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Multiple Low R Grounding, Different NGR Ratings

Multiple Low R Grounding, Different NGR Ratings

Multiple Low R Grounding, Different NGR Ratings

(OP)
Hi all,

I would like to request for your guidance. If we have a system as shown in "1" in the attached basic single line diagram.

1. Can we have mixed NGR ratings, say 100 A each on the 50 MVA transformer NGRs and 50 A each on the GSU transformer NGRs. (total limitation up to 300A).

2. Can we possibly re-arrange the system such that 2 identical transformers (and maybe a 3rd identical transformer) will share a common NGR and the same philosphy will apply to the 2 GTGs? The GTG GSU transformers are physically close to each other. The main 50 MVA transformers are physically close to each other.

3. Other options we are thinking of is a) make the main tranformer neutral floating and provide zig-zag transformer to both buses and b) make the GSU transformer 69 kV side delta.

The reason to all of the above is to limit the ground fault current to 300A so as not to exceed some GPR requirements due to possible transfered-potential issues. Any comments and suggestions on the above arrangements are highly appreciated. At the moment we are awaiting for a third party contractor to assess overall grounding design.

Thank you for all your usual support.

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