Current imbalance ungrounded power

[jimmy2times]

We have a strange problem occuring on ship. I amwaiting on some phase current readings to be taken which will answer my question for sure but in mean time i am trying to think something through (and not succeeding)

We have ungrounded ship power system. I have found information on shore power and it is a Dy11 11/0.48 kv transformer. I spoke to shore guys and they told me neutral is solidly earthed.

So this means we normally operate ungrounded but onshore are then a grounded power supply. There are two parallel 3core shore cables to the ship and a bonding cable which i assume has to take the function of a the combined neutral and earth conductor.

Our platform manegement system is monitoring load (scada) and it is showing uneven phase balance when on shore power. It may be instrumentation error but i was wondering if it is possible to get uneven phase balance as a result of first earth fault on ungrounded system?

Or if the supply arrangement on shore could be part of problem where the hull (now connected to neutral via the bonding cable) is sitting at an elevated potential from that of the transformer neutral. I looked back at this thread a while back and it was possible suggesting similar issue. Id be interested to hear what waross would say? (It will probably just turn out to be an instrumentation fault but im finding it interesting to think about)

thread238-150484

 

[waross]

Check for any current in the bonding cable.If possible, break the bonding cable connection and check for voltage between the bonding cable and the neutral. Even voltages a low as a part of a Volt are bad news.
The ship is well grounded by contact with the water. It does not need a bonding cable to shore.
There may be a line to line voltage unbalance on the shore power due to a phase angle error upstream. Does your unbalance change depending on the number of motors running? With a phase shift issue, the more motors running, the greater the current unbalance (up to a limit determined broadly by the relative effective impedances of the source impedance and the effective impedance of the motors.)


Bill
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