Power transformers

[pansf]

When we perform a DGA test in a power transformer (75MVA)and results indigate minor issues,
is it wise to perform an oil teatment (degasing, heating, filtering)? Because some engineers state that if
you perform such treatment you loose your transformer history.

 

[marks1080]

If the transformer were to fail due to bad oil/insulation you'll loose your history as well. If the unit has been trending bad for several years it is a good indication that something needs to be done.

 

[stevenal]

Degassing a gassy transformer amounts to treating the symptom while ignoring the cause. If you do find the cause and correct it, degassing can give you a new baseline to start from; else there is little point. Filtering/heating treatment deals with dielectric issues, but you did not state this was a concern.

 

[krisys]

The Dissolved Gas Analysis (DGA) is a a diagnostic test. It suggests the remedial measures to be taken to bring back the transformer to good condition. Bad oil is one of the major causes of the transfomr problem.

Maintenence of oil is normally done, when required. I do not understand, how the transformer history will be lost by treating the transformer oil. Same transformer, same active parts and even the oil. Of course, the oil is now more fit for the duty than earlier.

 

[ScottyUK]

Post your DGA results and any comments from the lab (save us doing the Rogers ratios manually) and maybe one of us can comment more objectively.

A watery transformer possibly will benefit from a thermo-vacuum oil processor but you need to understand the source of the water. You won't fully dry a 'wet' transformer but a prolonged vacuum at high temperature will remove a fair bit of it. There's less to gain by removing hydrocarbon gases. What you really need to be doing is locating and correcting the problem which is producing those gases. Sometimes easier said than done!

 

[oldfieldguy]

Transformer oil treatment is an interesting subject.

Advice: Have an analysis performed BEFORE any remediation work, then immediately upon completion of the work, and after a month or two of service following the work. (I'd strongly recommend that this testing be done by somebody other than the vendor who will perform the oil work, but then I have a suspicious nature)

What you're expecting is that the vacuum/hot oil treatment of the oil will drop your dissolved gas readings to very low values. After a month or two of service, any changes will be more significant.

You will expect your oil's physical properties to improve greatly. If the hot oil is circulated in the transformer, the affinity of hot oil for moisture will cause some of the moisture in the winding to migrate into the oil, where it is then removed by the treatment. Upon return to service, the relatively dry oil will continue to accept moisture from the transformer insulation and support materials, so you expect to see an increase in the oil's moisture numbers between the sample pulled immediately following the treatment and the sample taken after a few weeks. After a period of time, you expect the moisture numbers to stabilize. If you find them rising, then your transformer is likely 'breathing', drawing in ambient air due to changes in temperature and pressure. If you have a sealed and padded transformer, this would be likely to show up as having to continuously add nitrogen to maintain a positive pressure.

some service companies will drain the oil into a separate tank for treatment. This is a good time to pull a vacuum on the empty transformer, the idea being to eliminate some more moisture from the interior. Vaccum filling following the oil treatment should usually give the best total remediation effect.

And I'm glad I don't have to do this any more.

old field guy