Do you purge the air from draw lead bushings? 1

[stevenal]

New 12/16/20 MVA power transformer with positive pressure nitrogen oil preservtion system. The LV bushings are the Haefely "super draw lead" split conductor style. When installing these, we found a bleader screw at the top. If we open this, the transformer oil will rise to displace the air around the conductor. Is this advisable? I suppose that nitrogen will gradually displace the oil with time. Do regular draw lead bushings also allow this? No mention of this in the manual. Maybe the small amount of air involved is not worth bothering with. Thanks.

 

[electricpete]

I'm assuming this is a vertical bushing.

It seems to me that the highest electric stress is where the bushing passes through the tank and therefore the oil level must be well above that level.

If you have a conservator above the bushing level, then I can't imagine any problem with venting the bushing until oil comes out and all air is removed. The conservator will handle normal expansion and keep a small positive pressure to prevent air inleakage or bubble coming out of solution in the oil.

If you have a nitrogen blanket, I'm having a hard time imagining how you could every keep oil in the bushing.

 

[electricpete]

I should qualify my last post to tell you that I was just thinking out loud and I don't know the real answer.

I have only worked with large power plant transformers. I've never worked with the small transformers that had draw lead bushings. (I think they are used at less than 400A).

 

[stevenal]

Pete,
You assume correctly.

I was just reviewing instructions for some other transformers. Even the conservator designs say nothing about bleeding air from the bushing. These designs have bushing pockets that extend above the lid, with bleeder valves at the top of the pocket.

I know the oil will not remain. N2 coming out of solution will eventually fill the volumn above the normal oil level.

 

[NormGA]

Why don't you ask the transformer manufacturer? Quite often a transformer of that size comes with a field tech to supervise the installation. But apparently not this time. But if you don't check with the manufacturer and a bushing fails they could try to balk on the warranty.

 

[stevenal]

Already did, am awaiting the answer. Thought I'd get an idea of the intelligence of my question first. I'm hesitant to ask questions that might lead them to believe we don't know what we're doing. That also might lead to questions of warranty. If the brain power here doesn't know, maybe the question isn't so dumb. The crew tells me there is no similar way to bleed the HV (ordinary) draw lead bushings. Thanks.

 

[prc]

Draw lead bushings can be condenser type(usually =ormore than 52 KV ,American market exception} or LV bushings of plain porcelain type , DIN type or totally sealed oil filled type.Air releasing is required only in case of LV plain porcelain type or DIN type bushings.However this is also not necessary if oil filling was done under vacuum from conservator.
In Condenser type bushings it is not required as the tube is under full voltage and the oil inside the lead tube will not see any voltage stress.
In actual service ,if degassed oil was used the air inside the bushing will get dissolved and the tube will get filled with oilwith some time.At factory test bed LV bushings of above type will flash over during HV test if the air is not released andif air locks at the level where stress is max:viz, at the mounting level on tank.

 

[gmazumder]

Dear Stevenal,

you have not given two data, the voltage class of the bushing and the transformer has a conservator or not.

As far as i am aware it is not necessary to purge from the bushings if the tank has a n2 based oil preservation system as the N2 is over the oil, but there are instances where if the bushing bottom touches the oil level or ther oil level is slightly more than the bushing bottom then air purging will help and the bushing conductor will remain soaked in oil and that is good.

this is a small transformer by capacity standards and will not have a conservator if it has a n2 gas oil protection system. even in conservator based sytems if the conservator top is below the bushing top then you will not be able to purge air from the bushing and the transformer is operated and this is quiet normal for 220 KV/400 KV class transformers no iair purging is required.

gautam mazumder

 

[electricpete]

It sounds like some good answers so far.

Plese allow me to restate my opinion after a little more thought and consideration of the other responses.

Clearly the conservative approach would be to try to get as much oil as possible into the bushing. Even with nitrogen blanket present, the bushing is capable of maintaining within the bushing above the oil level (similar to how you can invert a glass in water and then pull the glass full of water above the water level (open end down below water level).

Venting the bushing after N2 pressure is applied to the tank (with bushing mouth below the oil level) would force oil up into the bushing and would keep it there when you closed the vent (when oil comes out).

It's conservative, and I can't imagine any objections to doing it that way. Whether it is a necessary activity or a waste of time is another question. Being a conservative type of guy, I personally would only accept advice from the transformer manufacturer or bushing manufacture (or someone like prc who sounds like he knows a lot about bushings) before following the apparently non-conservative but quicker approach of not venting.

 

[stevenal]

Gmazumder,
No conservator. The oil preservation system was described in the original post. BIL = 250.
Manufacturer said not to worry about it, saying dielectric strength came from porcelain and bushing oil. Seemed to miss my point about purging extra air. Might talk with the bushing manufacturer too.
Pete,

We all did the water glass trick as kids. Having short attention spans, we did not suspend the glass for an extended time. My intuition says that having regions of differing pressure (or partial pressure), and a method of gas transport (gas going into and out solution) that eventually the regions will equalize.

 

[prc]

Let me repeat what I said earlier.Whether or not to vent air from bushing tube depends on-
1)Whether oil filling was done under vaccum from TOP OF consevator or not-if done so the the inside of tube will automatically get filled up as in Torricelli or water glass experiment.
2) Whether the oil inside the tube is subjected to oil stress or not-normally low voltage bushings are subjected to stress.
3)N2 sealed conservator will not result in automatically getting the tube filled with oil-n2 pressure will be only of the order of 0.15 bar ie 0.15metres of oil head ,insufficient to cover the oil stressed area in the tube!
4)Always consult the trf mfger and he is bound to give correct advice.