Sealed-Tank Design vs Conservator-Tank Design in 20MVA power transformer, pros & cons

[GroovyGuy]

Howdy everyone,
I have a question or two. I currently have a project where I am out for tender for a couple of 20mva (CSA C-88) 55C ONAN 69kv delta - 13.8kv wye step-down transformers. I have asked for pricing for both a sealed-tank design and a conservator-tank type design. The pricing that I have received in real close for both design types (like within 2%).
BTW These transformers will be installed in a wet marine type of environment.
I did a quick search for the ENG-TIPS forums and found out that to my surprise that most of the transformers installed in this class of power transformer were in fact of the sealed-tank design. This surprised me in that I had incorrectly assumed that most of the installed base would be of the conservator-tank design type.

What is the experience that you all have had with this class of transformer? What are the pros & cons of the two design types?

Thanx much,
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[thermionic1]

IMO:

Sealed Con=N2 Maintenance, possibility of moisture ingress with temp/pressure changes. If you have a SCADA system, be sure to connect alarms for low N2/pressure.

Conserv: Con=More assembly issues if crews are not familiar with construction of conservator tank / bladder. If you have CT repairs or need to visually inspect, you are removing a quantity of oil first.

In the size you mentioned, I see more of the sealed type in the USA. I've seen teeny, tiny conservator units in Austria and Germany (like 400kVA).

Our distribution company specs 50/80 MVA in sealed units all of the time.

Our distribution company specs 200+ MVA in conservator units.

 

[prc]

Oil preservation systems for power transformers are of different types;
1)Free breathing through silicagel dehydrating breather + conservator
2)As above + a synthetic rubber balloon(air cell)inside conservator sealing the oil surface from breathed air
3)As (1)+ a synthetic rubber diaphragm horizontally fixed at middle of cylindrical conservator sealing air contact with oil
4) Completely sealed system- expandable tank and radiators taking up the expansion of oil without any conservator and breather
5)Without conservator + top of tank provided with space for oil expansion and with vacuum + pressure release valves
6) As (5) with out valve, but sealed with dry nitrogen with arrangement for automatically controlling nitrogen pressure through valves to a nitrogen producing system or cylinder.

Each has its own plus and minus.Today (1) and (2) are widely used around the world. Understand (6) is popular in US. (5)mainly used for distribution transformers. Since you said price difference only 2%, probably you got offer for (2)

 

[dpc]

In the US, the Nitrogen system is vastly more common in this size (they do have pressure relief valves and vacuum/pressure alarms). Conservators are not common here in US. I understand in the rest of the world things are different. The conservator units I've dealt with in the US have all had bladders (#2 in prc's list). No real issues with either type if properly installed and maintained.

Cheers,

Dave

 

[ppedUK]

For that rating of TX, I would want Buchholz (gas and surge) protection, so environment issues (wet marine) aside, I would usually go for conservator design. Sealed tank designs can have overpressure trip device, but it won't give you early warning of a developing problem.

 

[stevenal]

Leaks happen.

What happens in your marine environment when a bushing gasket fails with a head of conservator oil above?

If the same leak occurs on a sealed system, it will breath your marine air back and forth through the failed gasket as the transformer cycles.

An active N2 (prc #6 above)system will leak dry N2 into the air and send alarms to operators.

 

[cranky108]

The problem is a Buchholz relay would need to have a qunative test to be made as required by NERC. Can anyone name a test set that can give a definite pass fail test?

 

[prc]

Stevenal,you are right. With(2),there is vacuum outside the air cell and will be always full with oil. With any gasket leak in conservator, air will rush in and collect outside the air cell. Of course leak detector relay + plain oil glass window will detect such a situation.

Problem with (5) and (6) is chances for bubble formation with sudden change in ambient temperature.Air or nitrogen will dissolve in to oil to the extent of 8% by volume and solubility will change with temperature. Hence (5) (6) are not preferred in EHV or UHV transformers.

 

[dpc]

Since Buchholz relays were mentioned - sealed transformer with N2 blanket are generally equipped with "sudden pressure" or rate-of-rise relays that provide similar protection and sensitive detection of internal faults. I don't have a horse in this race, but always find it interesting that those outside North America just assume conservator type transformers are inherently better than sealed units. I have been involved with both types and have worked with sealed units at 800 MVA with 345 kV primary. I see strengths and weaknesses in both concepts, but either can be reliable if well-designed, well-constructed, and PROPERLY MAINTAINED.

 

[argotier]

As both main designs are valid, I would go with the one more common in your country/area. That way you will avoid surprises later on with the installation, maintenance and eventual need for spare parts.

 

[RRaghunath]

Having seen Conservator types all my career, I feel sealed transformers with Nitrogen cylinder attached are superior as
1) No messy thing with Silica gel regeneration / replacement. It is easier/simpler and much less frequent to maintain Nitrogen by just replacing the cylinder when there is low N2 pressure alarm
2) Less number of gasketted joints and hence less chances of oil leakage / less need for maintenance
3) Dry nitrogen blanket on top of oil surface helps keep oil dry by attracting moisture from oil and also reducing oil deterioration as there is no contact with oxygen (and thus eliminating / reducing oxidation of oil).
Thus, I feel the sealed transformers are more suited in tropical climates where transformers run hot, leakages are common humidity levels are high most of the year.