What is the %Z of this grounding transformer given this nameplate data ?

[bdn2004]

Below is the picture of the nameplate.... as you can see Z0 = 97.14 ohms per phase. Or can this be figured out from this info ?
If so what is the calculation ?

 

[Gr8blu]

Z% = V(primary) in volt / I(primary) in amp * 100

 

[RRaghunath]

It seems the subject transformer is meant to be used as Grounding transformer only.
Since there is no load that is going to be connected on secondary side, %Z is not relevant and hence not marked on the name plate.
Zo and the expected ground fault current is relevant and has been marked.

 

[bdn2004]

It actually does have load on it ... Aux Power they call it. It's connected to a 3-phase switchboard.

I've tried to draw out a completed faulted circuit to ground downstream on Phase B. Do I have this right ?

The ratings on this transformer appear to be rated currents for normal operation.
29A continuous phase current = FLA of the transformer
87A continuous neutral current = Approximately 3 x FLA
300A for 10 seconds, max rating.

 

[dpc]

There's nothing connected to the delta winding, so no load on the transformer. It supplies ground fault current during ground faults.

 

[RRaghunath]

It actually does have load on it ... Aux Power they call it. It's connected to a 3-phase switchboard.

I've tried to draw out a completed faulted circuit to ground downstream on Phase B. Do I have this right ?

The ratings on this transformer appear to be rated currents for normal operation.
29A continuous phase current = FLA of the transformer
87A continuous neutral current = Approximately 3 x FLA
300A for 10 seconds, max rating.View attachment 14475

The %Z value is relevant as you mentioned that the secondary Delta of the Grounding transformer is connected to an auxiliary power supply busbars. It is the %Z along with the source impedance at 15kV bus that will decide what will be the three phase fault current magnitude at 480V busbars.
IEEE 57.12.26 recommended value for this size transformer is 5.75%. (I have access to only 1992 version)
Value specific to this transformer can be known only from the design / test data from the manufacturer.
Manufacturer is expected to stamp the %Z value after the factory test, on the name plate prior to dispatch of the transformer. In this case, I can see the same is left blank.
Transformers are expected to withstand a three phase short circuit at its secondary terminals for a duration of 2s minimum. Had the name plate indicated short time withstand current for the windings @ 2s, we could have estimated %Z from that parameter and the continuous current rating. The name plate has only 10s rating.

 

[bdn2004]

Thanks, from my sketch no, there is no load on it. But I'm assured by an old hand that's worked there many years there is a switchboard attached to it that feeds critical HVAC equipment. There's another 500 kVA transformer on site like this one performing the same function on the other incoming utility feed... but it doesn't say "GROUNDING TRANSFORMER" like this one does. It's just a wye-delta three phase transformer. And it's impedance, stamped on it is 4.92... So it doesn't look there is something special impedance wise with these transformers.