Neutral Grounding (Zig-Zag) Transformer

[ProtectionEngineer]

Hi,

Can anyone help me to calculate the per-unit value for this neutral grounding (Zig-Zag) transformer?
Nameplate data: Rated 800 kVA, Zeo sequence Impedance 85 ohms per phase, H0 Current: 1000A for 10Sec, 30A Continuous.
Voltage 34500/19919V.

I need to know the per-unit value so that I can caluculate the phase-ground fault level.

Thanks.

 

[dpc]

The 85 ohm figure would seem to imply an associated grounding resistor along with the transformer. Does this nameplate data include the resistor?

 

[ProtectionEngineer]

There is no grounding resistor attached to the zig-zag transformer.

Thanks.

 

[dpc]

ProtectionEngineer,

Let me do some checking. I thought I knew the answer, but I've found conflicting information.

Basically, if you know the impedance in ohms, you just convert to pu using standard pu conversion equation.

But I'm a little unclear if the 85 ohms is each phase individually to ground, in which case the effective Zo is one-third of that value, or if that is the equivalent reactance through all three phases. And I can't get either approach to match up with the 1000A rating you found on the nameplate.

Maybe someone else has the answer, but I will do a little more checking.

 

[Rodmcm]

The rating gives you the answer, the zero seq impedance is 85 ohms per phase. The other ratings are purely withstand ratings ie the designers sizing for the application.

 

[dpc]

As best I can determine, the 85 ohms is a per phase value, so the Zo in your equivalent circuit is actually 1/3 of this, converted to the appropriate per unit quantities.

But this is still something of a guess. I'm still trying to get a more definitive answer.

No one buys these things anymore, so there isn't much data available.

 

[Guzzy89]

ProtectionEngineer

Here is the Z p.u. calculation(based on the transformer KVA rating):

Z p.u. = Z actual / Z base

Z actual = 85 Ohms
Z base = ((19919)^2)/(800 x 10^3)
Z p.u. = 85 / 500
Z p.u. = .170 = (Zero Sequence Impedance p.u.)

 

[Bung]

You might need to reconsider an almost entirely reactive impedance for the grounding. It is generally considered a bit of a no-no to have reactance grounded systems, due to the problems with power frequency overvoltages under arcing earth fault conditions. I don't recall all the details, but the mining industry in South Africa had some problems with it in the 1970s. There were a few papers in the SAIEE journal - people like a Dr Pretorius at the CSIR were involved with it back then, I think. Eastern Natal Region of Eskom used an NEC / NER combination at 11kV to keep earth fault levels to 300A, and the R componenet was to ensure that the system was not reactively earthed.

Bung
Life is non-linear...

 

[dpc]

I was wrong - you don't need to divide the nameplate impedance by three - just convert the 85 ohms to per unit for Zo.

I checked this with an expert and although I don't quite understand yet why this is, I believe this to be the case.

Sorry for the confusion - but thanks for the question - I learned something.

 

[Rodmcm]

The zero phase current splits into each of the phases by 1/3rd per phase, hence as pu values for currents are phase values then the zero pu impedance is calculated directly from the phase impedance value