zigzag not applied to HRG

[odlanor]

greenbook - IEEE Std 142-1991 estabilish on page 22 - 1.5.2
A zigzag transformer may be used for effective grounding, or an impedance can be................
........................................
This transformer is seldom employed for medium-voltage,
high-resistance grounding.

Does anyone knows why this restriction?

 

[davidbeach]

Not a restriction, but in the US a zigzag transformer is a very rare beast. Elsewhere in the world they are much more common.

 

[waross]

Three distribution transformers in wye/delta work well and are much more readily available in North America. The transformers must be rated for line to line voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Not sure what the question is. But if you use an HRG, you would not need a zigzag transformer.

Zigzag grounding can be used to get solid (effective) grounding, which is not the purpose of an HRG. HRG is used to restrict the fault current.





Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Also zig-zag xfmr is generally a "fix" to get a neutral (4-wire, grounded system) out of an ungrounded delta system, which are not very common in the USA. Most MV systems are wye connected already and "effectively" grounded or HRG grounded, depending upon the application.

Rafiq Bulsara

http://www.srengineersct.com

 

[odlanor]

This is a continuation of Thread238-284062.
We intend to ground 13.8kV delta system with resistor of 5A in order the first ground fault does not require process equipment to be shut down.
We intend to trip one hour after first ground fault ocorrence.
Tradicionally in Brazil , zigzag transformer is used in subtransmission delta system in order to ground delta and guarantee singlephase fault current be less than threephase fault.

 

[dpc]

I don't understand what your question is.

David Castor

http://www.cvoes.com

 

[waross]

In North America, you may see such a situation handled with three 13,800V primary distribution transformers. The secondary voltage is not important.
A grounding impedance may be inserted either between the wye point and ground or the impedance may be inserted in the delta.
The transformers need only be sized for the impedance limited ground fault current.
The advantage over a zig-zag is that the transformers are not special order and most utilities using delta distribution will have suitable transformers in stock both for initial installation and for replacement.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[odlanor]

as davidbeach said:
Not a restriction, but in the US a zigzag transformer is a very rare beast.
I guess one zig zag is less expensive and more easy installation than Three distribution transformers in wye/delta .

 

[dpc]

I guess one zig zag is less expensive and more easy installation than Three distribution transformers in wye/delta .

In theory that should be true, but in reality, the zigzag will be more costly because it will be custom built.

Also, you don't need three single-phase transformers - a three-phase unit will also work.

David Castor

http://www.cvoes.com

 

[prc]

waross,request clarification.When you say star/delta transformers delta on 13.8 kv side and secondary star is earthed through resistor.When you say impedance in delta, it is inside delta.My understanding correct ?

 

[waross]

Primary star point is earthed. The current limiting resistor may be installed in the primary earthing conductor or inside the secondary delta.
The secondary delta is not earthed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Thank you waross.In India( I think in UK also) Distribution transformers are usually delta /star connected. In US is it the reverse? Then the LV supply will remain unearthed system?

 

[rbulsara]

prc, you are confusing grounding transformer application with a regular distribution or power transformer application.

Rafiq Bulsara

http://www.srengineersct.com

 

[prc]

rbulsara, I understood from the above mails,in US they are using regular transformers to get neutral to the system.

 

[rbulsara]

prc,
Regular two winding design transformer,connected as a grounding transformer. It is still a grounding transformer by application, just not a zigzag one. This unit would not supply load on the secondary.

See section 1.5.3 in the attached link:

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

If it were to be used as distribution transformer to serve loads, it would typically be a delta/wye as you describe.

Rafiq Bulsara

http://www.srengineersct.com