree with Edison that 500vdc or 1kvdc is only a drop in the bucket for a 13.2kv motor. However the NEMA MG-1 standard says only 500vdc. There is no precedent or standard to apply higher dc voltage to a motor underwater that we know of. Based on discussions between motor shop and myself, we decided to go 10 1kvdc, but don't want to press our luck any higher.

PLAN: Motor will be dried in over for 12 hours, then repeat megger/PI at 10kvdc, followed by dc step voltage test to 28kvdc

QUESTIONS:
1 – What do you think is explanation for small variability in computed impedance between AC hi-pot (27,000 ohms) and pd test (22,000 ohms).
2 – More importantly, there was a larger variation unexplained in the submerged ac test. I expected the capacitive impedance to decrease by some factor less than 3 (maybe 2?) when we went from single-phase-to-ground tests (before putting into water) to the all-phases-together ac test (under water). After all the single-phase tests include not only ground contribution, but also phase-to-phase contribution. But what we actually saw was impedance approx factor of 4 lower. Could the presence of the water bringing a ground-plane closer to the endwinding explain this? (it makes sense it would have some effect although I don't know how much since water is demineralized and non-conducting)
3 - Does the sequence of testing make sense to you? I am thinking about requesting the 27,500vac/2E+1 factory ac hi-pot after drying following the water test next time. That would give us even higher confidence that as-left condition of winding is very robust. Shop says factory ac test before submergence test is standard procedure they have always done. The standards appear silent on this aspect.
4 – Silly question. What role does water play in revealing a failure in the endwinding if the water has low conductivity (demin water). To complete the circuit to ground, the current still has to travel to the core somehow. Does dielectric breakdown of water play any role? Or more likely, even though it is pure water compared to tapwater, it still has significant enough conductivity that will facilitate enough current through water to trip the test set with pinhole leak with no water dielectric breakdown required? I'm not sure but interested in opinions/thoughts on what role the water plays in detecting endwinding weaknesses. Do you think different conductivity of the water (like tapwater plus wetting agent) would give significantly different test results?

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[electricpete]

Correction: DC step voltage test after drying will be 32kvdc

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[electricpete]

Correction in bold:
Amps 1250
should've been
milliAmps 1250

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[electricpete]

I forgot my pictures. Here they are.

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[electricpete]

Related to question 4, I looked up properties of water. Dielectric breakdown strength is 88 times that of air (so clearly water is not introduced because it breaks down). Dielectric constant is 88 times that of air (may affect the capacitive picture somewhat).

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[electricpete]

Last correction (in bold) and I'm done:
Related to question 4, I looked up properties of water. Dielectric breakdown strength is 10 times that of air (so clearly water is not introduced because it breaks down). Dielectric constant is 88 times that of air (may affect the capacitive picture somewhat).

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[edison123]

Thanks pete for that submarine report.

Let me read and see if anything makes sense.

And lastly, why under water test ?

Muthu

http://www.edison.co.in

 

[electricpete]

Why do we request the submergence test?
The failure rate of our outdoor mv motors is much higher than our indoor motors. Several of the failures have been tied to moisture in some way and some suspected to be related to moisture. It is a humid coastal environment, we do everything possible to exclude water but sometimes evidence of free water is seen inside the motors.

NEMA identifies this submergence test as an optional test.

So we are now specifying it whenever we rewind these outdoor motors.

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[edison123]

Thanks pete. That sure is a tough requirement. Have your motor failures come down after doing this type of test and this type of sealed rewinds ? Have you tried other of forms mitigation like providing a hood just for the motors to protect them from elements ?

Muthu

http://www.edison.co.in

 

[electricpete]

Have your motor failures come down after doing this type of test and this type of sealed rewinds?

We have not data. This is the first one we've done. Another one coming in March/April.

Have you tried other of forms mitigation like providing a hood just for the motors to protect them from elements ?

Haha. I'm laughing because I feel I need to tell a long story to answer that.

There are 2 families of outdoor motors.

One family is six 4kv motors which have never had problems where we saw evidence of any water getting in, but yet have had 6 or 7 winding failures among 6 motors in the 22 year life of the plant.

The other family is the one in this thread .. eight 13.2 kv motors which have had a lot of problems with water getting in. We have had 8 or 9 winding failures among 8 motors in the 22 year life of the plant.

In contrast, our other motor families indoor (all have 0 or 1 failure in the same time period among families of 6 or 8).

As for the motor where we have seen water getting in, as stated above "we do everything possible to exclude water". Trying and succeeding are not always the same thing. We have identified numerous points of entry and corrected each one as we identified it. But the paths are subtle. The most recent one is described in
thread237-241996

The point of water entry in that case is leakage accross the rusty flange (you can see the rust on the flange surfaces on slides 2 and 9 of attachment to first post in that thread). This is assisted by the vacuum in that area created by the rotor fan action.

I'd say the motor design has some vulnerabilities to begin with. For one we have huge removable airboxes on top that form part of the inlet air re-direction and filtering. There have been a variety of improvements and lessons learned along the way, but the paths we discovered were subtle, not obvious, and I don't think we will ever achieve perfection.

The idea of building a hood is more challenging than you'd think. The motor stands high above a CW pit with nothing to brace off of. If you ran long braces from the side of the pit, you would restrict crane access to surrounding equipment. If you mounted the roof to the motor you'd have to mount it to the motor which could increase the weight and change the resonant frequency, and would also limit access required to the top of the motor... including crane on occasion. None of these are insurmountable, but it's a challenge and instead we focus on addressing the local vulnerabilities we identified on by one.

We don't give up on trying to stop the water, but if we can also provide a winding which "doesn't care" if it gets a little wet, that provides "defense in depth" (multiple barriers against failure). The cost to invoke this requirement is about 5% of the cost of a rewind. That is not just cost for the test itself but cost for the rewinder to assume the liability to rewind if it fails during test, which means he does pay much more attention to the endwinding connections of a motor that will be wet-tested. If we made the same investment 10 times and prevented a failure even once, I'd say it was a good investment (considering all the costs associated with failure, which are not limited to cost of rewind).

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

 

[electricpete]

Have your motor failures come down after doing this type of test and this type of sealed rewinds?

We have not data. This is the first one we've done. Another one coming in March/April.

Have you tried other of forms mitigation like providing a hood just for the motors to protect them from elements ?

Haha. I'm laughing because I feel I need to tell a long story to answer that.

There are 2 families of outdoor motors.

One family is six 4kv motors which have never had problems where we saw evidence of any water getting in, but yet have had 6 or 7 winding failures among 6 motors in the 22 year life of the plant.

The other family is the one in this thread .. eight 13.2 kv motors which have had a lot of problems with water getting in. We have had 8 or 9 winding failures among 8 motors in the 22 year life of the plant.

In contrast, our other motor families indoor (all have 0 or 1 failure in the same time period among families of 6 or 8).

As for the motor where we have seen water getting in, as stated above "we do everything possible to exclude water". Trying and succeeding are not always the same thing. We have identified numerous points of entry and corrected each one as we identified it. But the paths are subtle. The most recent one is described in
thread237-241996

The point of water entry in that case is leakage across the rusty flange (you can see the rust on the flange surfaces on slides 2 and 9 of attachment to first post in that thread). This is assisted by the vacuum in that area created by the rotor fan action.

I'd say the motor design is somewhat cheap/primitive has some built-in vulnerabilities to begin with. For one, we have huge removable airboxes on top that form part of the inlet air re-direction and filtering. The seal between the airbox and the motor is complex and not easy to access and a challenge to maintain. There have been a variety of improvements and lessons learned along the way, but the paths we discovered were subtle, not obvious, and I don't think we will ever achieve perfection.

The idea of building a hood is more challenging than you'd think. The motor stands high above a CW pit with nothing to brace off of. If you ran long braces from the side of the pit, you would restrict crane access to surrounding equipment. If you mounted the roof to the motor you'd have to mount it to the motor which could increase the weight and change the resonant frequency, and would also limit access required to the top of the motor... including crane on occasion. None of these are insurmountable, but it's a challenge and instead we focus on addressing the local vulnerabilities we identified on by one.

We don't give up on trying to stop the water, but if we can also provide a winding which "doesn't care" if it gets a little wet, that provides "defense in depth" (multiple barriers against failure). The cost to invoke this requirement is about 5% of the cost of a rewind. That is not just cost for the test itself but cost for the rewinder to assume the liability to rewind if it fails during test, which means he does pay much more attention to the endwinding connections of a motor that will be wet-tested. If we made the same investment 10 times and prevented a failure even once, I'd say it was a good investment (considering all the costs associated with failure, which are not limited to cost of rewind).

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[edison123]

Thx pete for doubly reinforcing the point.

Muthu

http://www.edison.co.in