Autotransformer presentation

[salem]

YNad1 400/220 kV, 400MVA , Z%=12.5 autotransformer
how we can present this transformer in fault calculations program where auto transformer is not available in the program library.

 

[Skogsgurra]

A Y/Yn transformer with 12.5 % uk should work. The only difference is that the auto transformer doesn't isolate secondary from primary. And that shouldn't influence fault calculation results.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

I'd be really curious to see a connection diagram for a wye-delta autotransformer. I don't believe such a beast could be built without introducing a major short circuit between the primary phases. At those voltages it is probably an autotransformer, but undoubtedly a wye-wye transformer; though it may have a delta tertiary.

 

[waross]

Hi David;
I don't think that anyone will seriously build it, but if you take three autotransformers and connect one end of the winding off each transformer to the centertap of the next transformer you will have a delta autotransformer bank. A little geometry shows the ratio as 1sqrt.28)/2, (1:2.646) rather than 1:2. I'll leave the phase shift for you to consider. In the event that you wanted to run one 550 volt rated legacy machine tool from a 208 volt supply it would probably work.
Respectfully

 

[davidbeach]

waross, interesting. An autotransformer with a low side delta, but I'm not sure what the high side is; it isn't delta since the ends of the windings don't connect and it isn't wye since there isn't a wye point. It is probably as close to a delta-delta autotransformer as one might get, but it certainly isn't a wye-delta. Custom built, the tap could be anywhere on the winding to provide what ever ratio might be desired. Lots of high side current circulating in the inner delta though, lots of copper and iron for a given MVA rating compared to a wye-wye. A paper exercise, not a practical transformer.

Back to the OP, he needs to check his nameplate again. My guess is that he has a Yyd and the 'd' is probably less than 35kV while the two voltages listed are the 'Y' and the 'y'. No phase shift between the H and M (Y and y) windings.

 

[Skogsgurra]

It may also be a mistake from the OP's side. Perhaps not an autotransformer at all? An induction regulator perhaps?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[salem]

OK, the transformer is an autotransformer with delta as tertiary winding
Again , the connection is YNa0(d1), 400/220/11 kV, 400/400/60 MVA

 

[Skogsgurra]

OK. I still think that you can use an ordinary Yyn model - with a tertiary (secondary, actually) D winding, if available in your library. The D makes triplene impedance low and is only needed if your fault currents contain harmonics. The transients always do.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ijl]

I recently made some fault calculations with a YYD-transformer (both Y's grounded, source at the primary Y, fault at the secondary Y, D not connected). It turned out that the existence of the D-winding greatly increased the fault current in the cases of a line-earth and line-line- earth faults. Thus, it seems to be important to include the D-winding in the fault calculations, even when the harmonics are neglected.

 

[Bahram7]

The Delta will let the circulation of zero sequence currents so you will get a line to ground fault current.
You can represent the autotransformer by a 3 windings YgYgD by calculation ZH, ZL and ZT from ZHL,ZHT and ZBT. In EMTP-RV (

http://www.emtp.com)

you can directly enter the nameplate imprdances.

Regards,
Bahram7

http://www.simtech-Intl.com

 

[waross]

A grounded wye / delta bank will transfer energy from the unfaulted phases back onto a ground faulted primary phase.
I believe that a delta tertiary winding will also transfer energy from the unfaulted phases into a line to ground/neutral secondary fault.
respectfully