Vacuum Pressure Impregnation, VPI, and Atmosphere Dip – The Trade-offs

Docta Reacta:  Today’s topic at PMI Institute addresses when Vacuum Pressure Impregnation, VPI, makes sense for your transformers, like a high voltage plate transformer or medium voltage trap reactor and when it does not and Atmosphere Dip is more appropriate.

Thank you for the applause.  Thank you! Thank you!  Wow, what a warm reception!  I realize that this is my first appearance on the new stage here at Power Magnetics Institute, and I am humbled to be your Docta of Technology.

OK, so let’s get down to business.  PMI uses both types of processes to apply varnish.  Let’s discuss the reasons for both.  As is considered good practice in the transformer industry, all our Dry Type Transformers and Reactors are varnish impregnated.

Varnishing is considered desirable primarily because:

  • It repels moisture.
  • The unit runs cooler.
  • They have much greater mechanical strength.

The two methods of impregnation that are used in the industry are Atmosphere Dip and Vacuum Pressure Impregnation or VPI.  Highly ducted low voltage coils are easy to impregnate and Atmosphere Dip is almost always adequate.  High voltage coils and unducted coils, however, are much more difficult to impregnate and Vacuum Pressure Impregnation is recommended.

Basically, complete impregnation guarantees the full advantage of the varnish.  Numerous studies have shown that VPI of High Voltage Transformers results in longer life.  We at Power Magnetics have heat run High Voltage Plate Transformers that were atmosphere dipped against the same units given a VPI; the units with VPI ran 20°C cooler (a considerable advantage).

Also, all our impulse tested Medium Voltage Trap Reactors, High Voltage Plate Transformers and Zig Zag Transformers have been given a VPI prior to final test.

As a consequence of all of the above, we at Power Magnetics, Inc. give Vacuum Pressure Impregnation, VPI, to all our High Voltage Plate Transformers, Medium Voltage Trap Reactors and Zig Zag Grounding Transformers.  We feel this has been a big contribution to our reputation for quality and endurance unmatched in the industry.

So why don’t we do VPI for all our products?  The answer is that not all products realize an advantage from VPI.  And it is a longer, more involved and therefore more expensive process.  So if there is no significant advantage, there is no reason to add the cost to the product!

I hope this answers your questions, but if not, please let me know.

What’s this?  Applause?! Thank you!  Thank you!

Docta Reacta here signing off until next time when I will do my best to inform, elucidate and amaze you!