Can anybody guide me how to select the percentage impedance of a 3 winding transformer. I have a transformer 3.3kV/22kV/0.4kV for an oil & gas application. Please guide me.

Thanks
Prakash

 

Prakash M... start with a Design Basis, as follows:

1) Call HV-side (22kV) the primary-winding, p.

2) Call MV-side (3.3kV) the secondary-winding, s.

3) Call LV-side (0.4kV) the tertiary-winding, t.

4) Knowing the maximum short-circuit duty for the MV system, and its VA capacity, calculate the impedance (Zps) as if the HV and MV were connected with a two-winding xfmr.

5) Knowing the maximum short-circuit duty for the LV system, and its VA capacity, calculate the impedance (Zpt) as if the HV and LV were connected with a two-winding xfmr.

6) Determining the impedance, Zst, requires trial & error. Start with Zpt = or > than Zps+Zpt.

7) Transform the Delta-configuation formed by the impedances calcuated in 4), 5), and 6) into an equivalent Wye-configuration, which will yield Zp, Zs, & Zt.

Caution: The above, at best, will yield an approximate answer because the R term has been ignored.

If you need assistance, let us know. Regards, Phil Corso