Hermetically sealed Oil Immersed Transformer trip due to low oil level (vacuum loss???) 1

[Oblsss]

Hello everybody,

I had to work on an incident in a Traction Substation where a Hermetically sealed Oil Immersed Transformer, aka corrugated tank completely filled without conservator, tripped due to low oil level after shut down (in reality it did not trip, it just could not be brought back in service).

During the inspection it was noticed that an internal piping from the transformer protection relay was punctured. We contacted the manufacturer of the transformer and they told us to change the transformer protection relay + gave us instructions on how to remove the old one and readjust the hermetic seal after the installation of the new one. Part of the instructions was to remove oil before removing the punctured transformer protection relay because of the nature of the Hermetically sealed Transformer, eg being completely filled with oil up to the top.

Partly because we had made some really persistent efforts reestablishing the vacuum seal before discovering the puncture but also because we wanted to see if the transformer was filled up to the top, we removed the relay without removing oil and found out that the level was indeed at a low point (we could see the gap from the oil level through the hole (relay flange at the top of the transformer tank). When we mentioned this to the manufacturer, he told us that it was normal and that the atmospheric pressure, now inside the tank through the punctured relay or even the removal of the relay, was pushing the level down. After the installation of the relay and with the use of a vacuum pump, the level was brought back to normal.

My questions are:
1) Should the oil level drop up to a trip point when the hermetic seal is lost?
2) Should you be able to remove an instrument, protection relay or bushing without removing oil when the hermetic seal is lost?
3) Since the oil level was brought up to normal when the hermetic seal was reestablished, is there any chance that the transformer was installed with less oil mass that it should?
4) Is the task of maintaining a hermetic seal for the life of a transformer unachievable ?

Extra hints:
- There was not any leakage of oil in the transformer.
- The power of the transformer is 3.3 MVA with extended overload capabilities (2h duration 150% and within this 2h period One (1) more overload of 60sec. duration 300%).

Thank you!

George

 

[prc]

There are different types of hermetic sealing. I assume your case is a transformer completely filled with oil without a gas cushion over the oil level, but with a flexible tank corrugation to take care of extra oil volume from the oil expansion due to heating. Approx 0.8 % volume increase for every 10 C temperature rise.
1) Yes oil level will drop as the atmospheric pressure will push down the oil level.
2)Yes possible; but vacuum oil filling as per OEM's instructions.
3)No
4)Yes

 

[Oblsss]

Thanks for the answer prc!

"Approx 0.8 % volume increase for every 10 C temperature rise."

Do you mind sharing a source for this information? Does the same apply for a 10 C temperature decrease? According to your experience, 0.8 % volume increase for mineral or synthetic oil?

For this transformer, the insulating fluid is silicone oil.

Thanks!

 

[prc]

1. Volume Expansion coefficient
- Mineral oil- 7.5 x10-4 (+-10%) K-1
- Silicon Oil – 10.0 x
- Natural Ester - 7.4x
- Synthetic ester -7.5 x
(clause 9.4.4 of EN 50588-1-2015Medium Power Transformers 50Hz with the highest voltage for equipment not exceeding 36 kV- Part 1:General Requirements)
2. Designers normally take 1 % per 10-degree rise. It will contract to the same extent when temperatures come down.
3. For more details on Silicon oil, refer to ASTM D 2225 & IEC 60076-14