ZNyn transformer zero sequence impedance

[PrebenJakobsen]

Hi experts
I am working on a windfarm substation where the interfield voltage is 33kV. The WTG transformers and the export transformer is delta connected on the 33kV side.
On the substation we want to place a grounding transformer which is also used for 400V auxillary supply.
On 33kV side it is Z connected and grounded with a 19 ohm NER, and on 400V side it is Y connected and directly grounded.

Now we are simulating this transformer in different programs and gets very different results for the line to ground fault. My question is:

* Will the transformer provide a low impedance path for ground faults on 33kV side?
* Will the transformer provide a low impedance path for ground faults on 400V side?
*Will it be possible to transfer power to a 1phase - 230V load?

Best Regards
Preben Jakobsen

 

[veritas]

Hi Preben

How about a diagram please showing all the detail?

Thanks.

 

[PrebenJakobsen]

A quick sketch
Regards
Preben

 

[dpc]

If you put 19 ohms between neutral and ground, I don't think it will provide a low impedance path for 33 kV ground faults.

I think if you need a grounding transformer, put in a dedicated grounding transformer with a completely separate transformer to get your 400 V station service voltage. Combining these two functions into one special transformer is not going to save you any money in the long run.

To get 230 V and 400 V, I'd put in a three-phase station service transformer. Or a single-phase step-down transformer.



David Castor

http://www.cvoes.com

 

[PrebenJakobsen]

Hi dpc
The 19ohm NER will limit the current, but my concern is the zero sequence impedance of the transformer.
The theory says it will constitute a very large impedance if the amp-turns on one side is not balanced by amp-tuns on the other side.
We have calculated the current in four diffrent programs with different results.
Space on an offshore platform is very expensive and i would like to use one transformer.
Regards
Preben

 

[veritas]

Hi Preben

To achieve what you want to, you need a star-delta with the star on the 33kV side and the delta being the LV. You are then welcome to put in a NER in the 33kV neutral. Modelling is tricky. With a star winding on the LV side (earthed or not makes no difference to HV EF's) you are not going to achieve the ampere-turns balance you desire as you correctly pointed out.

The tank-delta effect now comes into play, where the tank of the trfr effectively acts as a " delta tertiary". In this case the zps for 33kV faults impedance can be anything from 5 to 20 times the pps impedance (all depending on trfr construction). But this is undesirable and can lead to nasty consequences which I have seen.

I have done star-delta trfrs used as grounding trfrs in SKM's Powertools for Windows. If one of your packages is this one, I can show you how to model it.

The delta winding can then supply your LV load but you will need a voltage displacement function to detect EF's as your delta has no natural neutral to earth. You may also want to consider earthing one corner of your delta.

Hope this helps.

Kind regards.

 

[dpc]

Ignoring exciting current: Current can not flow in one winding unless it can flow in another winding. I'd suggest you discuss grounding transformer application with qualified transformer manufacturer and make sure you have accurately modeled the winding configuration and equivalent circuit of the transformer you think you want. The software is only as accurate as the models they use.

If you have access to a Westinghouse (or ABB) T&D book, take a look at Table 7 in the Appendix - specifically Equivalent Circuit A-10. I believe the transformer Z0 will be the same as the impedance between the two zig-zag windings.

David Castor

http://www.cvoes.com

 

[prc]

Znyn transformer will work and I have seen this applied in many parts of the world.

When a LG fault occurs in the unearthed 33 KV system,fault current passes through the zig and zag windings of the Zn winding.These will be in opposite directions in a core limb and hence ampere turns will get balanced.( see J&P Transformer Book ed.12 Chater6 ) So zerosequence impedance is the impedance between zig and zag of the windings.Transformer manufacturer can furnish this value.

 

[PrebenJakobsen]

Hi prc
As I understand the Z connection it is inherent self balanced, and will provide a ground fault path on 33kV.

On the 400V Y connection Z0 will be infinite according attached figure from an ABB hand book.

What about 1-phase power transfer in normal operation?
Three-limb transformers with Yyn coupling can be loaded app. 10% on the neutral point, but i can't find any data on a ZNyn transformer.

Best Regards
Preben Jakobsen

 

[rockman7892]

prc

With a zig-zag transformer you mentioned that for a L-G fault on the 33kV system, fault current passes through the zig-zag windings on this transformer. Where does fault current go after passing through these windings? Can it get back to the source? In other words what is achieved by this zig zag is it more for detecting ground faults or can the fault current as a result of it be used for tripping protection. Can you please briefly explain how this special transformer works in this application?

Also this may be a silly question but I want to learn so I'll go aheand and ask it. Why not provide the generator as shown with a delta primary and wye secondary on the 33kV system so that the 33kV system can be grounded?

 

[prc]

Jacobsen-You are right.With ZnYn connection single phase loading is not possible or limited.But I have seen spec in some countries for grounding transformer with this connection with stipulation-secondary should be suitable for 3 phase 4 wire system. 100-250 KVA rating is generally used for station loading, 3 phase motors and single phase lighting loads put equally in three phases.With single phase loading, half the primary winding will contribute balancing AT and the other half(being in another phase) will act as reactor. This half will have some compensation by Amperes induced in tank surface.I do not know the extent of single phasing possible.

rockman-Please see attached connection diagram from J& P Transformer book ed11.0(1983)where the current flow is shown.

Your question is not silly. I also have the same query.Some of our experts may be able to explain.Your query is very relevant as normally (esp in large gererators ) the step up transformers is always delta/grounded star to avoid zero sequence currents entering in to stator windings during a LG fault on lines.

 

[PrebenJakobsen]

Thanks for the replies.
We are trying to get Z0 data from vendor, but I am worried about voltage rise on the other phases in case of a 1phase fault.
I will try to persuade the customer to use a transformer with compensation winding (ZNdyn).
Best regards
Preben Jakobsen