Transformer power factor under vacuum

[SecondGen]

What effect, if any, does negative tank pressure have on transformer power factor? I've never known of it being an issue but another tech on site seems to remember hearing it in a PF seminar. Thoughts??

[Jrmcritical]

What effect, if any, does negative tank pressure have on transformer power factor? I've never known of it being an issue but another tech on site seems to remember hearing it in a PF seminar. Thoughts??

In theory, it 'shouldn't' have any effect on power factor as power factor is simply a capacitance measurement. With that said, under no circumstances should this test ever be performed under vacuum, as it can greatly (and gravely) increase the chances of flashover.

I've attached Megger's "Application Guide for Power Factor Testing of Power And Distribution Transformers." It's a great reference and explains a bit more in depth about the relation of pressure vs. vacuum and it's associated hazard.
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[SecondGen]

In theory, it 'shouldn't' have any effect on power factor as power factor is simply a capacitance measurement. With that said, under no circumstances should this test ever be performed under vacuum, as it can greatly (and gravely) increase the chances of flashover.

I've attached Megger's "Application Guide for Power Factor Testing of Power And Distribution Transformers." It's a great reference and explains a bit more in depth about the relation of pressure vs. vacuum and it's associated hazard.
366

Excellent document, thanks for this. Screenshot below for those interested.

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[Kalbi_Rob]

Excellent document, thanks for this. Screenshot below for those interested.

367

And this always brings up a question that I've never could answer.

If flashover happens in transformers under vacuum, why is this different than a vacuum circuit breaker? As the salient feature of vacuum as an arc quenching medium is that as soon as the arc is produced in a vacuum, it is quickly extinguished due to the fast rate of recovery of the dielectric strength in vacuum. I realize the medium of an oil filled transformer is different than a vacuum CB, but it always brings up the question. Obviously, all manuals point to never testing under vacuum.

But then again it might be the DC component that is the problem/cause.

[SecondGen]

And this always brings up a question that I've never could answer.

If flashover happens in transformers under vacuum, why is this different than a vacuum circuit breaker? As the salient feature of vacuum as an arc quenching medium is that as soon as the arc is produced in a vacuum, it is quickly extinguished due to the fast rate of recovery of the dielectric strength in vacuum. I realize the medium of an oil filled transformer is different than a vacuum CB, but it always brings up the question. Obviously, all manuals point to never testing under vacuum.

But then again it might be the DC component that is the problem/cause.

I asked the exact same question to the crew today. Only thing I could think of is that the vacuum pressure is much, much greater in a bottle than a transformer.

Now what I wonder is: if a flashover can occur at 250V when testing under vacuum, why doesn't it occur at 35kV in normal operation when the transformer is under vacuum?

[Jrmcritical]

And this always brings up a question that I've never could answer.

If flashover happens in transformers under vacuum, why is this different than a vacuum circuit breaker? As the salient feature of vacuum as an arc quenching medium is that as soon as the arc is produced in a vacuum, it is quickly extinguished due to the fast rate of recovery of the dielectric strength in vacuum. I realize the medium of an oil filled transformer is different than a vacuum CB, but it always brings up the question. Obviously, all manuals point to never testing under vacuum.

But then again it might be the DC component that is the problem/cause.

I like where you're going with the possibility of DC current being the reason for a heightened risk of flashover, and thinking about it more in depth it makes sense. For example, think of any capacitor. It allows you to "Store" DC current. So, let's take a power factor test to its most basic form - an insulation resistance test. As were turning a conductor into a capacitor eventually it will reach saturation. If voltage is high enough, similar to a Leyden jar we'd have a flashover. This isn't possible with AC power as it can't be stored (although it can be obtained from stored DC current).

When looking at a flashover with AC current it's not because of stored energy (as AC current can't be stored) its due to an arc either because of a short circuit, switching under load, or breakdown of insulation (which is basically the same as a short circuit)

[Hireup]

I asked the exact same question to the crew today. Only thing I could think of is that the vacuum pressure is much, much greater in a bottle than a transformer.

Now what I wonder is: if a flashover can occur at 250V when testing under vacuum, why doesn't it occur at 35kV in normal operation when the transformer is under vacuum?

Maybe it's not so much the medium but what may be in the medium?

Water boils at room temp under vacuum and a vacuum may lift moister in to something it shouldn't be around?

Let me know if I'm on to something here..

[Wanderer20001us]

Maybe it's not so much the medium but what may be in the medium?

Water boils at room temp under vacuum and a vacuum may lift moister in to something it shouldn't be around?

Let me know if I'm on to something here..

It appears to be a characteristic of Nitrogen/Helium (maybe all gases). There looks to be a low breakdown point at partial vacuum that would explain the issue. Breakdown voltage continues to lower from atmospheric to a low point prior to increasing again when approaching perfect vacuum (ie vacuum bottles).

[NBatson]

What effect, if any, does negative tank pressure have on transformer power factor? I've never known of it being an issue but another tech on site seems to remember hearing it in a PF seminar. Thoughts??




Is the transformer empty and under vacuum? If so then cancel your plans to do any testing. If its full of oil and just reading negative pressure on the gauge then carry on. Hopefully this isn't a here's your sign episode and I'm not just stating the known.

[SecondGen]

Is the transformer empty and under vacuum? If so then cancel your plans to do any testing. If its full of oil and just reading negative pressure on the gauge then carry on. Hopefully this isn't a here's your sign episode and I'm not just stating the known.

Power factor testing an empty liquid-filled transformer?

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[NBatson]

Power factor testing an empty liquid-filled transformer?

383



Doble Power Factor Test. Obviously not the recommended way but it can be done under reduced voltages.

Delta windings

161kv+ -10kv
115-138 -5kv
34-69 -2kv
12-25 -1kv
-12 .5kv

wye windings

12kv+ -1kv
below12kv -.5kv

[SecondGen]

Doble Power Factor Test. Obviously not the recommended way but it can be done under reduced voltages.

This is interesting, I don't think we've ever done it at my shop. What is the main purpose of testing without oil?

[NBatson]

This is interesting, I don't think we've ever done it at my shop. What is the main purpose of testing without oil?

There are different scenarios but an example would be:

If you pull the core and coils you could test those separately to see if the root cause was in the core and windings or if its insulation parts in the tank.

Another example could be if your testing the 480v secondary side and your suspecting faulty bushing its hard to guard off that little bushing. You could drain and test it strait to the coil to place the blame on the bushing or to rule it out.

We had the opportunity to take the doble training class and it was super full of information. I'm still new to the testing field so I really enjoy stuff like that.

[SecondGen]

There are different scenarios but an example would be:

If you pull the core and coils you could test those separately to see if the root cause was in the core and windings or if its insulation parts in the tank.

Another example could be if your testing the 480v secondary side and your suspecting faulty bushing its hard to guard off that little bushing. You could drain and test it strait to the coil to place the blame on the bushing or to rule it out.

We had the opportunity to take the doble training class and it was super full of information. I'm still new to the testing field so I really enjoy stuff like that.

That makes sense, I figured it was most likely used in diagnostics or specialized applications. Have you ever done it that way in the field? Just curious about what kind of numbers you get.