X/R ratio of a ZigZag Txfo

[tem1234]

Hi,

I am wondering this question. Is the X/R ratio of a ZigZag transformer will be the same as a standard transformer?

I have the figure in IEEE that show standard X/R ratio for Txfo depending on is MVA, but i suspect that a ZigZag have a X/R ratio greater than this for the zero sequence impedance.

The ZigZag is approx. 4 MVA.

Thanks

 

[saladhawks]

Impedance & X/R ratios for 3-winding transformers should be obtained from transformer nameplate and manf. test reports. There is quite a bit of variability of data in 3-winding transformers due to the specific winding configuration and the capacity of each winding.

Why do you need this information?

 

[tem1234]

Hi Saladhawks

This is for Short circuit current calculation. This isn't a big deal, it was more for my curiosity. The impedance is know, just the X/R ratio that i was wondering. It's not a 3 winding txfo, it's a grounding txfo.

The transformer isn't built for now, so it's difficult to have real value, but I think i have never seen a X/R ratio on a txfo nameplate so far. It's not a standard requirement. Most of the time, we use standard X/R ratio for transformer in SC calculation.

thanks

 

[dpc]

I guess I would expect it to be similar to a normal two-winding transformer of equivalent physical size and voltage.

 

[RRaghunath]

tem1234,
Zigzag transformer name plate mentions Zero sequence impedance of the zigzag winding. This controls the earthfault current in the system where-in the zigzag transformer is connected. The earth fault current would be 3*((Vline-line/sqrt3)/Xo).
If you have a auxiliary winding (in the same zigzag transformer) to supply loads, then, the zero sequence impedance of the transformer for faults in the auxiliary power supply system is quite high (typically 50%) restricting the earth fault current magnitudes severely.

IEEE C62.92 discusses the subject in detail.