grounding old distribution system

[kookypedia]

Guys,
I have to change the old gear in the following system and design the grounding system for that.
Problem is that the old system is not grounded and I can not touch the old T1.
Any idea for grounding the new system? Adding new 2MVA transformer looks an expensive option!

 

[rcw retired EE]

High resistance grounding with a zig-zag or wye-delta grounding transformer. GE, Eaton, Siemens all offer them. I-Gard has a good system. Check Post Glover grounding resistor web site also for good background. We installed many of these in an older refinery with only minimal shutdown.

If you can't use high resistance, use a zig-zag grounding transformer.

 

[waross]

How about three 50 or 75 KVA distribution transformers with 13.8kV primaries.
Connect them in wye:delta. The secondary voltage is relatively unimportant as long as the three are the same.
Ground the wye point through a 2760 Ohm resistor and you will have a 5 Amp High resistance grounded system.
You may also ground the wye point directly and insert a suitable resistor in the delta.
5 Amps at 13.8 kV = 69 KVA
You could probably accept that overload on 50 KVA transformers. The overload will only be present during ground faults.
Another option is corner grounding.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[kookypedia]

It looks like if I do a low resistant zig-zag grounding, the highest amp that will flow to the ground is 400A.
Is this mean, I should design my grounding current for only 400A?!

 

[waross]

Sorry. I calculated based on grounding the 13.8 kV side.
For 480Volts,
400 Amps x 480 Volts = 192 KVA grounding transformers. These will probably accept a substantial overload for short time duty. With this in mind, you may select smaller than 200 KVA grounding transformer(s).
5 Amps x 480 Volts = 2.4 KVA
A zig=zag transformer may be a special order.
You may be able to source 13.8 Volt distribution transformers in stock somewhere. They will probably be cheaper than a zig zag transformer and more readily available should a transformer need to be replaced in the future.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[kookypedia]

Bill,
The delta/delta is for utility (we can't touch it). We also, don't want to pay for another main transformer (2MVA) just for grounding purpose.
What do you mean by grounding transformer? Do you mean Zig-Zag?

 

[VTer]

Did you talk to the utility? Corner grounded delta is an option.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[kookypedia]

This is a 40 years old system and I'm not sure if utility is willing to do that...
Sizing ground grid for 5A of zig-zag doesn't look right because, at least we have lightning to consider.
Do you have any idea how to size the ground grid if I only have a HRG zig-zag?

 

[wroggent]

If it's a 40 year old transformer maybe they would be willing to replace it on their dime?

 

[kookypedia]

How we really should talk with the utility? If we are not paying, are they still responsible for grounding their Transformer?

 

[VTer]

Not sure how you can change the system that includes utility equipment without talking to them. You should really discuss your plans with them and ask for guidance and see what they say.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla