Two questions about our azimuth thruster - marine tech

Thread Starter

StrongPenguin

Joined Jun 9, 2018
253
I'm struggling a bit with learning to use GIMP, so please bare with me for not coming with a drawing just now. I'd reckon you could picture this in your head, while I'm sitting at the drawing table.

We have two diesel generators (and two shaft generators, but irrelevant here) producing power to the main bus bar. 690 volts, max. 1000 kW each. For everything I am saying here, only one diesel generator is online.

We also have an azimuth thruster 880 kW, which has a frequency converter, which receives power from the main bus bar. Before the DC link of the frequency converter, there is a LCL filter (don't know the values, but the inductors are pretty big! So are the caps..) I think I read somewhere, that the LCL filter is to protect the DC link and give it a stable feed of AC current, but also to protect the main bus bar from all the noise the DC link does. But today I could only find articles saying it's to protect the main bus bar (where everything on the ship is somehow connected to), and not the DC link. Thoughts?

And another thing. I've noticed, that if we give almost full power to the the azimuth, and all of a sudden take all the power off, we get a kVAR effect error on the diesel generator on line. Is this because the propeller is now still spinning at full speed and thus acting as a generator feeding into the main bus bar, which the generator voltage controller can't figure out to handle?
 

Janis59

Joined Aug 21, 2017
1,224
Not a tiny ship seems. My captain licence permits to drive only a 28 meters long ocean ships however they have hardly smaller engines. So, have nothing to say but Your hypothesis may be right (or no). Generally, kVAR alarm must diagnostice that there is capacitative OR inductive overflooding in line under observation (other words overvoltage or overcurrent). Can it be You havent any cos(fi) correction substation? If it stays there, probably defective at too overloaded conditions. The high kVAR conditions may hit all electricity system by V or A stress, thus may happen "sh* absolute" in midst of Ocean. If there the kVAR indicator are swollen in line between freq converter and servomotor, then seems You diagnosticed right; but anyway the condition may damage that motor as well the converter. Probably kind of resistor matrix for short-time use and magnetic launcher may be tried to fix the bug? Governed by alarm signal, I mean. For resistor may try to use some kind of water heater. If in water tank 4190 W*sec will give a 1C, thus the 1 m3 and 100 C will give a 400 MW*sec what is probably enough, however the MW-scale heater can be problem.
 

Thread Starter

StrongPenguin

Joined Jun 9, 2018
253
@Janis59 Sorry for the late reply, a stepper motor in our azimuth for the internet dome wen't out, leaving us in the dark.
Glad to hear someone agrees with me :) The ship is not that big, 80 meters long and 8000 kW total. No we don't have any phase compensation on our main bus bar. The azimuth has a capacitor battery, don't know if that counts as phase compensation.
The error doesn't last very long, it's usually gone when we get to the engine control room, which fits perfect with the azimuth producing a peak during load off.
 
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