NER on delta-star transformer

[Chap79]

I'm currently designing an NER for a delta star transformer. It is a 3500kVA, 11kV/6.6kV transformer (Ifault = 308A). I've calculated the required values. It however varies between 30ohm (127A) and 127ohm (30A). My question is do I go for the lower current or the higher 1? I know that a low value of current will be easier to find the resistor? Secondly is there something about a 10% to the fault current that I must consider? Thanks

 

[pwrtran]

The full load current @ 6.6 kV side is 308A, not the fault current.
Assume a typical Z=5%, then Isc=308*100/5=6160A at the secondary bushings. If taking the source impedance and secondary bus impedance into account the actual fault level is lower than above estimated.

Not knowing your application one cannot say you must need a NGR. However, if you have to have a NGR then you must have a fault detection system to indicate and clear the L-G fault within reasonable time. Another thing is you cannot have any single phase load without installing a isolation transformer to create its own reference ground.

 

[Chap79]

Yes you are correct it is the full load current. I must design the NER for the neutral of the secondary side(star) I want to limit the current to a value of 10% of the full load current. This will be to prevent nuisance trips (unbalancing? any other reason) Is this statement correct? Limit the current to 40A and work out the resistor from there?

 

[pwrtran]

This will be to prevent nuisance trips (unbalancing? any other reason) Is this statement correct?

Not prevent the nuisance trip due to unbalancing, it is to reduce the lose time due to phase-to-ground fault, which means you can have the feeder still live during a limited ground fault for certain time that allows you to do the troubleshooting without shutting down the entire production line.

To limit the line-to-ground fault current to 40A, which is probably less than your load current, your overcurent protection will not work. You must have overvoltage 59 element.

 

[Marmite]

The normal practice where I'm from is to choose the Neutral resistance to limit the earth fault current to the full load current of the transformer.
Regards
Marmite

 

[Chap79]

@ marmite According to standards step-and-touch potentials must be lower than 50V which makes this practice impossible. I dont know if this practice is only applicable in our country? I want to set my value to 10% of the full load current, but need advise on the pro's and the con's?