Star/Delta Isolation Tx For shore supplies

Hi All,

First post so take it easy!

We have a requirement to design an installation using a star/delta isolation transformer to feed shore supply sockets to power and charge new tram/trains (Believe first of their kind in the UK). I am trying to get an understanding of the implications, and what we need to consider in the design as this Tx will be installed within the building supplied from the buildings existing LV system. The trains on board converter has a DC link to ground which could cause earth leakage hence the request for a Star Delta Tx to prevent tripping of the buildings LV supply.  

From my understanding we will need to install insulation monitoring for first fault conditions and on 2nd fault condition will isolate the circuit (An IT System). The train does not require a neutral or CPC (But will we need a CPC to comply with BS7671? If so how does this connect to a delta winding?).  But where I am getting confused is the earthing side of things. Everything within the building including the rails the trains sits on, is bonded back to the building MET. Is there a risk of different potentials within the area? or because they are bonded back to the building MET they will remain at the same potential as this system supplies the Isolation Tx? I may be over thinking this but have never come across this before so eager to educate myself.




  • please forgive a daft question here. all stray currents must return to their source,

    Depends on the nature of  the "source". If it is galvanically isolated from its upstream supply. and has its own "N-PE" link or equivalent, then yes stray currents should return there. But if it's more akin to just a simple bridge rectifier supplied by L & N then the return path would have to involve the N-PE link on the upstream a.c. side.

        - Andy.

  • thanks, Andy. fair point - I guess we need to look a little closer at the details of the installation, but I still suspect that any decent AC/DC converter will provide sufficient isolation to avoid issues of tripping the upstream AC


  • I'd not be too sure - I have seen VSD designs where the 3 phases hit 3 diodes up to make a DC bus " B+" and 3 diodes down to a "B-", and there are loads  with an earthed centre tap between B+ and earth, B- and earth., used to create the zero to synthesize the 3 phases coming out to the motor windings by gating pulses from B+ and B- in the right moment.

    Beyond a certain  power level the weight of a 50Hz transformer drives some seemingly odd design decisions, in that it is easier not to isolate but to let things flap about instead. This is part of the problems with car chargers and EMC - a 1:1 transformer at 50Hz is not a practical object for the boot of a small car at the power levels needed for fast charge..There should be room in a train, but I'm not sure if it would be designed that way or not.


  • thanks, gents. I know it's the last resort of the desperate engineer, but do we need to get the drawings out? Dave

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