Auto-transformer vs Wye-Wye-Delta

[MontanaEric]

Are auto-transformers the same thing as a wye-wye transformer with a buried tertiary (wye-wye-delta)? If not, how is the tertiary connected in a wye-wye with a buried tertiary?

 

[prc]

Cuky, with YNyn+tertiary winding(I thought that is what you referred to),there will be no chance for zero sequence flux causing tank heating. When tertiary is eliminated ,this is a concern to be checked.

 

[RRaghunath]

prc, I worked with 200MVA auto transformers 400/132kV in 1980s with an Indian utility (National Thermal Power Corporation). This transformer had tertiary Delta.
Also, a 75MVA, YNyn0 transformer 400/33kV also had Delta tertiary (in the same power station).
Later, there was a report that said up to 100MVA, it is not necessary to have Delta stabilising winding and the heating of tank can be managed with appropriate design measures (attaching non-magnetic metallic plates to the tank walls is one such measure I believe).

Rompicherla Raghunath

 

[FPelec]

prc,
in Italy the standard connection scheme for HV/MV distribution transformers is Yyn0; there are some thousands of these units, used in primary distribution centres, spanning from 16 to 63 MVA.
The network neutral on the HV side (either 132, 150 or 230 kV) is solidly grounded in power plants and interconnection susbstation, where EHV/HV autotransformers (Yna, 3-limbs without tertiary) are installed, so that there is no need for further grounding the HV network neutral in primary distribution centres.
The medium voltage neutral of HV/MV Yyn transformers is instead grounded at primary distribution centres through a Petersen coil. As the transformers core is always 3-limbs, the zero sequence magnetizing impedance is low enough to allow for the circulation of the zero sequence current when a fault happens on the MV network.
Even in case of long-lasting faults, a proper design of the flux shunts installed on the tank allows for a safe operation of the transformer, without excessive heating of the tank surface. A dedicated type test with neutral current injection (60 s - some 100s of amps) is performed to validate the design.
The main advantage is that the Petersen coil can be directly connected to the star point of the MV winding (no need for a zig-zag neutral grounding transformer). The HV winding can still be designed with a non-uniform insulation as the HV network is solidly grounded.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[Mbrooke]

But what happens when the Petersen becomes shorted? Or a resistor is needed in parallel? Obviously real world practices proves me wrong in my following statement, but going by my own experience I would either use a delta tertiary or a delta primary on the HV/MV transformers.

 

[prc]

Raghu, I know those transformers as I designed them in 1970's for Korba STPP. Station start up unit was Ynd and not YNynd. In India we use YNyn without tertiary for step down units up to a rating of 100 MVA as above that rating 5 limbed units were common those days.(1970s)Some exceptions were those I mentioned in my earlier mail. Thank you F Pelec for the info from Italy. I wish experts give their views of any negative points for Ynyn for step down units. Nothing has come to our attention in India so far.