Zigzag TX rating - Westinghouse T&D vs IEEE C62.92 2

[tofulover]

Hi all,

I am trying to size the kVA rating of the zigzag TX. The parameters are:
11kV, Ineutral = 400A, 60sec rating.

Based on Westinghouse T&D book, the rating is calculated to be 11kV/sqrt(3) x 400A x factor of 0.104 = 264.2kVA.

Based on IEEE C62.92.4:2014, the rating is calculated to be 11kV/sqrt(3) x 400A / sqrt(3) x 7% = 102.7kVA.

Not sure if I am calculating this right and why there is such a big discrepancy? If this is incorrect please advise the right method?

Thanks in advance.

 

[VTer]

The kVA rating should be I_neutral continous/3*Vll*sqrt(3). If that is a short time withstand current than you need the Z0 value.

"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

 

[RRaghunath]

The IEEE calculation is right, I suppose.
Unlike the neutral grounding transformer which is connected between neutral terminal and ground, the zigzag earthing transformer is connected to the line terminals. So, this transformer is to be treated as three phase transformer.
Zigzag transformer is defined by its zero sequence impedance and needs to be specified while ordering the transformer. This decides the allowable maximum magnitude of earth fault current in 11kV system.

 

[tofulover]

Thanks VTer. If I calculate the value using the formula you mentioned, i.e. 400A/3 x 11kV x Sqrt(3) = 2540kVA. Surely that is too big a size?

Thanks RRaghunath. I also think the IEEE is a bit more correct - but I am not sure why there is such a big discrepancy with the Westinghouse T&D which is a reference most of us trusted?

 

[dpc]

I'd suggest talking to a specialty transformer manufacturer. In the USA, Uptegraff is still in business I think. As you probably know, a standard delta-wye transformer will work just fine as a grounding transformer and will be much simpler to source and replace. Any zig-zag transformer will likely be a one-off and there will be a lot of engineering and manufacturing costs to absorb unless the manufacturer has a close match they have already built.

 

[tofulover]

Thanks dpc.

 

[prc]

Both formulae are correct. It is explained in Annexure A of C57.92.4 -2014. The T&D book formula is for a star/delta transformer used as a grounding transformer. C57.92.4 formula for zig-zag connected transformer. (see A1&A2 equations of C 57.92.4)
A1- line to line voltage (kV) x neutral current x (1/root3) - star/delta transformer
A2- line to line voltage (kV) x neutral amps (1/3) - zig-zag transformer
The last digits - 0.104 and 7 % are for correcting continuous ratings into the short-time current ratings.
I find the T&D book also used the A2 equation (See Page 120) and not A1 as claimed.

 

[RRaghunath]

Ramachandran Sir, IEEE Std - you mean C62.92.4 I believe. It is mentioned as C57 in place of C62.

 

[prc]

Thank you Raghunath; I meant C62.92.4; sorry for the mistake.

 

[VTer]

Clarification to my previous post. Per C57.32-2015 table 14, for 1 min rating the continuous duty in percent of thermal current rating is 7%.
For you zig-zag xfmr the equivalent kVA should be ~103kVA.
Sorry about the confusion.

"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

 

[tofulover]

Thank you all. It is clear now. Appreciated the help.