Subtransient reactance in percent on generators MVA base. Most synchronous generators have subtransient reactances in the 9-20% range¹. The subtransient reactance is used for ½ cycle, 5 cycle and 30 cycle short circuit calculations in accordance with ANSI Standards.

Reference:
¹ Short Circuit Calculations, General Electric , GET 3550.

Transient reactance in percent on generators MVA base. This value typically ranges from 15-30%. The transient reactance is not used in ANSI standard calculations except for hydro and salient pole generators without amortisseur windings (Xpu = 0.75X’).

Zero sequence reactance in percent on generators MVA base. This value may range from 3-15% for typical generators. Zero sequence values are used in all ground fault calculations.

Locked rotor impedance for induction machines. When actual data is not available, use 16.7%.

Generator neutral ground resistance in ohms. This is the most common method of grounding generator neutrals and is usually given in amperes. The impedance is found from the following equation.

R = Vln / I

If the generator is grounded through a grounding transformer with a secondary resistance, this resistance must be converted to the primary winding. If you know the Amp value for the resistor, you can enter the amp value in the Amp Class field and use the Calculate button to find the resistance.

Generator neutral ground reactance in ohms.

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.

IEC

These options are enabled if you have selected the Show fields and X/R calculations based on IEC 60909 option in Tools > Options > Equipment.

Resistance of the synchronous machine.

Range of generator voltage regulation.

Ratio of fictitious resistance of the synchronous machine when calculating peak short circuit current and the sub-transient reactance (direct axis).

Generator reactance to resistance ratio, which is used to determine resistance values in short circuit studies. Typical range is 30-150 for most generators greater than 10 MVA.