So at work today I had to sync our backup generators to the main power system without loosing power anywhere. I did this last year, had a slight glitch with the length of time it took but all in all it worked OK. At the time I thought things were behaving a little mysteriously but it was all within limits so I went with it.
Last time DG1 took AGES to sync. Like more than 45mins. We had kind of given up wondering if it was even working and when we came back to it, it had synced and all was good. Then DG2 was started and synced within about 5 secs. The procedure was amended to say “Sync DG2 first as DG1 takes a long time to sync”.
Today DG1 was chosen as the first one again. Once again it took AGES and we’d left it be. The mechanic and Chief engineer were doing some tinkering on it so we were going to let them finish then turn it off. I just happened to be walking past the control panel and noticed it had synced! How bizarre. Ran up on load then turned off just before lunch.
Ran up DG2 after lunch thinking “this is gonna be a piece of piss”. Nope It wasn’t. The exact same thing happened. Now I forgot to mention that with DG1, the sync scope was coming right and saying OK but the word BLOCKED was present. This was also happening on DG2. Whilst DG1 was not working properly I had started looking into the manuals and drawings. I found a picture that showed the voltage tolerances. AHA!
It just so turns out the word BLOCKED was in the Voltages part of the page, and hello these is a differential indication! BUS:457v GEN1:450v VDIFF:7/13
So I presumed these two voltages needed to be closer (by the way documentation is a mission to get hold of!). One way was adjust the AVR, the second adjust the bus voltage. Turns out the AVR is set from the digital controller/protection relay so that is difficult to change without messing up settings that will probably never get changed back. Second way is to adjust the Bus voltage but how does one achieve this when it is fed from a transformer from the 11kv bus with two massive turbines feeding it without affecthing the rest of the ship or platform?
I then realised the load in the engine room (where the switchboard and generators were) was only about 550kw and is usually up over 1MW. Less load = higher voltage. More load = less voltage. So I started up a 500kw pump and the generator synced immediately! Only drama now is that the bus voltage was still originally higher than the generator so the current was VERY high…note to self - “only run DG’s synced to STG’s for very short duratinos for the purposes of switching only!”.
So next time your generators won’t sync…try starting the fire pump!