Interesting one. You may find that bits of BS7671 do not suit this, for the same reason as it does not suit parts of railway traction installations or mines, or ships. Need to be clear which bits though, as getting an act of parliament through in a hurry just for this job may not be so easy  (as some of those other cases have done )

It is not clear from the description if the isolation has to be star delta, or if windings that are star to star and just not to distribute the neutral would be equivalent. The advantage is the star centre to earth voltage can be used for fault 1 detection. If not a psuedo neutral must be created for the detection using 3 equal capacitors or resistors or  an additional cunningly wound transformer, but it is extra parts.
You may not need a CPC on the train, but I presume you intend to detect a cable fault on the secondary side of the TX so some sort of loose earth reference via the fault detection will be needed. Do you have any sort of diagram you can upload, it may be that the mental picture forming in my head is complete tosh!
 Mike

Thanks for the reply, it is an interesting one.

Forget to mention the supplies are 3 Phase and the request for a Delta Secondary Winding is from the manufacturer.

I cant share any diagrams as such but here's a breakdown of the supply If i have time later on i will sketch out:

Site Delta/Star 11kV/400v Tx>Main Site Switchboard> Building Switchboard>Proposed Isolation Tx Delta/Delta?>Panel Board> 63A TP Sockets> Plugs into train

The on board converter is insulated but referred to ground in the DC link intermediate point which could leak currents to earth (This is from the manufacturer), so if the train is supplied via an unearthed delta secondary winding then there should be no tripping but we would need to monitor the cable for faults to earth no so like you said would need some sort of reference? Would this cause an issue on the actual building supply even if we install an Isolation Tx? As everything is bonded back to the building supply which is a standard Delta - Star arrangement?

Thanks

K

The on board converter is insulated but referred to ground in the DC link intermediate point

Hmm. Do they mean they connect one side (or the centre tap if bipolar DC ) of the DC bus to the chassis ? that is common for high power inverters and VSDs, but awkward in this case, as then each phase takes it in turn to connect to chassis when it is the most negative of the 3 phases (assuming negative side of the DC is connected to chassis, it could be positive, but usually isn't;) .
Is so really you want the isolation  transformer in the box  near the convertor, as the cables that supply it have a very different relationship to earth depending if the converter is plugged in or not - unplugged it is IT. plugged it has more of the properties of TN, but as if the N was commutating round the phases visiting each one once per  50Hz cycle.
A consistent ADS is very hard then as which ever state you design it for the other one looks like a fault.
I'd be asking for clarity on that.

Mike

The on board converter is insulated but referred to ground in the DC link intermediate point

Hmm. Do they mean they connect one side (or the centre tap if bipolar DC ) of the DC bus to the chassis ? that is common for high power inverters and VSDs, but awkward in this case, as then each phase takes it in turn to connect to chassis when it is the most negative of the 3 phases (assuming negative side of the DC is connected to chassis, it could be positive, but usually isn't;) .
Is so really you want the isolation  transformer in the box  near the convertor, as the cables that supply it have a very different relationship to earth depending if the converter is plugged in or not - unplugged it is IT. plugged it has more of the properties of TN, but as if the N was commutating round the phases visiting each one once per  50Hz cycle.
A consistent ADS is very hard then as which ever state you design it for the other one looks like a fault.
I'd be asking for clarity on that.

Mike