3 winding transformer

[mbrooke]

How does a 3 winding transformer impedance play out vs 2 windings?

Take two comparisons for example:


115kv/13.8kv 50MVA Z 10%

vs

115kv/13.8kv/13.8kv 100MVA


I take that any one of the secondaries on the 100MVA will not have the same short circuit value as the 50MVA.

Reason I ask because I've seen very few 3 winding units and have no idea how they work.

 

[Julius Right]

In IEC 60909-4 it is a calculation example for the impedances and equivalent circuits of a three-winding network transformer YN,yn,d5, SrTHVMV = 350 MVA:
The following data are given or found from measurements:
UrTHV = 400 kV; UrTMV = 120 kV; UrTLV = 30 kV
SrTHV = 350 MVA; SrTMV = 350 MVA; SrTLV = 50 MVA
It seems to me SrTHV<SrTMV+SrTLV
They don't expect to work never full load on both MV and LV windings.
That means SrTHV=max(SrTMV,SrTLV )
However, there are still some unclear data:
If the transformer is provided with OFAF that means there are 3 apparent power rated: OFAF;OFAN,ONAN which should be the "rated" one?
What is the "rated power between side A and B”? It could be the minimum of A and B [usually it is B].

 

[Bugman1400]

The two lowside wdgs typically divide the full rating. So, the 115kV H wdg is rated at 100MVA, the first 13.8kV X wdg is rated at 50MVA, and the second 13.8kV Y wdg is rated at 50MVA. Of course it is listed on the nameplate. Also, in addition to, an impedance between highside and lowside, you also have an impedance between the two 13.8kV lowside wdgs. If all three wdgs are sourced, and there is a fault on one of the 13.8kV wdgs, you will get contribution from the other 13.8kV wdg through the X-Y impedance, as well as, from the highside H wdg.

 

[mbrooke]

The two lowside wdgs typically divide the full rating. So, the 115kV H wdg is rated at 100MVA, the first 13.8kV X wdg is rated at 50MVA, and the second 13.8kV Y wdg is rated at 50MVA. Of course it is listed on the nameplate. Also, in addition to, an impedance between highside and lowside, you also have an impedance between the two 13.8kV lowside wdgs. If all three wdgs are sourced, and there is a fault on one of the 13.8kV wdgs, you will get contribution from the other 13.8kV wdg through the X-Y impedance, as well as, from the highside H wdg.

Typically the impedance is such that between the 115kv and one 13.8kv winding the same as a 30/40/50MVA transformer? Thats all I'm concerned about because I want to increase substation capacity but using the same space (the pad and oil sump can take a 100MVA) while being within existing short circuit ratings.