when do we do pressure test on power transformers?

Question

one of our transformers tripped and we want to fault find

Answer 1

I'm going to have to assume you're using an oil-filled transformer - there's no real reason to pressure test an air-type!

Manufacturers of transformers do a pressure test before the unit is ever released for shipment ... it has to arrive at your facility "good to go". Most will include an operation-and-maintenance handbook, stipulating how often and what type of preventive maintenance is called for.

One such example is found at the link I've given ... it indicates doing the pressure test once per year. In some cases, this is insufficient - particularly when something out of the ordinary has been happening. The real condition is to make sure the check is done regularly, and done the same way every time. Otherwise, trending such data is virtually useless.

On the good side ... the presentation does give you some other ideas on what to check to see if your transformer is healthy.