We had an event today onboard our ship. Two of our thrusters were running, and only one diesel generator. This is possible during calm weather, but with a bit of wind, it can not be done, because the thrusters have to work more to keep the ship in position.

So the wind increased and the navigators decided to start a second generator. But the stand-by generator did not connect quite so successfully to the main bus bar. It connected, was connected for about 5 seconds, then got thrown off again, and we got the alarm "DG1 Abnormal MSB open"

First off, I think it took about 2 minutes to synchronize, which usually takes like 30 seconds, at most. Then when the generator got in for those 5 seconds, the "Produced effect" indicator got in reverse, counting from -1 kW and tapping out at -130 kW, 5 sec later.

Now, this generator usually counts reverse at first, but after a few seconds, it goes to 0 kW and then starts to produce power. But we usually connect it while there is low load on the main bus bar, around 500 kW.

This counting in reverse thing, is it because the voltage on the generator is so low, that the main bus bar (higher voltage at this point) starts to actually drive our generator/motor, instead of the generator supplying power? That is the only way for the generator to become a consumer, right?

If the voltage is low, then does it have something to do with the revolutions on the generator? I don't remember if we experienced a major drop of frequency, when generator 1 tried to connect. We probably did.

 

This counting in reverse thing, is it because the voltage on the generator is so low, that the main bus bar (higher voltage at this point) starts to actually drive our generator/motor, instead of the generator supplying power? That is the only way for the generator to become a consumer, right?

If the voltage is low, then does it have something to do with the revolutions on the generator? I don't remember if we experienced a major drop of frequency, when generator 1 tried to connect. We probably did.

That sounds about right. It is possible it could not synchronize with the other generator or get up to speed or it was under excited. This is why power plants keep their generators turning and synced up with the rest. They take the load off so they barely use any resource but once they need it, they just reconnect to the grid and they can control their output a lot faster without as many problems. I am not sure what generators would be on your ship but most are the same concept. If you have a negative power reading then it is consuming off of the grid.

 

We tried again today, only at much lower loads. Only Read -15 kW, until it caught on and started producing.

The voltmeter was very stable at 690V during the entire process. I expected a drop, but did not notice anything.

What did surprise me though, was that the voltages reached 690V way before we got 60 Hz. Guess I need to find my old school books again.

 

That is quite a powerful ship with three thrusters each rated at over 1,000 brake horsepower.

If you read the operating manual starting procedure – I suspect there might be some mention about reducing the system load to avoid what you experienced.

 

As I read it the 1st generator is seriously overloaded before you attempt to add the second, I cannot beleive a 1000Kw generator is delivering over 1700Kw without either or both bad voltage sag or frequency reduction, in fact I am amazed it had not simply given up altogether! Expecting a second unit to synchronize to such a buss condition is asking a bit much IMOP.

 

@Hymie I've excluded 2 other thrusters and both main propellors. It's quite powerful

@fourtytwo The listed trusters are noted with max power. The load was +800 kW, which should be very doable.