Impedance Selection For a Three Winding Transformer

Yes, your understanding is correct. Typically, a lower impedance is seen on the H to other 2 windings, and a higher number between X & Y. If your spec calls for 14%Z, then the manufacturers will design for 14% H-X and H-Y, and whatever results from this for X-Y. Really, it's the impedance from H to X or Y that matters most, as it will affect available fault current in relation to the grid connection.

For your conceptual clearance; If R1 is the primary winding resistance with leakage reactance XL1 and the secondary winding values R2, XL2 respectively, the secondary impedence is Z2 =R2 + jXt2 and for assessment purpose the secondary impedence will be transfered to the primary side by multiplying with turn ratio N1 / N2

Thus total primary impedence seen from the primary side is a sum of primary impedence and secondary impedence referred to the primary.

Your statement that "One of the secondary winding connects to GTG unit and the other secondary winding connects to STG unit" is incorrect practically the transformer is meant for the load of the auxiliaries related to GTG and STG.

Actually, the OP is probably correct. I have seen generating plants where a 3-winding transformer is used as the GSU (generator step-up) for a Gas Turbine Generator and a Steam Turbine Generator, eliminating the need for two separate GSUs. The auxiliaries associated with each generator are then tapped off that winding's bus.

The auxiliary power for the plant is tapped from GTG bus (11KV) of the GSU ( 3- winding transformer). We do not have any tap from the STG bus. The STG connects to secondary2 of the GSU(3-winding) through a generator breaker. The GTG unit connects to the Secondary1 of the GSU through a generator breaker. But for the GTG circuit at the 11KV side we have a tap off at the 11KV SWGR for the station auxiliary loads(GTG and STG units).

The reason for using the 3-winding GSU for the proposed plant is due to space constraint at the substation side. I have also seen 3-winding transformer in other plant but for application wise this is my first design to implement a 3-winding transformer for the proposed power plant. I was not sure about the impedance selection between Primary and Secondary of the 3-winding transformer and thus request a support from the FORUM. Reply from PeterT helps me for further discussion with the transformer vendor.

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