Zigzag transformer - Zero sequence impedance

[RRaghunath]

Zigzag transformer is supposed to offer low impedance for zero sequence currents. Any typical value??

IEEE 62.92.4 has a typical calculation for sizing the zigzag transformer and arrives at the required transformer per phase zero sequence impedance value as 5ohms.

Thanks in anticipation.

 

[Bahram7]

It all depends on the desired level of L-G fault currents. I've seen 6.75 and 7.5 Ohms at 25 kV.

Bahram7

http://www.Simtech-Intl.com

 

[RRaghunath]

Thanks Bahram 7 for the response.

The 33kV system I am working is supposed to be solidly grounded. This means that the earth fault current magnitudes shall be 95% of the phase fault currents (very effectively earthed - IEEE 62.92.1) and it means the zero sequence impedance (phase-earth) needs to be lower than 5 ohms when the 132/33kV transformer is 50MVA with 12.5% impedance.

Do you think the same is achievable!

Thanks once again for sharing.

 

[jghrist]

I am studying a 34.5 kV system that has a 6859 kVA, 33 kV zig-zag grounding transformer with an impedance of 93.3% (148 ohms). The system is not effectively grounded and the impedance was probably specifically designed to limit fault current.

 

[Kiribanda]

rrghunath,

It sounds like you are working with IEC voltages. You can get typical zero zequence values for various transformer connections in Table 1 of IEC 60076-8 "Application Guide for Transformers".

Hope this helps.

Kiri

 

[dpc]

That should be achievable. They are typically specified based on desired fault current and/or ohms/phase and also fault duration. I would talk with some transformer suppliers.

You can also use a standard delta-wye transformer as a grounding transformer.

 

[RRaghunath]

Thanks all for the responses.

I would check in the IEC for typical values as suggested by Kiribanda. Thanks.

dpc, the standard of my client specifies 132/33kV, YNd1 grid transformer with Zig zag earthing transformer. I do not have much experience with sizing / specifying these transformers. Thanks.

 

[RRaghunath]

Kiribanda,

I reviewed the Table 1 in IEC 60076-8. The table doesn't include zero sequence impedance values for Zigzag transformers.

 

[Rodmcm]

Sorry guys but maybe I am a little confused here. The purpose of a zig zag transformer is to provide an earth fault path. By design it will have an impedance but you can adjust this impedance, the easiest way by the introduction of resistors in the earth leg of the zig zag. You generally specify the requirment.It seems to me that you should be looking at why you need earth fault current, options for obtaining it, and how much you need. For a system that requires >95% earth current does not need an earthing transformer, what it needs is a good earth mat.

 

[dpc]

If the system is otherwise ungrounded, you will certainly need a grounding transformer of some type if you want to see much ground fault current. This is the main function of the grounding transformer - to provide a reliable ground source to a system that is otherwise ungrounded, remotely grounded, or poorly grounded.