Directional Earth Fault / ring operation - 11kV Vessel / isolated ground network

[J.Key]

Hello, all, I have a question/topic to discuss. We are currently building a ship with 11kV distribution. We have one DG running with neutral point resistor. The generator is feeding the main busbar where we have 2 feeders with MV circuit breakers. Both of the feeders are then connected to a single bus at aux. 11kV SWBD (yes that is "ring operation" and it shouldn't be like this). both feeders have directional earth fault protection (measured Io by core balancing CT, direction by U0/I0 phase difference), which is tripping both CBs at the same time. I am convinced that this is due to residual current imbalance redistributed to each feeder. i.e. generator is feeding both CB, but the ground loop is then leading I0 a bit differently and this I0 CT detects Earth fault. Do any of you have some experience with this? I would like to find some books or articles to be able to present to the vendor to make requested changes.

 

[RRaghunath]

The directional earth fault protection should have been at the other end of the feeders, looking towards the feeder (away from the aux switchboard busbar). In such a case, the protection would isolate the faulty feeder alone allowing the other feeder to continue to supply the aux switchboard.
As it stands in the diagram, tripping of both the feeders for an earth fault in one of the feeders or in one of the outgoing feeders from the aux switchboard (after coordination margin, if provided).
This seems to be an application problem.

 

[J.Key]

Hi, as far as I know, EF should be used at the beginning of the protected cable on the side of the power source. That is a reason we have DEF there.

Both upstream CBs trip at the same time for DEF2low, if we supply only with one CBs everything is OK, The issue is when we feed with 2 of them.

 

[RRaghunath]

What you said is right Mr. Key if the feeder were radial type. Here the situation is different.
As you said, it would be an appropriate solution for you if you can operate only one feeder ON at a time.
You may have auto changeover scheme if you like to switchon the second feeder when needed.

 

[J.Key]

Hello RRaghunath, the SIEMENS SWBD was added quickly without our approval or questions, so they just did single bus, if we were consulted we would tell them that ring operation is prohibited (as we have the similar system for ships ring-net system (split ring, short time parallel function). The issue here is that accepted our suggestion to split aux. bus. however I would like to uderstand physics behind the EF trip.