Using Wye-Delta ?Transformer for grounding transformer on 13.2kV System

[rockman7892]

I'm looking at a project that has a distribution bus on the secondary of a 13.2kV delta transformer which (wye primary winding has to do with generator and grid connections) has a zig-zag transformer specified to be connected to the distribution bus to provide a ground fault sources on the delta system. The specified zig-zag transformer is specified to be a 3.2MVA transformer with a grounding resistor connected to the neutral.

It is proving difficult to find such a transformer however I believe that a standard wye-delta transformer can be connected with the wye side connected to the system bus in order to achieve the same result? From what I have read it seems to be that these wye-delta grounding transformers are typically single-phase transformers connected together for a 3-phase transformer? Can a standard 3-Phase distribution type transformer be used to provide this grounding source?

From what I've seen the kVA rating of the transformer is typically sized in order to support the ground fault current that will be dictated by the resistor size. After calculating the transformer size based on fault current you can then apply multiplier to kVA rating based on C57.92-06.200 for clearing faults in a specified period of time (in order to reduce kVA size).

Is this as straight forward as calculating a standard 3-Phase distribution transformer size or are there certain reasons why the ZZ transformer is a better choice?

I cant seem to find what the specified resistor size was for zig-zag transformer but am assuming it is intended to be a LRG system (I'm getting more info)

 

[waross]

The most important parameter is that the individual transformers must be rated for line to line voltage.
In the event of a ground fault the transformer may see up to line to line voltage.
The delta voltage is unimportant as long as three identical transformers are used.
A lower resistance grid type resistor may be installed "in the delta" rather than at the wye point.
Whichever way you go, the power dissipated in the resistor will be the same.

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

 

[xnuke]

The wye-delta grounding transformer will have a larger (by √3) VA rating than the zig-zag grounding transformer, so it will probably cost more. Grounding transformers may have different test requirements than distribution transformers - see the standards I reference below.

By the way, since you said "wye primary winding has to do with generator and grid connections," you should know that having a wye-connected primary tied to a generator is usually a bad thing - the SLG fault current will usually exceed the three-phase fault current rating of the generator, and you can't reduce it using resistance grounding of the generator because that resistance ends up in parallel with the low zero-sequence impedance of the transformer. The first SLG fault you get and you'll likely destroy your generator either mechanically or electrically. GSU transformers are typically delta on the primary (generator) side for just this reason - they have an infinite zero sequence impedance, so generator neutral grounding resistance is effective in reducing ground fault currents.

You might want to review IEEE Std 32 and the relevant sections of IEEE Std C62.92.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

I haven't seen a zig-zag transformer since my school days when our instructor took us on a short field trip to see what one looked like.
I expect that they are special order with a long lead time.
About 20 years ago, I had an application for a zig-zag in the third world. I couldn't find a sales rep who knew what a zig-zag transformer was.
I found another solution.
Compare that to three "Off the Shelf" distribution transformers.
And, if it fails, do you want to repeat the long lead time?
BTW Comparing the VA ratings of a distribution transformer with a zig-zag transformer for a cost comparison may not be comparing oranges to oranges.
I agree with the delta side of the transformer facing the generator.
All three phases will contribute to a fault, however two phases will be at a displaced phase angle. This will cause rotor heating, but not as much as the same fault with a wye/wye transformer.

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

 

[rockman7892]

waross

Are standard 3-Phase distribution transformers made with L-L rated voltage windings or do you have to specifically use single phase transformers for this reason?

When a L-G fault occurs the L-N voltage across winding will be L-L voltage as you stated. Would this also not be the case when a standard wye secondary of D-Y distribution transformer is impedance grounded, or does the delta primary help hold the respective L-N voltages on the transformers secondary under L-G fault condition on the secondary?

Does grounding transformer impedance matter much if the NGR will be the predominant impedance in the zero sequence circuit?

xnuke - Does the wye-delta have a higher kVA rating (compared to zig-zag) due to the fact the L-L voltage is used in the kVA calculation with the "wye" winding as opposed to L-N?

 

[davidbeach]

A two bushing transformer is made for line-line connection while a one bushing transformer is made for line-neutral/ground connection. You can get either.

 

[rockman7892]

davidbeach

Based on your answer it sounds like single phase transformers need to be used to make-up this wye-delta transformer. I'm not familiar with a 3-Phasse distribution transformer that would have other than 3 phase bushings plus neutral bushing and thus only have L-N rated voltage across the primary wye winding.

Can you perphas purchase a 3-Phase transformer that would be rated for 13.2kV L-N on the primary wye winding?

I'm just trying to understand if it is possible to buy 3-phase transformer or if you must purchase single phase units to construct 3-phase unit.

 

[davidbeach]

As far as I know you can buy a three-phase transformer with whatever configuration you want. Some configurations may be more of a special run than others, but wye or delta is just a spec item.

 

[waross]

You could use a 25,000 Volt three phase transformer.
The transformer KVA must support the current passing through the NGR.
Base your KVA on the NGR current.

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

 

[prc]

1) The latest standards on Zig-zag grounding transformers - IEEE C57.32-2015 (replaced old std 32) IEC 60076-6-2007 Reactors Part 10 (replaced old 289-1988)
2)There are two types of grounding transformers - (a) Three phase units to get a neutral on unearthed 3 phase system ( it can be single winding zig-zag or normal Ynd transformer) (b) Neutral grounding transformer (single phase two winding transformer to reduce the rating of NGR by connecting the same on secondary side of transformer
3) Apart from US market, grounding transformers are quite common in other places and are readily available. Advantages of zig-zag over conventional Ynd unit- it requires nearly 1.732 times less copper for the same current density and hence much cheaper. Normally grounding transformers require very less continuous rating and hence conventional units will become superfluous on rating basis.
4) Only R will decide the neutral current and transformer impedance will not affect the current.
5) Apart from above standards, Beeman's classic book on Industrial Power system has many more details on grounding transformers.

 

[davidbeach]

Zig-Zag seem to be common in the IEC world and rare in the ANSI/IEEE world.

 

[xnuke]

I helped specify some zig-zags a couple of years ago when a client wanted to use some 34.5/12.47 kV Δ/Y step-down distribution transformers in reverse temporarily and needed the 34.5 kV side effectively grounded. They were designed with a specific zero-sequence impedance to limit ground fault current and still provide an appropriate coefficient of grounding, and had a 10-second rating. I just did the design calcs, but the PM told me they priced out both the custom zig-zags and an equivalent off-the-shelf she-delta with a neutral grounding resistor and the zig-zags cost less so they were selected. I was told they worked fine.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dpc]

If this is used in the US, I would go with a three-phase wye-delta transformer over a zig-zag. The major transformer suppliers have no interest in providing a zig-zag transformer and replacement time will be long. These were exceedingly rare in the US for a long time but are slightly more common now due to their application in wind farm substations. There is no downside to the wye-delta other than a slightly higher cost (theoretically).

Recommend Industrial Power System Grounding Design Handbook by Dunki-Jacobs, et al. Although may be out of print.

 

[rockman7892]

Per Power System Grounding design Handbook the following calculation would be used for this application assuming we wanted to limit ground fault to 400A:

Transformer kVA = (V_L-L)(Total Gnd fault/3)(Factor of 3 to account for 3-phase transformer) = (13.2V)(133A)(3) = 5.3MVA

Per C57.92-06.200 if gnd fault is cleared in less than 2 seconds than derating of the transformer by a factor of 11.3 is allowed which will give a value of 5.3MVA / 11.3 = 470kVA or 500kVA. Does this seem correct for this application? This seems very small compared to the specified 3.2MVA zig-zag that was originally specified. Perhaps the derating factor is not correctly applied?