TX Zero Seq impedances

[CDG16]

Hi, All
Would appreciate some guidance on the following. I have the zero seq impedances (in ohms) for an auto TX. I would like to convert these values to p.u. for calculation purposes. I've seen formulas where they calculate Zoh, Zom and Zol. They then calculate Zbase and from there the p.u. impedances. However, I've seen on a test sheet that someone used for Zol a faxtor of (HV Volts / MV Volts) and for Zom (HV Volts / MV Volts) squared. Which formulas are correct? I personally believe that the first ones are correct, but the voltage factors are putting doubt in my mind.
Thanks in advance for all your responses.

 

[DanDel]

The standard calculation for getting pu values from ohmic values is:

Zpu = Zohms/(V^2/VA)

Zohms is the impedance in ohms, in your case the Zero Sequence Impedance.

Zpu is the impedance in pu, agin this would be the calculated Zero Sequence Impedance in pu.

V^2 in the system voltage squared

VA is the base VA for your pu calculation(typically, this would be something like 100MVA, or 100,000,000VA for this equation)