500 KV Auto Transformer with Oil to SF6 Bushing Test

Stop trying to take shortcuts and follow recommended procedure..it is what it is...

Agreed! The humongous replacement value of that transformer makes it well worth treating it with great respect! There's a reason the manufacturer gave those directions. If you don't follow those directions, any warranty is worthless.

Hello RedNeck.

You must be feeling "fully charged" - living so close to the homestead of Nikola Tesla.

Sticking to 'specifications' does not protect you from making errors or incorrect interpretations. The man has a point, . . . do you take some short-cuts and get the transformer tested, . . . so that it can be put to service and generate some revenue? Or do you take a couple of weeks to disassemble the GIS and more, just so that you can stick to specifications?

An engineer is supposed to be able to weigh the costs and benefits of things he does and make a decision. This man is not an engineer - as he can mot make a decision.

It is obvious that he is not an engineer or he would not be seeking advice from sources of unknown qualifications.

The manufacturer is the best source of information for this.

He should consult with them for the possibility of alternate methods;they are the most qualified for their product.

Their specifications may err on the side of caution for liability and safety concerns.

The courts are full of companies and families of people that have taken short cuts.

As I said,"Do not gamble if you cannot afford to lose."

I have seen first hand the results of short cuts in electrical causing actual "shorts" and it is not pretty.

If he feels lucky,then by all means use an unapproved ad-hoc method for testing.

It sounds as though the Original Poster has contacted the manufacturer already and simply doesn’t like what the manufacturer says.

it’s only to be hoped that no other CR4 reader is in the vicinity when something goes BANG!

In my very limited but real experiences, the destruction will continue for longer than just BANG!

no i didnt contact with manufacture

what i want from CR4 before taking any decision to find some one illustrate to me the effect of sf6 on power factor and capacitance test

Sounds like you have never testa a sizeable transformer before. Every transformer has bushings and you HAVE TO TEST IT WITH THE BUSHINGS. It is not possible to do it otherwise.

It does not matter if the bushing is oil-filled or SF6 insulated, . . . you have to test it with the bushing.

You do the bushing test separately, . . . and subtract the readings from the transformer readings. The HV bushing must have a "Test-Tap" to do the measurement.

Normally, . . . the HF6 bushing will measure a power factor (DF) considerably lower than an oil-filled bushing and lower than the transformer power factor (DF).

Overcome <...didnt contact with manufacture...> in the first instance.

NO,not BANG,more like BOOM! with mushroom cloud and X rays of the vicinity.

https://www.youtube.com/watch?v=-UGh9jOChHI

The update is appreciated.

Thanks for your reply and your value note about the grounding. I didnot mention that the grounding and neutral connections are at oil to air bushings, so it's no problem to disconnect the ground and complete the test. My concern is the effect of sf6 on the output value.

Yes, your biggest problems are measuring the C&DF as well as the insulation resistance. HOWEVER, as each of these test consists of THREE PARTS - Hi-Gd, Hi-Lo, and Lo-Gd, . . . you can perform the Hi-Lo test accurately, as this configuration is not influenced by all the additional items of the Hi or the Lo side.

Furthermore, . . . you can perform the test Hi-Lo as well as Lo-Hi, in which case you should obtain the same answer, . . . same capacitance & DF.

As the Hi-Lo insulation is the most important one in a transformer, and is found to be good, then the Hi-Gd and Lo-Gd insulation can also be considered to be good.

I have zero practical experience with transformers like this one, so don't hesitate to correct me.

But the title says "auto transformer". In my experience that term refers to a transformer where the low voltage winding IS part of the high voltage winding. If that is the case, how can one measure the insulation resistance between two parts of the same winding?

dkwarner, you are correct - I missed that in the title of the post (it was not mentioned in the OP's descriptive text). Since this is an auto-transformer, there is only one test of the HV/LV winding insulation to ground, not 3 as I mentioned (HV-G, LV-G, HV-LV).

If the OP can disconnect the LV bus, and use the HV's grounding disconnector switch as a test point, he may be able to do what he suggests. He would have to disconnect the ground connection of the disconnector from actual earth so that he can do the test. That still leaves the added impact of the SF6 GIS that is connected to the HV bushings of the transformer.

I don't know what type of insulation power factor test equipment the OP is using, but Doble Engineering (world leader in such testing) recommends having the transformer winding disconnected from anything else that can contribute to the test readings. It's not just the SF6 gas in the bushings & GIS that impact it. There are also physical insulating stand-offs that support the conductor inside the GIS at some points, as well as the GIS bushings between sections themselves. All of these will contribute to the insulation power factor readings.

Even if the OP gets a baseline reading including the GIS contribution, if there is a change you might not be able to tell whether it points to a problem with the transformer or the GIS. Either one could be a large expense to diagnose and repair, but I'd say the transformer is likely the more difficult. My recommendation is still to test the transformer separately.

On a separate note, I think ABB's insistence that the transformer be tested annually is excessive. Other, independent (e.g., NETA) maintenance test standards would recommend that the electrical tests be done at about 3-year intervals, with insulating oil sampling and analysis done annually. Condition-based maintenance procedures would vary that interval based on operating conditions, previous history, etc.

Gentlemen, . . . the problem is "heysam" who does not know how to describe what he has. A 500kV/24kV, 500MVA generator transformer is NEVER connected as an AUTO-TRANSFORMER.

One might fear for the future, then...

three phase bank transformer 167 MVA for each phase

"haysam", . . . it would appear that you are not capable of describing the transformer you have. It may be best for you to post the NAMEPLATE of the transformer before things get to far out of hand.

thanks for your support i will send it

but my question is how the sf6 affect the power factor and capacitance test to take decision