[BC] Transformer conditioning

[Clive Warner]

<First of all, why would this type of testing and conditioning not be done
at
<the FACTORY?

** The transformer is indeed perfectly made and tested at the factory; I've
seen it.
But then they are packed in crates and put into stores. And such items are
usually stored for quite a while.
However it is usual for a spare to be kept at the transmitter site. In which
case the same thing applies.
It is normal for these open-frame transformers to have moisture in the
windings.

<What is involved in this "conditioning"? And how many radio stations have
<the equipment to "properly test" the insulation?

- It can be a bit difficult. I learned about this myself the hard way;
someone should have taught me *before* I burned up the transformer of a
brand new out-of-the-box 10KW transmitter. I found a local rewinding shop in
the depths of a Lagos slum and unbelievably it came back better than new for
just $500. But when I got back I got the treatment. About conditioning
transformers.

<Would you care to share your knowledge on this subject? I for one would
like
<to know more.
<Jerry Mathis

With pleasure. You must drive the moisture out of the windings before you
ever apply full voltage. It can be tricky. It depends on what you have
available. I have:

a) Wrapped the transformer loosely in aluminium foil and surrounded it with
dozens of ordinary light bulbs wired in parallel.
b) Reversed the transformer, connecting the high tension side to 240 per
phase and then shorting the 'secondary' through an electric fire as a small
load. The current isn't that much but it will warm the thing up eventually.
c) Blown air inside using a fan-heater or similar.
Then waited about 2 - 4 days.

It is essential to have a good insulation tester. My own personal one is a
Comark, it is the electronic type with an analogue meter, can reach 20KV.
What you do is, you note the insulation resistances between primary and
core, secondary and core, primary and secondary. Then you start warming it
up. After three or four hours you take the measurements again. You keep
doing that. Plot the result and you will see a characteristic curve that
happens with all these open-frame transformers:

The typical sequence of events is:

The resistance looks OK when it is out of the box for the first time.
After several hours the readings start to drop.
The readings keep dropping until typically, at 1-2 days, they reach a low
point. The resistance in fact will be low enough to cause a catastrophic
insulation failure at that time, if the transformer had been on power.
The readings gradually recover as the warmth drives out the moisture, until
after several days of good, high, insulation resistance, the transformer can
be put into service.

Now, I only learned this when I became an installation engineer. Your
average station engineer doesn't exactly see many transformer failures. But
on installations, you're putting transformers in, month in, month out. And
them at the factory don't take too kindly to 'the cowboys in the bush'
blowing up the transformers . . .

I think it's proper that the station engineer should know how to sample any
oil filled transformers and flash-test the sample for minimum breakdown
voltage, and to be able to prove the insulation rating of any HV junction in
the building.
Always assuming there will be transmitters in the future . . .

Clive