Substation Transformer under Vacuum

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bzr157

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Our substation transformer 34.5 k / 460 V, 3 MVA, 60 Hz is operating at negative 2 Psi (vacuum pressure), is this a normal phenomena? What is the normal operating pressure of this kind of unit? What are the impacts in the longer term if this will continue to operate at this status? Thanks
 
You need to find the manual, but in general, transformers operate on the positive side. If the transformer was filled in an high ambient temperature area and is now in a low ambient area with little or no load, the pressure may be in the vacuum range.
 
This is from the following document http://www.cooperindustries.com/con...rces/library/210_3phTransformers/S2101510.pdf page 7

Seems that depending on the situation, negative might be ok.

The pressure-vacuum gauge indicates whether the gasspace in the tank is under positive or negative pressure.The pressure will vary depending on the transformertemperature. If the transformer is de-energized oroperating under light load in low ambients, the pressuremay be negative.Note: If the indicator reads zero and does not changeunder any load condition, the transformer shouldbe checked for a possible leak in the seal.If sufficient air has been absorbed by the liquid duringshipment or storage, the transformer may operateindefinitely in the vacuum range, depending upon theloading conditions. This, in itself, is not cause for concern,provided the pressure vacuum gauge does not remain onzero for any length of time an indication of a leak.The unit may be equipped with pressure vacuum switcheswith two SPDT contacts for remote alarm on positive andnegative pressure. For wiring and contact ratings, refer tothe schematic furnished with the trans-former.When required, the pressure gauge is furnished with apressure regulator that will automatically regulate the tankpressure between 7.0 psig positive and 3.0 psig negative.The pressure regulator is fitted with a valve and fitting totake gas samples.
 
bzr157 said:
Our substation transformer 34.5 k / 460 V, 3 MVA, 60 Hz is operating at negative 2 Psi (vacuum pressure), is this a normal phenomena? What is the normal operating pressure of this kind of unit? What are the impacts in the longer term if this will continue to operate at this status? Thanks
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If it is running under very light load conditions that would not be much of a concern, pressure will rise with load. However if it is loaded then you need to top it off with some nitrogen and do not attempt to draw a sample when it is under vacuum!!
 
don_resqcapt19 said:
You need to find the manual, but in general, transformers operate on the positive side. If the transformer was filled in an high ambient temperature area and is now in a low ambient area with little or no load, the pressure may be in the vacuum range.
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I’ve worked on 100’s of power transformers, I’ve never come across one with vacuum.

My first thoughts are a blocked silica gel breather. With low load the oil contracts and will cause a vacuum.
I’ve had a tank “belly” due to pressure rise, this is a new one on me.

As Don said read the manual.
3MVA transformers don’t come cheap and failure is even more expensive when down time is added in.
 
I sold a number of unit subs, sec unit subs, and power center transformers.
The tanks of these transformers are basically seasled and as such the pressure could be negative or positive from time to time.
Common accessories were a vacuum pressure Gage, rapid (sudden) pressure rise relay, pressure relief device, dial hotspot indicator, liquid level gauge, dial type themometer, and alarm contacts on the gauges.
The is something called a drain valve commonly with a sampling Feb ce to be used to sample the liquid. One of the more important things that needs to be done is to have a sample of the violent anaslized for contaminates and moisture which are big enemies of liquid cooled transformers.
 
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Tony S said:
I’ve worked on 100’s of power transformers, I’ve never come across one with vacuum.

My first thoughts are a blocked silica gel breather. With low load the oil contracts and will cause a vacuum.
I’ve had a tank “belly” due to pressure rise, this is a new one on me.

As Don said read the manual.
3MVA transformers don’t come cheap and failure is even more expensive when down time is added in.
Click to expand...

We had 4 out of 12 of them under a vacuum on our last testing job, not that uncommon.
 
templdl said:
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The is something called a drain valve commonly with a sampling Feb ce to be used to sample the liquid. One of the more important things that needs to be done is to have a sample of the violent anaslized for contaminates and moisture which are big enemies of liquid cooled transformers.
Click to expand...

Sampling is important but never open that valve with a vacuum, air will suck into the sample valve causing air bubbles to rise between the windings which will likely cause arcing, rapid pressure rise, and possible explosion.
 
zog said:
Sampling is important but never open that valve with a vacuum, air will suck into the sample valve causing air bubbles to rise between the windings which will likely cause arcing, rapid pressure rise, and possible explosion.
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Zog, How did you get a sample from those under vacuum? Add artificial loadbanks?
 
zog said:
Add N2
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I think it best that only those who are qualified do this. Maintaining a transformer is not something that should be done by the inexperienced. Hopefully your reference to N2, dry nitrogen gas added to the transformer to negate the negative pressure will help illustrate the seriousness of this sampling procedure.
 
zog said:
Sampling is important but never open that valve with a vacuum, air will suck into the sample valve causing air bubbles to rise between the windings which will likely cause arcing, rapid pressure rise, and possible explosion.
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not to mention that a vacuum will allow moisture in, never a transformers friend.
the one's i've bowsered all were left with a positive pressure after filling.

it would make sense tho, if the transformer were filled hot, sealed, and cooled
to ambient, you'd almost always have a vacuum.
 
Thanks for your inputs. At first I was doubtful if the gauge itself is already malfunctioning, in fact it reads negative 4 Psi. Then I have an opportunity to check it when we conducted some test like IR and DBV. After the test, it is still negative but at -2 Psi only.

Also, this transformer operates at about 40 to 50% only, can this be considered a light load causing it to yield a negative pressure?

@zoga: is it ok to add N2 on the transformer while on service or at no load condition just to be safe?

Manual would be a good reference in maintaining an equipment but unfortunately I don't have one in hand. Also this transformer is now at her 40 years in service.
 
zog said:
We had 4 out of 12 of them under a vacuum on our last testing job, not that uncommon.
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Now you’ve got me interested, any chance of some information on them. I can see the advantage in locations where it is humid, a silica gel filter would soon becoming ineffective.

If you don’t mind I’ll PM my mail address, even now I’m retired I’m still interested in something new. If you can’t send info don’t worry about it.
 
I have a set of old Westinghouse Electrical Maintenance Hints that includes 4 volumes, #4 of which addresses Power Application Maintenance.
My reference indicate that it is important to have a positive pressure in the tank preferably not more that 5psi. It is important to note that as negative pressure is not desirable as it may lead to moisture contamination of the oil noting that a negative pressure can pull moisture laden ait into the tank. The tank should have a bed of dry nitrogen gas on top of the liquid which would be reflected in a positive pressure on the VP gage.
Other than this publication it appears to be very hard to find how the VP gasge relates to the proper application on a liquid filled transformer.
In the OP's case it would be prudent to get a sample of the oil and have it analyzed and then charging the tank with a layer of dry nitrogen..
I strongly recommend that a good engineering service provider do this work.
 
templdl said:
In the OP's case it would be prudent to get a sample of the oil and have it analyzed and then charging the tank with a layer of dry nitrogen..
I strongly recommend that a good engineering service provider do this work.
Click to expand...

Charge then sample, you try to sample under vacuum and you have a big problem.

The OP needs to call a transformer expert to come look at this transformer. This is not something you get instructions on how to do on a discussion board.
 
Sealed and blanketed transformers are ubiquitous in the low MVA range. We only really run into free breathers on very old units or larger conservator styles.

The only problem with a sustained vacuum is what happens if you get a leak. Any leak will ultimately contaminate the oil, but a leak in the wrong spot during a deep vacuum has the ability to draw in a lot of moisture. Andd once you start getting partial discharge failure due to oil contaminates you're all done, so it's really worth avoiding.

Also as mentioned, unless you want to rapidly destroy your transformer, never sample oil under a vacuum.
 
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