Power Transformer 220/110 (69) kV

[gwgroup]

I am currently designing a core type three-phase transformer, with three-legged core, 220/110(69) KV, 60/70 MVA, Y-Y connection. Can I do using serie-parallel winding for the secondary with reconnection. Is neccesary in this case, ysing a delta winding for reduce the third-harmonic voltage. Thanks.

 

[jbartos]

Question: Do you intend to have a two winding transformer or a three winding transformer with delta winding? The three-winding transformer is not necessary; however, the third winding (delta) is often designed for the transformer voltage stability. The delta winding will enable a path for circulation of triplen harmonics.

 

[gwgroup]

In the papers of GE "The whys of the wyes", we understand that only the Y-Y bank of single-phase units exhibits a third-harmonic voltage as 60%-65%, then is neccesary a terciary delta winding. But the core type three-phase transformer exhibits 2% or less third-harmonic voltage, for this in not neccesary a terciary delta winding. Thanks jbartos.

 

[jbartos]

Comments: I agree that the tank of a 3-phase, three-legged core-type Y-connected transformer acts as a delta winding with high impedance to other windings. There is relatively small third-harmonic line-to-neutral voltage and a separate delta tertiary winding is not required to reduce it.
Normally, transformers are designed for intended applications. The Y-Y transformers may have peculiar applications, e.g. tie two delta transformers. There may be other factors to be considered beside the triplen harmonics, e.g.:
Stabilization of neutral
External load may be supplied from tertiary
The transformer may be autotransformer
Etc.

 

[gwgroup]

In a Y-Y bank with grounded neutral and supply system isolated, without delta terciary, the limitation following may be noted:
a) The third harmonic hazards.
b) Possible neutral inversion.
c) Overexcitation

Thus problems doesn´t exist if both system and Y-Y transformers Grounded.