Mike. I think that's a single-phase 25kV transformer, so any 3-phase 400V supply would have to hook in somewhere else, but I can't see where. Without knowing where that feed comes in and how it's configured, it's hard to trace any stray current paths. If it's coming back in through the auxiliary converter bottom right, then we could have an issue with stray currents from the DC mid-point earth connection which Kieron circled in his earlier diagram. I don't know much about the railway, but more generally I think that there are a few options, including various isolating transformer types (anything you like, so long as it's not earthed both sides), or simply making sure that your protection settings are above likely leakage levels (or immune to DC and/or high frequencies). Dave

Kieron

This is clearly complex, otherwise we'd have an answer for you by now. It feels like you know exactly what's going on, but the rest of us are playing catch up, so please bear with us 

To move the debate forward, and help answer your question, I'm going to make some assumptions and then offer some suggestions. I'm happy to be shot down if it gets us closer to a solution 

I assume that there's a 3-phase 400V supply straight into an on-board AC/DC converter, where the centre point of the bridge is earthed. That earth connection provides a path for various stray currents to return to the AC source. This looks like the front end of a UPS or VSD, where I'm not aware that we get many issues with nuisance tripping, so I still question the need for an isolating transformer 

If you do go for an isolating transformer then, at the risk of stating the obvious, it should be dedicated to the flying lead to the tram. You then need to avoid danger from electric shock, which I think means automatic disconnection. I'd think very carefully about introducing any impedance into the neutral-to-earth connection, because it moves towards an IT system, and I don't think that the tram is class II insulated

I'd challenge the manufacturer on why they recommend so high a neutral resistance that it's effectively floating, and ask them what would happen if the supply neutral were solidly earthed

hope this helps

Dave

Hi Dave

Spot on

That is exactly where I am with this, unfortunately I can only go with what the manufacturer has recommended, where they have told us that there will be DC leakage and it might cause tripping of the supply, Id be more concerned about the DC leakage inhibiting the protective devices.

My thinking is that the best way forward is to provide an Isolation Tx, just for the shore supplies, which will isolate the DC leakage from the buildings LV network, this will have a neutral earth resistor at 2.4kOhms as per standard shore supply installs and then use DC immune RCDs on each shore supply to prevent the DC leakage inhibiting the protective device.

I probably could have worded my OP better by saying how do you design a supply to equipment from a secondary delta winding of a Tx to comply with BS 7671.

Thanks for the help is really appreciated

K

thanks, Kieron

Yes, standard solutions are always a good place to start. Unfortunately, I'm still a little confused, because I can't see how you can have a delta secondary and a neutral earth resistor. Unless you have a star secondary, you don't have a neutral point which you can then connect to earth

Dave

Kieron

Looking back at previous comments, I think you're looking at an isolating transformer with a delta winding towards the rest of the fixed installation and a star winding towards the tram. If you then connect the star point, i.e. the neutral, to earth, you then have a conventional five-wire system. Then, like many motor supplies, you can take three phases across to the tram through a suitably protected cable, and separately make sure that the body of the tram has a suitable connection to earth. I guess that the traction circuits use the connection between rail and track, but I don't think that it would be disproportionate to create an additional flying earth lead to the chassis

Dave 

Apologies for not being clear, if we were to propose the neutral earth resistor solution it would be a star secondary. Even so with this method the actual connection to the train is purely 3 phases, no earth, no neutral.

I was wondering if the manufacturer is dead set on a supply from a delta secondary, how would this look in terms of protection of the cable if it came into contact with exposed parts etc and compliance with BS7671. I'm assuming the train would be required to be class 2?

Thanks

K

I replied same time as you, the manufacturer requested a delta winding towards the train not the building supply.

K

thanks, Kieron

I agree with you, supplying the converter from a delta (or isolated star) would create an IT installation and require class II insulation. I think that Network Rail have got themselves into trouble over this for 650V signalling supplies

with a more conventional star secondary with a decent connection from neutral to earth, there's no problem with just taking three phases direct to the converter - many motor drives do the same. you will need to make sure that there is a good earth connection to the body of the tram, in the same way that we make sure that the metal shell of a motor is effectively connected to the earthing system. just like a motor, this could be a separate CPC, and doesn't have to run with the cable carrying the three phases

Dave

I'd think very carefully about introducing any impedance into the neutral-to-earth connection, because it moves towards an IT system, and I don't think that the tram is class II insulated

I don't quite follow that - I thought one of the advantages of an IT system was that you could have Class I equipment - and still have continuity of supply on first fault. If everything was Class II you wouldn't need ADS at all (on first or 2nd faults) whether IT or TN.

   - Andy.