zigzag transformer? 1

[far243]

why do we use zigzag transformer to craete earthing for big plants?
for example:
4 frame 6 gtg 42mw
4 delta /star transformer 50mva
short circuit 23 ka
max load 100mw
bus rating 2500A 3 SECEND

 

[RalphChristie]

If you'll do an advanced search, you'll find a lot of info regarding zigzag transformers.

But briefly, a zigzag transformer creates an artificial ground on a delta system. (Thus, a zigzag transformer is connected on the delta side of a transformer) Sometimes a combined auxiliary and zigzag transformer are used, the zigzag transformer for a grounding point, and the auxiliary transformer as the substation supply. (Local supply for lights, plugs, battery charger, etc in the sub)

 

[dpc]

The main advantage of a zig-zag transformer over a delta-wye transformer is you get more bang for your buck (available ground fault current) in terms of transformer core size. But they are becoming pretty rare, at least in the US. A standard delta-wye transformer can be used just as effectively to ground a system, but it will end up being larger than the zig-zag for the same level of required short-circuit current.

The zig-zag unit is wound such that normal phase voltage sees a very high impedance, while zero sequence voltage sees a very low impedance.

Actually, there is nothing "artificial" about the ground established by a grounding transformer. It's real and it's got the iron in the transformer to prove it if you ground one of your phase conductors.

 

[RalphChristie]

dpc

What I actually meant with "artificial" is that fact that you have to add a piece of equipment (in this case the zigzag trsf) to obtain a grounding point/method. I totaly agree with your statement that the grounding point is real.

A zigzag transformer is also called an interconnected-star earthing transformer or a Neutral-earthing coupler (NEC) In South Africa there is still a lot in use and they are easily obtainable.

Regards
Ralph

 

[dpc]

Thanks, Ralph.

It's interesting that these transformers still seem to be popular everywhere except the US. I had to specifiy one for a client a few years back and all the major transformer suppliers here just laughed at me. They weren't interested. There are still a few small "custom" transformer shops such as Uptegraff that will make them.

I generally just recommend the use of a standard delta-wye transformer because they are so much easier to get and end up being cheaper.

 

[RRaghunath]

Ralph,

Auxiliary/ZIgzag transformers are widely used in Sultanate of Oman, where I worked for about 3years.

There the 132/33kV transformers are Yd connected with auxliaray / zigzag transformers providing the neutral point for 33kV system and also power supplies for the substation (precisely the way you explained).

Just to share.