[BC] oil filled trannies

[Clive Warner]

John wrote:
>Clive, I have 3 oil filled transformers and diode
>stacks for our Collins 250 kW rigs, that have been
>sitting outside in crates for YEARS.  What needs to be
>done before they are put into service?

OK. Assuming they are still in good condition (the diode stacks I mean, the
transformers are not likely to have come to any harm!) then:

1. Transformers.

Uncrate. I think it is better to prep them before moving them, as this size
of trannie requires moving techniques, as you know.
Remove the sample cover plate and dip for an oil sample using a clean, dry,
dipper. At all costs avoid dropping anything inside... or, if there is a
sampler spigot, just draw a sample into a clean dry container.
Seal the container immediately to avoid additional humidity getting into the
sample.

Fill the test chamber of an oil tester to the required level (this is
usually a small square container of thick glass with a pair of 1 sq.cm
electrodes separated by 1 cm), connect the test voltage wires, and slowly
increase the voltage until flashover. Note the reading. Repeat three times
and calculate the average.
Compare your flashover voltage with the manufacturer's stated minimum for
the oil/transformer. If it's equal to, or above, you're home free and can
proceed to remove the old trannie and move in the new one.
(Several hours of work, usually, involving long crowbars, motion skates,
lifting jacks, rope, and a crew. Might be able to use a 4WD as motive power
with a bit of ingenuity and a few pulleys attached to convenient anchors!
This is real engineering and makes you wish you'd paid more attention to the
physics teacher explaining how levers work.)

If your oil fails specification, you will need to drain it into 50 gallon
drums, and then either:
a) contact an oil supplier to arrange reconditioning (your local electrical
utility may be able to help, too; they'll have lots of oil-filled devices!)
b) hire a reconditioning machine and do it yourself (takes days, smelly,
hot, I hate that job.)
c) dispose of it in an ecologically accepted way and replace with fresh oil

2. The diode stacks will simply need careful cleaning if they've accumulated
any dust or debris while in storage. Before you put them into service, test
each diode in the stack for forward/reverse function. If everything looks
fine, put an insulation tester - preferably a modern one rather than a
'Megger' - across the stack, and measure the insulation resistance.
I have very, very rarely lost a diode or two, and cannot recall ever losing
an entire stack, so you should be on pretty safe ground there.

If this is a new install, don't forget to pressure-test the insulation of
the HT terminations such as wall boxes, especially if done with pitch. I
have seen an 11KV wall box blown to pieces because of moisture ingress. But
I must admit, I really, really hate testing those, because the machine is so
damn big and dangerous. (Much bigger than the oil tester).
Nowadays I understand the terminations are usually done using cold-poured
epoxy, which is cleaner, quicker, and better all-round. I hope the days of
making plumber's-solder wipes are over...

Best wishes
Clive