Three Winding Transformer - Impedance Tab

Transformer impedance is in percent on the self-cooled MVA of the winding and nominal voltage rating. The strict definition is the percent of rated voltage impressed on the high voltage winding to produce rated full load current in the short circuited low voltage winding.

In this dialog box, Zps stands for Primary to Secondary, Zpt stands for Primary to Tertiary, and Zst stands for Secondary to Tertiary.

Manufacturers provide impedance data in many different formats depending on the test facility and transformer size. The data entry format for EasyPower was chosen from one of the most common testing procedures.

You can specify the impedances in any MVA base: the primary, secondary, or tertiary.

Figure 1: Impedance tab of Three-Winding Transformer Data Dialog Box

Option Descriptions
Option Description

R1

Positive sequence winding resistance in percent.

X1

Positive sequence winding leakage reactance in percent.

R0, X0

Zero sequence winding leakage impedance in percent. If you don't know this value, enter the positive sequence impedance (Z) for shell transformers (see the Form field on the transformer's main dialog box). For core transformers, use approximately 85% of Z. If you enter this value as zero (0.0), the positive sequence impedance is used.

MVA Base

The winding MVA based upon which the percent impedance value is specified.

Grounding

Use the Grounding area of the Impedance tab to enter grounding impedances. They only apply to wye grounded connections. The units are R + jX in ohms. If you only know the ground amperes of the circuit, divide the line to neutral voltage of the circuit by the amperes to get the impedance. This can be achieved in EasyPower by entering the Amp Class first and then clicking Calculate.

R

Transformer neutral ground resistance in ohms. This is the most common method of grounding the transformer neutral winding. Grounding resistors are usually given in amperes. The impedance is found from the following equation.

R = Vln / I

If the transformer is grounded through a grounding transformer with a secondary resistance, this resistance must be converted to the primary winding. Only wye grounded transformers are modeled with grounds. Mid or corner tapped delta windings are not modeled.

jX

Transformer neutral ground reactance in ohms.

Amp Class

This is the current in amps through the ground impedance at the rated voltage. You can enter data in this field directly in amps or calculate it based on the voltage and ground impedance R +jX using the Calculate button.

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