Modeling 3-Winding Transformer

The data entry process for 3-Winding Transformer is straightforward and simple if you have the correct data information, understand the application of the transformer data in PTW calculations, and know the objective of your studies.

An example of a large 3-winding transformer on the following page contains the test data sheet and nameplate. This information is to be used for modeling the equipment within PTW.

Each subview of the Component Editor is used for entering the equipment data:

3-Winding Transformer Subview

The connection of three phases for each winding needs to be determined and selected from the drop menu.
From the data sheet and nameplate, the connections are:
• Primary: Wye-Ground
• Secondary: Wye-Ground
• Tertiary: Delta

Nominal kVA
The Nominal kVA entered for each winding in this section is used to evaluate the selected size or full load ampacity of transformer against the design amapcity (calculated by the Demand Load Study) in Equipment Evaluation module.

The Nominal kVA fields will also be used as the Base kVA for calculation of the PU impedance of transformer in the overall impedance network ONLY IF the Impedance kVA Base field in the Transformer Impedance subview of Component Editor is left blank.

In other words, the Nominal kVA in 3-Winding Transformer is used only for evaluation and will not change the transformer impedance when Impednace kVA Base is filled in Transformer Impedance subview.

The kVA rating of the transformer is represented in the data sheet and nameplate as three separate rating for each winding depending on the operational cooling class OA/FOA/FOA. For instance, if Max Load condition is modeled for a load flow and short-circuit study, then the Max FOA rating can be used:

Primary: 400,000 kVA, Secondary: 400,000 kVA, Tertiary: 94,333 kVA

But if Min Load condition is to be used for the study then OA rating can be used instead:

Primary: 240,000 kVA, Secondary: 240,000 kVA, Tertiary: 56,600 kVA

Transformer Test Sheet

Transformer Nameplate – Ratings and Impedances