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.

 

Thanks

K

Parents
  • Thanks All for the replies.

    Forgive my lack of knowledge on the Electronics/DC side of things, looks like I have some reading to do.

    The purpose of the isolation Tx is to prevent any leakage current from coming onto the existing LV services at the building, typically we were looking to follow the network rail standard (This isnt a network rail project) which uses a delta/star Tx with a 2.4kOhm resistor at the star point to limit the earth fault current, this allowed for enough fault current to trip protective devices. We suggested this but the manufacturer of the converter recommended that a 100kOhm resistor is used or a Delta secondary winding ungrounded. We feel that using a 100kOhm resistor and monitoring the faults at such a low level would likely cause nuisance alarms or trips.

    Using a Delta secondary we feel we would need some sort of earth reference to protect the swa cabling if it were to come into contact with a line conductor, to do this surely we would need an earthing transformer in addition to the Iso Tx which is basically a star wound Tx? I think what I am asking is how would you supply a 63A TP Socket from a Delta Secondary winding of a Tx?

    Below is a diagram of the converter (Circled is the DC link grnd), the manufacturer has said the following "the infrastructure ground and the rail ground are on the same potential. The high frequency current will flow back to the midpoint of the DC-link (connection between sine filter and capacitor bank mid points)" which I read as what Dave mentioned above. The site exposed and extraneous conductive parts including rails are all bonded back to the MET so would be at the same potential.

    Appreciate the replies

    K

Reply
  • Thanks All for the replies.

    Forgive my lack of knowledge on the Electronics/DC side of things, looks like I have some reading to do.

    The purpose of the isolation Tx is to prevent any leakage current from coming onto the existing LV services at the building, typically we were looking to follow the network rail standard (This isnt a network rail project) which uses a delta/star Tx with a 2.4kOhm resistor at the star point to limit the earth fault current, this allowed for enough fault current to trip protective devices. We suggested this but the manufacturer of the converter recommended that a 100kOhm resistor is used or a Delta secondary winding ungrounded. We feel that using a 100kOhm resistor and monitoring the faults at such a low level would likely cause nuisance alarms or trips.

    Using a Delta secondary we feel we would need some sort of earth reference to protect the swa cabling if it were to come into contact with a line conductor, to do this surely we would need an earthing transformer in addition to the Iso Tx which is basically a star wound Tx? I think what I am asking is how would you supply a 63A TP Socket from a Delta Secondary winding of a Tx?

    Below is a diagram of the converter (Circled is the DC link grnd), the manufacturer has said the following "the infrastructure ground and the rail ground are on the same potential. The high frequency current will flow back to the midpoint of the DC-link (connection between sine filter and capacitor bank mid points)" which I read as what Dave mentioned above. The site exposed and extraneous conductive parts including rails are all bonded back to the MET so would be at the same potential.

    Appreciate the replies

    K

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