Funny blackout or ship's alternator inside

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Funny blackout or ship's alternator inside

Funny blackout or ship’s alternator inside

Here is the brief story – ship is experiencing unexpected blackouts with Generator No.1 with no any alarm on PMS activated. The generator may work a long time without any trouble, but it also can cause blackout few times per day. The crew became crazy – there was no any real reason for trouble…
Sometimes after the blackout the same generator was found active and loaded, but other time the standby generator was started.
First impression was to check the fuel system. No problems were found…
Second decision was to exchange the governors between the generators. Funny result – exchanging the governors made one of them to behave as it doesn’t have governor at all. But it is a different story…
After a long time of troubleshooting inside mechanical part the decision was to check the electrical part. Nobody believed the trouble was there because the generator was working for weeks with no any problem. A lot of modules have been exchanged, a lot of e-mails written. Finally the electrician decided to go inside the alternator part. And what did he find?
It was a B&W generator and this is the top part of it.

Pay attention to the cables connected to the current transformers on the picture (yellow circled). Two of them were touching each other. Vibrations and friction were destroyed the insulation and both cable cores start touching each other on random time base.
On the drawing below is clear what happened – short connection between those two cables was zeroing the exciting voltage and the output of the loaded generator became zero. And this was the cause for the blackouts.

But why in some cases after the blackouts they found the same generator taken load again, and in other cases the standby generator was started and loaded?
Believe or not, the answer is in ACB. There is a time delay for opening builded in it. In this particular case it was 25 milliseconds. 
So, if the short circuiting was less than 25 ms, the ACB remained closed and after restoring the excitement the same generator was loaded because it was still connected to the consumers. But if the short connection stayed more than 25 ms, the ACB opened and the standby generator executed its program.

End of story. It happened once to other people, it may happen to you. You have been informed! :)
From Marelsys Post for Reference.

Bharatiyulam

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