Hello everyone. I have client that requested we take some oil samples from their 11kV transformers to send for analysis. 2 of their transformers, when the sample taps were opened, were drawing in air and not draining oil. The taps were closed up again to avoid drawing anymore wet air into the internals. Both transformers have not got a breather on which I think has led to a vacuum being created in the transformer. What I need to know is:
1) is it possible to create a vacuum by not having a breather? (the breather pipe has a bung on it)
2)I don't want to remove the bung and risk large amounts of air to rush in causing air bubbles in and around the cores/tapchanger and introducing unwanted moisture. So is there another way of removing the vacuum bearing in mind the transformer would have to remain live due to production reasons?
3)If the transformers were left with a vacuum inside for approximately 8 months until a site shutdown, could this cause any damage/oil to breakdown inside them?

Any advice greatly received!

Assuming you are talking about fairly standard transformers ie 11kv/415v 1000kva.
These transformers often dont have silica gell breathers but have a breather hole with a blanking plug for transit. Once landed and before being energised these plugs should be removed, it sounds like this has not been done on these units.
I dont know the exact circumstances but I think I would remove the plugs and let the transformers breath as they are ment to.