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What’s cooking?

I saw a story on the news about a generator fire on a ferry at Swartz Bay but it was light on details.  One thing they did say was that a generator caught fire while they were transferring from shore to ship’s power.

There are a few ways things can go wrong when doing this but the first thought that came to mind was that the ship’s generator might have been brought online before it was properly synchronized with the shore power supply.

You can take multiple AC sources, generators and shore power, and use any combination of them to supply your electrical needs in the most effective, efficient way to suit conditions;  bringing them on and offline as needed to provide uninterrupted power to your electrical system.  So long as a generator’s output is synchronized with the main system before it’s connected, it can be brought online seamlessly.  When it’s done properly it’s a smooth operation.

The excitement comes when you hook the generator up without synchronizing it first because it will be pulled into phase with the main bus rather violently.  Depending on how out of phase the supplies are, and the size of the equipment involved, the side-effects can range from minor to catastrophic.

Another scenario that could occur in this situation:

If the generator’s engine failed while it was connected in parallel with the shore power (during the switch-over) then the generator itself would remain in phase with, and be driven by the shore power connection; effectively behaving like an electric motor.  I guess that’s why it’s called “motoring.”  There are ways to protect against this (reverse power relays) but I don’t know how things are commonly done on ships; all I ever did on a ship was cook.

It just happens that there will be a marine engineer and an electrician at the Thanksgiving supper I’ll be attending this weekend so I can pick their brains.

Anyway…In this situation the consequences can range from minor to catastrophic depending on how the generator is regulated, whether the generator field remains excited, and just why the engine died.

I’m no electrician, but as a welder I do get to play with generators now and then…and I do love to speculate…

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