Damage to CT PT while testing a 20 MW Generator.

You haven't stated what the CT was rated for and what the short circuit current was. But, you may have induced such a large voltage inside the CT PT coil that the insulation broke down between the windings and shorted it all out.

"Cause of the fault?"

The designer of the test?

Some more information about the "test", how it was performed and what happened would be handy in determining possible causes of the failure.

I don't know the nature of the test but one does not usually apply a direct short to the output of 20MW generators while the generator is excited. The currents generated can easily destroy or damage all sorts of things, up to and including any one near by.

I was present when a generator of about this size was connected to a buss with a rat's nest, including rat inside the duct. The short circuit current was sufficient to make a valve wrench jump up off the floor and flip in the air. The magnetic field erased the magnetic strips on all of my credit cards in my brief case which was sitting in front of the breaker cabinet.

The rat was very effectively electrocuted. It was not hard to find the fault in my case as the buss duct was burned and smoking where the nest was located.

Finally, you don't usually damage a Current Transformer (CT) by shorting it. If a CT secondary circuit is opened while the primary is energized, the secondary voltage can easily exceed the insulation break down voltage. When the arcing starts inside the CT, a lot of energy gets dumped inside the transformer, often causing an explosion resulting in lots of very hot semicircular bits of copper wire lying about and a great deal of smoke.

I have seen this happen a few times when people miss-wired metering circuits or opened the wrong circuit while the generator was on load.

The potential transformer (PT) can be damaged by over current or excessive voltage. Usually there are fuses to protect the PT from over current but if one is running a test, it is easy to over-voltage the PT causing insulation failure which can lead to shorted turns or an open primary or secondary. PT failures are seldom as exciting as CT failures.

Failure of the circuit protection devices to remove the supply to the fault connection.