Ground Resistor Calculations

[sky2020]

I am designing a low-resistance grounded system for a double-ended substation. How to pick the grounding resistor for 50MVA D/Y 120KV-13.2KV transformer 7.74% impedance?

My calculations:

50*1000/sqrt(3)/13.2 = 2200A, pick 10% = 220A; R = 13.2*1000/sqrt(3)/220 = 34 ohm. This gives 440A from the two transformers.

How does the charging current come into play? How does the CT ratio come into play?

 

[topgone]

I am designing a low-resistance grounded system for a double-ended substation. How to pick the grounding resistor for 50MVA D/Y 120KV-13.2KV transformer 7.74% impedance?

My calculations:

50*1000/sqrt(3)/13.2 = 2200A, pick 10% = 220A; R = 13.2*1000/sqrt(3)/220 = 34 ohm. This gives 440A from the two transformers.

How does the charging current come into play? How does the CT ratio come into play?

Remember that limiting the available bolted fault current in the neutral by inserting a resistor will also determine the value of transient overvoltage during faults. Choose an NGR allowed current slightly higher than the total charging current possible in your system.