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
Connection
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


Rated Voltage
The Rated Voltage for each winding needs to be entered in these fields. These are the same voltages used as the rated voltage for test and measurement of short-circuit impedance values and can be extracted from test sheet and/or nameplate.

The Rated Voltage for the transformer can be read as follows:
Primary: 345,000 V, Secondary: 138,000 V, Tertiary: 14,400 V


Transformer Nameplate – Tap Changer


Tap %

The Primary and/or Secondary Tap % can be entered in their respective field based on the actual operational settings of these taps.

The Max Tap, Min Tap and Tap steps in % for this transformer can be calculated as follows based on the nameplate:

Max Tap %
= (Max Tap Voltage – Rated Voltage) x 100 / Rated Voltage
= (379,500 – 345,000) x 100 / 345,000
= +10 %

Min Tap %
= (Min Tap Voltage – Rated Voltage) x 100 / Rated Voltage
= (310,500 – 345,000) x 100 / 345,000
= -10 %

Tap Step %
= Max Tap % / No of Steps
= 10 / 16
= 0.625 %

Assuming zero % tap as the operational setting, the Tap % fields for Primary and Secondary can be left as zero.

The 3-Winding Transformer Subview can be entered below for both the Max and Min Load operating conditions. The Max Load state can also be used to take care of both.

Max Load operating conditions


Min Load operating conditions



Transformer Impedance Subview
Positive Sequence Impedance
The % Rpos and % Xpos data fields need to be calculated and entered for each pair of windings base on the referenced Impedance kVA Base field. Depending on the value entered for Impedance kVA Base, the % Rpos and % Xpos data values can vary.

By default, the Impedance kVA Base is equal to Nominal kVA - but it can be changed if the provided data in the test sheet or nameplate has a different base kVA.

The printed data sheet provides the nominal positive sequence impedance. The hand-written test values provide both the Nominal kVA and 100 MVA impedance base. Therefore the positive sequence impedance can be entered based on
either set of data. The end results will be identical.

Alternative 1 – Transformer Nominal kVA Base
Primary-Secondary:
Impedance kVA Base = 240000, % Rpos =0.15, % Xpos = 6.908

Primary-Tertiary:
Impedance kVA Base = 56600, % Rpos =0.191, % Xpos = 9.848

Secondary -Tertiary:
Impedance kVA Base = 56600, % Rpos =0.214, % Xpos = 9.2

Alternative 2 - Impedance kVA Base = 100000
Primary-Secondary:
Impedance kVA Base = 100000, % Rpos =0.0625, % Xpos = 3.878

Primary-Tertiary:
Impedance kVA Base = 100000, % Rpos =0.337, % Xpos = 17.399

Secondary -Tertiary:
Impedance kVA Base = 100000, % Rpos =0.378, % Xpos = 16.3

Zero Sequence Impedance
The approach for entering the data for the % Rzero and % Xzero field for each pair of winding is similar to positive sequence. In our example, there is no zero sequence data provided in the test sheet or name plate. The positive sequence value will be used instead.

The Transformer Impedance Subview can be entered below using either one of the alternative methods. As previously mentioned, both will lead to the same results.

Alternative 1 – Transformer Nominal kVA Base


Alternative 2 – 100000 kVA Base