What is the corrosive sulfur issue?

What is the corrosive sulfur issue?


What is the whole corrosive sulfur issue
that transformer owners may have heard about? Well, it’s an issue that started
somewhere around the late 1990s, around the year 2000. There were some large expensive
transformers throughout the world, more so in hot climates and they were
relatively new and they suddenly failed and there was no indication that they
were going to fail from all the regular oil testing. So they did autopsies, forensic investigations to
see what was the problem. And they found a sort of a black powder
like coating on some of the copper conductors and also this black substance
inside some of the paper insulation. When they checked it out in the lab,
they found out it was copper sulfide. So that is what is created when you have
corrosive sulfur compounds in the oil and those compounds react with the
copper. They make this copper sulfide. The problem is copper
sulfide is conductive. And so the insulating properties of the
paper are diminished greatly by having this conductor inside
the paper insulation. And
that’s why these units fail. So the big issue at the time was, well there was a laboratory test to
test for corrosive sulfur in the oil. But these oils of these transformers
that had failed from this, they passed the test. So the industry realized that the
conditions on the laboratory tests for corrosive sulfur were
not stringent enough. And so ASTM and other organizations
changed the requirements of the test and made it more stringent. And the test is now performed in the lab
at 150 Celsius for 48 hours and that’s stringent enough to separate and find
the oils that do have this potential problem. If a transformer
owner does have this problem, then, and also I should say if it is in a
transformer that in our recommendation is running with a top oil temperature gauge
of at least 60 or 70 Celsius and it is doing this for at least six months of
the year, then if you have corrosive oil, it might be worth mitigating. And our recommendation for mitigation
would be the best thing you could do is replace the oil with brand new oil
that passes the corrosive sulfur test. And that also has in
it a metal passivator. A metal passivator is a chemical compound in liquid that dissolves in the oil, and it basically coats out
onto the metal surfaces, especially the copper. And it provides kind of a protective
shield because when you retrofit the transformer getting rid of the old oil, you’ll still remain between
5 and 10% of the old oil. So even with the new oil
that’s not corrosive, you will have traces of the old oil. So that metal passivator provides some
insurance and protection against the remains of the corrosive sulfur oil. In some cases that may not be
economically feasible for the customer. So the second best recommendation we
have is to keep the current corrosive oil if you have to, but add
the metal passivator to it. So that will provide some
protection from further damage.

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