Theory of electrical HV impulse affect on steel lamination insulation 1

[genRman]

Could someone provide information with regard to the theory of how HV elecrical impulses travel in transformer motor and generator core steel laminations, from the windings through the insulation coating?

I am presently involved with form wound winding interturn tests and have noticed that a 0 - pk wave of >0.5micro s but less than 1.2 micro s rise time has some influence on the level of insulation seen on the core lamination insulation material after this test.

Any ideas?

Thanks

 

[electricpete]

That's a new one on me. People do impulse testing on transformers and surge testing on motors without any thought to damaging the core.

 

[genRman]

Electricpete,
the test work I am involved in is inter-turn insulation testing of inserted coils in the core, but ones which are not yet connected in groups of phases.

Your comment may mean that this is a new area for consideration in a very specialised field.
Thanks anyway

 

[prc]

Impulse voltage applied on the winding of trf,esp HV and EHV windings causes an induction of high surges in the inter laminar insulation of core sheets .It is thought that this surge causes shorting of burrs in the sheets and causes an increase in the core loss(no-load loss) of trfs.So core loss measured soon after impulse test will be higher and many customers insist to measure this loss after impulse test. But this is a reversible process and losses come down to original values after some time.So latest ANSI C57 -12.90 states that this is an unexplained phenomenon and losses need to be measuresed only before the impulse test.

 

[genRman]

PRC,
Thanks for your posting.

Could you please provide detail on exactly how the core loss is expected to be reversible?
my investigations thus far have shown burr and pressure induced shorting of the laminations but I donnot understand how this could be reversed as I understood once the short is there, a permanent path is established due short circuiting the lamination during the high stress levels induced by the 0 - Pk impulse test and will be maintained ever after.

Thanks for the ANSI standard, I will look into it

 

[prc]

I am also not sure how it happens.But the said std says that this is an unexplained matter and the losses do reverse.
Now some historyuring the olden days the change in losses was appreciable due to poor insulation provided on the core laminations .But with todays modern core lamination insulations (C-5 over C-2)the increase in losses is negiligble often less than 0.5%.