Increasing Excitation Current On Dry Type Transformer Testing

It sounds like the internal losses of the transformer have increased. Shorted turns due to insulation failure is the first thought that comes to mind. Woody This would cause excessive primary current and overheating. Enjoy. Woody

Have you done any additional tests on the windings, such as DC winding resistance? I assume the Doble power factor test results were consistent with past tests? However, dry type transformers always have noticeably higher PF values than equivalent size oil-filled, so sometimes it's hard to be sure if the numbers are good or bad.

If you are using Doble equipment, I also assume you are a Doble client. What does their application engineer say? They should be able to interpret the results for you and recommend additional tests or corrective action.

power factor, dc winding resistance and capacitance have not changed much over all the tests we have done, only exciting current. From the manufacturer I have gotten the answer that they don't test excitation current at the factory. Doble engineering appears to be more focused on large oil filled transformers so small dry types (1000kva, 4kv) are kind of off their radar, I think. I have seen this occurring on more than one transformer over a series of years. The transformers all go back in service no problems and the electricians have cleaned them quite well as, like I said, pf is good. I am pondering if something has changed in the doble software or hardware over the years.

The excitation test is very simple - you are just putting enough voltage on the winding to establish the magnetic field ("excite" the transformer), and measuring the current that is needed. The only thing that will change it is something that affects either the current path (turn-to-turn shorts, etc.) or the magnetic path (core damage, winding distortion). That can happen even when the insulation overall is good.

Has the transformer taken an external fault at any time you are aware of? That could have dome something to distort the windings on the core. Also, is it on all phases or just one or two?

Doble may focus mostly on larger units, but your client engineer should give you the attention you are paying for. You may need to push a little for them to refer it to one of their "gurus." Some of the client engineers are younger and newer, but there are some really smart people there that have been around this a long time. Would you be willing to tell me who your engineer is? You can message me directly if you want.

Good luck!