Sizing an NGR 3

[Coca1995]

I am using a 4.16 kV to 25 kV transformer at 5.6 MVA, 7% impedance Del, wye for my project. I want to size an NGR for this transformer. Thing is, 25 kV is too high for my standards and this is the first time i am working on it. My initial calculation for NGR gives 130A Earth fault current and the NGR value would be 111 ohm for 10 sec. However, i googled it so many times i am genuinely confused. Different posts show different ways of calculations and different answers. Please help

 

[Kiribanda]

On your 25kV system I believe you have 25kV cables and other equipment having some capacitance.
That means you have to estimate the TOTAL charging current and then multiply by 3.
Then select the NGR current slightly above this calculated value so that during a single phase to ground fault the resultant GF current
is the vector sum of NGR resistive current (IR) & total cap current (3ICO). Once you draw the vector diagram
you will find that it is approximately 1.414 time the calculated charging current.

 

[dpc]

High-resistance grounding or low-resistance grounding? If High resistance, the 111 A is almost certainly not right. Generally it will be 5 amps or less. If you are wanting to use high-resistance grounding on an extensive distribution system, that will be problematic.

 

[RRaghunath]

25kV is not for motor power supplies.
25kV is more of a distribution voltage. What is the reason for restricting ground fault current at 25kV level? Is it to limit the size of cable shield!
130A, 111 Ohm NGR seems correct as 130A is the rated current of the 25kV winding. This allows the neutral CT to be of same ratio as the phase CTs and thus allows Restricted Earth fault current protection to be effective.

 

[Coca1995]

great. thank you guys, this helped me a bit. I now get it. Thank you so much

 

[wcaseyharman]

Typically I’ve seen high resistance NGRs at that voltage paired with a neutral grounding transformer, unless it’s a low resistance system using a neutral reactor.