Setting Earth Fault Relay

Hello : I understand that you have a grounding transfo connected to a 20KV system, presumably 3phase 3wire delta ? What is the KVA rating of your upstream source?How did you calculate your 12.5amp line-to-ground fault?

The connection of earthing transformer is zig-zag not wye-delta.Also 12.5 A isn't line-ground fault, it is primary setting of earth fault relay in base of system earth fault analyses.

REGARDS

MSJ

Unfortunately, nowadays with our world information transmission system (internet), we usually neglect the basic conceptions and deep thinking about new problems.
With more deep thinking, we can find that the mentioned evident equation (S = 1.73 VI) is not correct for zig-zag earthing transformers.
Indeed, each leg of a grounding transformer carries one-third of the neutral current and has line-to-neutral voltage. So in a grounded wye - delta transformer, the total power rating including all three phases is the neutral current times
The line-to-ground voltage:
S= 1.73 VI
But a zig-zag transformer is more efficient than a grounded wye - delta transformer.
In a zig-zag, each winding has less than the line-to-ground voltage,
by a factor of 1.73, so the bank may be rated lower:
S = 1.73/1.7.3 VI = VI
Therefore, the continuous rating current of E/T is 5 A and rating neutral current is 15A, and we can choose 12.5A as correct earth fault primary current setting.


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