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05-11-2012, 11:22 PM #1
OT - 3 Phase connection and earth leakage breakers.
OT as not machine tool related.
Some of you helped me in a recent thread with inverters causing harmonics in the ship supply system here:
I have another problem regarding the same vessel.
We connect to shore supply when alongside and it is available to save running ship's generators, the shore cable and associated equipment is rated to 180A but be normally draw about 120-130A. The power is passed through a shore converter which allows anything in and automatically converts it to the correct frequency and voltage.
We are berthed in Malaysia and the maximum shore power available on one socket is 100A; normally in this situation we connect into 2 sockets which come into a connect into a 'joiner' box, into which we connect the ships 180A plug. Each plug on this jetty has an earth leakage trip as well as the standard 3 phase current trip. I am finding the earth leakage trips will not make at the same time, ie, make A and B trips and vice versa.
Each plug is wired with 3 phases and earth (no neutral), each plug is wired with the same phase to pin and I have checked continuity through the joiner box
Can anyone think if I am missing something why the earth leakage breaker should be fighting.
If I pare the ship's load back to about 95A I can successfully connect through one plug though if I get a transient load such as the fridge compressor cutting in, the main breaker trips on overcurrent.
All help and comments greatly appreciated.
05-12-2012, 08:07 AM #2
Most ELCBs are using Residual Current Detection, meaning they look at the current of all 3 phases and if they don't summ to zero, it trips. So anything that makes the currents unbalanced will do it. Most likely your shore power connections are coming off of separate isolation transformers as a precaution against a moored ship causing back feed issues and damaging the systems in other moored ships. so when you tied them together, the slight lag in the transformers energizing will mean the current is unbalanced for a few cycles, and that is enough to trip the breakers.
It could also be that the 3 phase circuits in the plugs are "rolled" with respect to each other, meaning the ABC rotation is correct, but not the same phase at the same pin. Wouldn't make any difference to two separate loads/ships, but when you try to combine them, you are creating a phase shift. That's easy enough to check with a meter, just take the two live female ends from the shore power and measure the voltage across the same pins. Should be zero, if not, see if any combo is zero, that's your correct configuration. If none of them are, you may not be able to do this at all.
05-12-2012, 09:32 AM #3
Because you have a power combiner like an Auto Phase would make me think
that the supply is coming from one shore transformer. This has a bunch of relays
and most likely a PLC to decide which phases equal 0 volts and then couples them
together. Then you go on to your main shore power breaker and then on to your
voltage/ frequency converter.
I agree w/ Jraef about load imbalance being a problem and causing the
residual current devices (RCD) to trip but because of the frequency converter
remember that the shore side AC is first turned into DC and at this point has no
relation to shore power. This then is "remade" into the correct power that the
yacht was designed to operate on by the AC inverter side of the frequency
converter and is totally isolated from shore reference.
I have on occasion seen the start up capacitors trip the shore power
breaker but not very often. If you have a fairly sensitive AC current or ground
fault meter onboard you could measure the leakage going back to the dock via the
earth wire. You could then test different loads to see which has the most
leakage. Also you might ask at what value are the RCD's made for. If you have a
megger onboard check the shore chords for insulation breakdown. Relative
humidity can also be an issue as well as salt build up in the shore power stuff
that is exposed to weather and/ or wash downs.
Depending on which make of frequency converter is installed aboard you
should be able to limit how much dock power is used to prevent a input trip.
There would be a pre-alarm the the limit is being reached.
(Atlas Marine Systems tech rep USA)
05-12-2012, 10:17 AM #4
Most GIFs are pretty simple devices: Just a turn or two of each conductor through a ferrite core.
Then there is a multi-turn sense winding on the core as well.
As long as all the current going through one power winding is quite equal to the
the current in the other winding(s), the sense winding sees no magntic flux and hence
produces no voltage.
At the point where there is a current deficit in one of the windings, the sense coils produces
a voltage that trips the interruption. The idea being that current that is "missing" from one
of the conductors has gone to ground via a person or other leakage path. Typically they
trip with between one and five milliamps of difference current.
So in your case where you deliberately parallel the lines, there is probably a tiny difference
in the resistance of the parallel connections which is enough to get current from one service
to try to return through another service. Hence the trip.
I suspect that if all the load is turned off, there will be no trip while the services are paralleled.
It will only be when some substantial load is applied that the the trip happens.
I susepct the only way around the problem (short of breaking the wiring onboard into two
separate services) would be to parallel the services via two, three-phase isolation transformers.
In that way you could keep the primaries separate but still parallel the secondaries.
05-15-2012, 05:08 PM #5
Thanks for your attention and answers.
As is often the case I took the path of least resistance (no pun intended). The boat next door but one moved yesterday and they had a single 125A socket; I extended my cable and plugged into that socket as soon as they moved.
The best thing about this solution is that I don't have a 5 plugs splitting 1 cable into 2 sockets with the various glands and gaskets for water ingress, you should see how hard it can rain here and water seems to get in no matter how well you think you've bagged it up.