High Resistance Ground conversion

[ccov]

IQuestion converting an ungrounded delta system to a HRG setup, understanding the advantages has to offer:
Existing System: 23kV delta / 480V delta (ungrounded) 2000KVA transformer - 3 wire secondary bus - 3 wire 2500A 600V switchboard with GF indicating lights. All motor/xfmr loads.
1. Starting with the transformer. New delta/wye transformer. How you size the grounding resistor?
2. Will the HRG neutral need carried to the switchboard? IOW, would a bus and switchboard replacement be required, or can the existing 3 wire setup continued to be used?
Thanks for a great site and the wealth of knowledge shared.
I also have an unrelated question:
On an ungrounded wye secondary, during a line to ground fault, what happens to the voltage / how does it compare to a line fault to ground on an ungrounded delta system?
Thanks...

 

[davidbeach]

1. Resistor sized to provide the desired current at 277V across the resistor. It will need to be sized to carry that current continuously.

2. There is no HRG "neutral" as a neutral. The center point of the wye needs to be carried to the location of the resistor and no where else.

3. No difference.

 

[dpc]

You mention a new delta-wye transformer - You don't actually need a new 2000 kVA transformer - a small grounding transformer can be added to the existing 480 V system. This could be delta-wye, or a zig-zag connection.

http://www.i-gard.com/pdf/application guides/Conversion Guide.pdf

Resistor should be sized so that it will pass ground current at least equal to the capacitive charging current to limit the risk of overvoltage. For a 480 V system, this will PROBABLY be an amp or less.

 

[burnt2x]

ccov,

1. Start by calculating the capacitive charging of your 480V system. The resistor should be such that the ground current it passes be at least equal or greater than the charging current.

If you don't have the charging current data, use 1A/ 2000kVA of your source as a rule of thumb, i.e. your charging current will be 1A (2000kVA trafo).

2. The resistance can be calculated by using the voltage-to-neutral (277V) divided by the calculated charging current.

Resistor = 277V / 1 V = 277 ohms, 250 ohms could the job (resistor let-through = 277V / 250 ohms = 1.08 amps which is greater than the expected charging current)

hope this helps.

zigzag grounding will therfore be 1A rated (831 VA), which can be