Grounding transformer

[bikenut44]

Is there a reason why grounding transformer are wye-delta? If I had some auxiliary loads that I wanted to power from the secondary, could I make it wye (gnd) - wye (gnd)?

 

[davidbeach]

No. If you wanted to do that you'd need a wye-delta-wye. Without the delta it won't function as a grounding transformer.

 

[Marmite]

Or an interconnected star/star. Quite common where I'm from for supplying substation low voltage auxiliary services from the earthing transformer.
Regards
Marmite

 

[davidbeach]

What do define as an "interconnected star/star"? I've not run into that term.

 

[Marmite]

Interconnected star is another term for Zigzag. What I described is a zigzag with an auxiliary star(Y) winding. Vector group ZNyn1 or ZNyn11 for example. Attached for info is a GA drawing for a 33kV version.

Regards
Marmite

 

[magoo2]

I think the interconnected star/star is also called a zigzag transformer.

 

[waross]

A zig-zag may be a special order transformer.
Three distribution transformers rated for line to line voltage may be used for the wye:delta grounding bank.

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

 

[bikenut44]

A zig-zag was my first design preference, but the customer is arguing to establish the ground by ordering a wye-wye unit transformer (2000kva). I modeled the wye (gnd) - wye (gnd) in SKM and did not get single line to ground current. I tried drawing out the zero sequence diagram to understand why no ground fault current but I must be missing something. Can you explain why there is no ground fault current.

 

[davidbeach]

You'd have to have a zero sequence "source" on the secondary. The wye-wye will pass zero sequence currents from one side to the other, but won't source zero sequence current. Getting a wye-delta-wye may be easier than it appears. A wye-delta-wye can be built on a three legged core while a two winding wye-wye requires at least four legs. In a prior life, I found that it took a tight spec to get a dry type wye-wye of any size that wasn't actually a wye-delta-wye; that third winding can be less expensive to manufacturer and ship than would be the larger core necessary to get a true wye-wye.

 

[bikenut44]

So why does a wye-delta work? The delta can't pass the zero sequence current, so why does it provide a grounding path?

 

[davidbeach]

Look at the zero sequence diagrams for the various transformer configurations. The wye side of a delta-wye transformer will provide a complete circuit in a sequence diagram; the delta side won't. The wye-wye, if both wyes are grounded, will connect the zero sequence networks across the transformer but does not, in and of itself, complete the circuit; something else will have to do that.

 

[prc]

As per IEC (60076-1)and IEEE (C57.12.70&80) the correct term for the proposed connection is zigzag or Z connection. It consists of two halves in series,but the two halves in different phases (or limbs of core) . To get a grounding transformer only Z winding is sufficient. Only in case of auxiliary supply requirements, an additional star winding is required. Compared to a star winding, Z winding requires 1.15 times more copper and a star /delta unit will require double the quantity of copper in star winding. Hence the economy and advantage of Z connection for grounding transformers. During an LG fault,Z connection will allow fault current to flow in three phases together to return through neutral.