Two Winding vs. Three Winding Transformer 1

[cyriousn]

We have a 325MW renewable site that was studied with a Y-g 345kv / Delta 34.5kv 350MVA two winding transformer but we are kicking around the idea of changing it to a three winding transformer to reduce the secondary current. If we kept the impedance the same between the primary and the two secondary windings wouldn't this essentially be the same transformer from a studies standpoint?

Also, I would assume it would be easier to manage the secondary cable connections if there was two secondary windings. The design currently calls for (4) MV switchgear lineups.

 

[waross]

The ohmic impedance and the percentage impedance will be higher with two 50% capacity windings.
The inductive reactance depends on the number of turns and the placement of the windings and may not change much.
So, the ASCC will be less due to higher resistance but the effect will not be linear due to the similar inductive reactances.
The X:R ratio will also change.
The first cycle peak transient depends much on the air core impedance and the ohmic resistance and see a greater reduction than the reduction in the ASCC.

 

[cyriousn]

The ohmic impedance and the percentage impedance will be higher with two 50% capacity windings.
The inductive reactance depends on the number of turns and the placement of the windings and may not change much.
So, the ASCC will be less due to higher resistance but the effect will not be linear due to the similar inductive reactances.
The X:R ratio will also change.
The first cycle peak transient depends much on the air core impedance and the ohmic resistance and see a greater reduction than the reduction in the ASCC.

I believe the ISO would accept a difference of the total impedance of 10% or less from the originally studied transformer. We would have to develop the transformer spec. to match it as best as possible.

 

[prc]

Yes. You can have two secondary windings with equal percentage impedances. In India, we have several 315 and 400 kV transformers with 400/33-33 kV YNynyn units at Solar firms. Some of them have 5-limbed core construction without a stabilising delta winding,
In 2021, the Central Electricity Authority of India standardised ratings and specifications of collector transformers (Www. cea. nic. in.). The specifications, “Standard Technical Specifications of Power Transformers for Solar Park Pooling Stations” (260 pages), can be downloaded from the above website.

 

[bacon4life]

It might depend on which studies you are referring to. Even if the impedance changes beyond 10%, it might not make a practical difference to the grid.

For voltage control purposes, two windings would change some things.

 

[wcaseyharman]

On the system side yes, but I would think you would reduce your low side bus fault current (which doesn’t seem like a bad thing).