Vulnrability of HV generator insulation at high temperatures

[jt2001]

Hello,

We routinely bake stators during overhauls after completing a steam wash to remove contamination.

I am interested in getting a better gauge of time required at temperature in order to ensure all moisture is removed.

Completing a PI test intermittently seems to be a logical way to monitor moisture levels.

I have some concerns about stressing the windings at baking temperature (100-140C). Is there a good rule of thumb for determining a safe voltage to complete the PI at these high temps?

Is it unsafe or stressful to the machine winding to use normal line voltage to complete a PI whilst the temperature is so high?

Kind regards,

JT

 

[che12345]

"..#1 stressing the windings at baking temperature (100-140C). Is there a good rule of thumb for determining a safe voltage to complete the PI at these high temps? #2. Is it unsafe or stressful to the machine winding to use normal line voltage to complete a PI whilst the temperature is so high?"
I have the following opinion for your consideration.
General: the test I understand is Insulation resistance (IR) test. Not PI, usually mean primary injection.
1. As regards to IR test, there are tones of information in the net by insulation resistance meter manufacturers. Most of them also suggest the test voltage and the factor to be taken into consideration when the device is under certain higher temperature.
2. IR test is usually conducted at a lower than the device voltage. Test conducted within the recommended volage is Safe and NOT stressful while within say 140 [sup]o[/sup] C .
3. BTW: IR is usually done with DC at a lower voltage than the device rated AC voltage. Another test is the dielectric test which is done using AC, usually at a higher voltage.
Che Kuan Yau (Singapore)

 

[electricpete]

Is there a good rule of thumb for determining a safe voltage to complete the PI at these high temps?

I wouldn't ordinarily think of elevated temperatures bas degrading the insulation's ability to withstand dc voltage at normal IEEE43 levels(as long as temperature remaiins below the insulation rating). I would on the other hand has reservations about putting IEEE43 dc levels onto a motor that was very wet, so it seems prudent to dry for a reasonable time before first check.

The EPRI medium motor voltage repair specification states:

The stator shall be dried out in an oven, with the temperature controlled to the range 210-230F. After dryout, insulation resistance and polarization index tests shall be performed to confirm that the winding is dry [references IEEE43].

Note the max temp mentioned by EPRI (230F) is far lower than the max temperature you mentioned 140C = 284F). I'm not sure the basis of the EPRI limit but I value their recommendations in general. What happens if you dry out at higher temperatures I'm not sure. I vaguely recall a concern about water leaving the insulation to quickly and causing steam damage but I think maybe that applies more to transformers.

 

[waross]

PI = Primary Injection?
Google Polarization Index.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Gr8blu]

There are two concerns with drying temperatures. First - heating too fast "bubbles" the liquid, which may damage the insulation. Second - cooling too quickly allows moisture to re-enter the insulation system.

A fairly low-temp heat cycle (< 250 F) is all that's needed to get the moisture out (water boils at 212 F at sea level conditions). The lengthy part of the process is the cool-down period (which may be 1x to 2x the time of the heat cycle).

To damage the insulation thermally, you'd have to exceed the insulation system rating: for modern Class F materials, that would by around 155 C (312 F).

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[jt2001]

Waross, Polorization Index is what I am refering to.

Gr8blu, That answers my question regarding the thermal issue.

My next question would be; What is the trend betwee the ultimate failure voltage of a insulation system and temperature?

So a 15kV stator with class F insulation is suitable to withstand up to 15kV between 20 - 155C.

There is a maximum yield or failure voltage of the stator at 20 degrees which is much higher than the nameplate voltage.

Is there a correlation between temperature and this failure voltage? I am assuming this failure voltage is lower (considerably) at 155C than it is at 20C.

Kind regards,

Jack

 

[waross]

Waross, Polorization Index is what I am refering to.

Yes, You know that and and I know that and your intended meaning was clear from the context.
My comment was intended to clarify your meaning for others.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

"..I am interested in getting a better gauge of time required at temperature in order to ensure all moisture is removed. Completing a PI test intermittently seems to be a logical way to monitor moisture levels...".
1. Following information may be useful. Note: both Polarization index (PI) and Absorption ratio (AR) are taken at about the same temperature, within a short time.
2. a) Polarization index (PI) = R [sub]10min[/sub] / R [sub]1min[/sub] > 2..4.
b) Absorption ratio (AR) = R [sub]60s[/sub] / R[sub]30s[/sub] >1.4..1.6.
Che Kuan Yau (Singapore)

 

[edison123]

Modern HV machines insulation is non-hygroscopic and any low IR vales is most likely due to surface moisture, oil and other contaminants. Dry-out has to be done after thorough cleaning (and checking of PI) and slowly over 3 to 8 hours depending on the machine size and maximum winding RTD's temperature of 110 deg C.

Megerring is usually done 2.5 KV to 10 KV and hence should not damage the insulation, which is designed to operate at 140 deg C (for class F) at much higher AC voltages.

Also, if the one-minute IR value exceeds 5000 Megs, PI is not a useful assessment parameter per IEEE 43, which answers all your questions.

Muthu

http://www.edison.co.in

 

[Gr8blu]

Testing insulation is a three-fold process.
1) insulation resistance (insulation to ground), also known as "meggar" test. Used to determine presence of moisture inside the insulation, or - more likely - surface contamination which "tracks" to a ground plane or another phase/polarity.
2) surge test (turn-to-turn). This stresses the insulation between individual turns inside the coil structure, to ensure things are okay. This is preferably done using a multi-pulse device (such as a PJ tester). It is NOT the same thing as what is typically referred to as an "impulse tester" - which only triggers a pulse once for every "button push".
3) dielectric (aka "hipot") test. This applies high voltage across the ground wall insulation in attempt to find small voids or deformations of the insulation build. Test may be performed using either AC or DC source: AC value is generally taken as 2x (rated) + 1000 volts, for "as new, already been vacuum pressure impregnated" windings. DC value would be AC x 1.7 volts. If the winding has been in service, it would be tested at proportionally lower stress (voltage) levels.

The meggar test is done at a specific voltage which is related to the nameplate line voltage. It does not vary with temperature. However, the RESULT of the test can vary with temperature (and humidity), which is why it should be corrected to a known condition (typically 40 C) for trending purposes.

Surge and hipot testing is typically performed at room temperature (roughly 20 C). Because the RESULT of a meggar test goes down with increasing temperature, it is not recommended to perform the hipot test at significantly elevated temperatures.

Refer to the following standards (Note that there are similar IEC standards out there as well.):
IEEE 43 Recommended Practice for Testing Insulation Resistance of Rotating Machines
IEEE 522 Guide for Testing Turn Insulation of Form-Wound Stator Coils for Alternating Current Electric Machines
IEEE 95 Recommended Practice for Insulation Testing of AC Electric Machinery (2300 V and Above) with High Direct Voltage
IEEE 112 Standard Test Procedure for Polyphase Induction Machines
IEEE 113 Guide: Test Procedures for Direct Current Machines
IEEE 115 Guide: Test Procedures for Synchronous Machines

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