Generator Parameter Calculator

Turn generator nameplate and manufacturer data into the parameters used for power system studies. Calculate full load amps, per-unit resistance, X/R ratios, and DC time constant.

What are generator parameters used for?

Generator parameters can be used to model generator characteristics on a time-current characteristic (TCC) plot and to calculate the generator's fault contribution for arc flash and equipment duty studies. Typical TCC elements:

Full load amps
Overload curve
Short-circuit withstand capability point
Short-circuit decrement curve

Generator Parameter Calculator

Enter the available generator manufacturer's published/nameplate data in boxes 1 and 2 to calculate the values to the right.

1. Generator rating

kVA, kW & power factor

Select Calculate for one value to be calculated from the other two. Leave all three unchecked to enter every value manually.

Rated kVA Calculate

Rated kW Calculate

Power factor (0 < PF ≤ 1) Calculate

Rated 3-phase L-L voltage (V)

Frequency (Hz)

Connection

Neutral grounding

2. Impedance and reactance

Reactance base (kVA) Same as rated kVABase kVA of the below reactance values.

Ra (Stator resistance)

Ra already given in puChanging units clears the entry.

X″d — subtransient (pu)

X′d — transient (pu)

Xd — synchronous (pu)

X₂ — negative sequence (pu)

X₀ — zero sequence (pu)

3. Additional manufacturer data

Optional. Td″, Td′, e_t, I_f and I_fg are used for the fault currents and decrement curve. Ambient and temperature rise are for reference only and do not affect calculated values.

Td″ — subtransient time (ms)Direct-axis short-circuit time constant.

Td′ — transient time (ms)Direct-axis short-circuit time constant.

e_t — prefault terminal voltage (pu)Defaults to 1.0 pu. Leave I_f and I_fg blank to use no-load excitation.

I_fg — no-load field current (A)Field current at no load, rated volts.

I_f — loaded field current (A)Field current at the prefault load.

Ambient (°C)

Temperature rise (°C)

4. Study base conversion

Optional. Converts the reactances and Ra to the base used in your system model.

Study base (kVA)For example, 10,000 kVA for a 10 MVA system base.

Study base voltage (V)Leave blank to use the rated voltage.

Example based on: CAT 3.0 MW generator data sheet.

Calculated values

Enter your generator data to calculate results.

Show calculations

Full load amps: — A
Rated apparent power: — kVA
Rated real power: — kW
Power factor: —
Zbase (base impedance): — Ω
Ra (Stator resistance): — pu
x/r (subtransient, X″d): —
x/r (Neg-sequence, X₂): —
x/r (Zero-sequence, X₀): —Approximate: uses Ra because R₀ is rarely published.
Tdc (DC time constant): — msUse only if not provided by manufacturer.

Bolted fault current at the terminals

Ibase (base current): — A
3-phase, subtransient (I″k): — A
3-phase, transient (I′k): — A
3-phase, steady state (Ik): — ANo-load excitation (I_f / I_fg = 1).
Peak asymmetrical, ½ cycle: — A
Line-to-ground, subtransient: — ASolidly grounded wye, reactances only.

Estimated decrement curve

Enter rated voltage, kVA (or reactance base), X″d, X′d, Xd, Td″ and Td′ to plot the decrement curve.

Symmetrical rms (AC)
Total asymmetrical rms (AC + DC offset)
Full load amps

Constant excitation, no AVR field forcing. Use the manufacturer's curve when available.

Decrement values

Time (s)	Symmetrical (A)	Asymmetrical (A)	× FLA

Formulas and calculation basis

Ratings and base values

Full load amps: FLA = rated kVA × 1,000 / (√3 × V_LL)
Rating conversions: kW = kVA × PF
kVA = kW / PF
PF = kW / kVA
Base impedance: Zbase = V_LL² / (base kVA × 1,000)
Base current: Ibase = base kVA × 1,000 / (√3 × V_LL)
Ibase = FLA when the reactance base equals rated kVA.

Resistance, X/R and DC time constant

Per-unit stator resistance: For a Wye line-to-line entry: Ra (Ω/phase) = Ra (Ω L-L) / 2
Ra (pu) = Ra (Ω/phase) / Zbase
When Ra is entered in pu, it is used directly.
X/R ratios: x/r (subtransient) = X″d (pu) / Ra (pu)
x/r (neg sequence) = X₂ (pu) / Ra (pu)
x/r (zero sequence) = X₀ (pu) / Ra (pu)
DC time constant: Tdc (ms) = X″d (pu) / (2π × f × Ra (pu)) × 1,000

The zero-sequence x/r is approximate. It divides X₀ by the positive-sequence stator resistance because the zero-sequence resistance R₀ is rarely published and usually differs from Ra.

Fault currents

Three-phase bolted fault: I″k = e_t × Ibase / X″d
I′k = e_t × Ibase / X′d
Ik = Ibase × i_d
Steady-state component: i_d = (e_t / Xd) × (I_f / I_fg)
I_f / I_fg = 1 when either field current is blank.
Line-to-ground bolted fault: I_LG = 3 × e_t × Ibase / (X″d + X₂ + X₀)
Solidly grounded wye only.
Peak asymmetrical current: I_peak = √2 × I″k × (1 + e^{−π / (X/R)})
X/R is the subtransient x/r.

Fault currents assume a bolted fault at the generator terminals with prefault terminal voltage e_t. I_f is the regulator field current at the prefault load and I_fg is the field current at no-load rated voltage, so loading the generator before the fault raises the steady-state current. Resistance is neglected in the current magnitudes. Resistance- or impedance-grounded neutrals limit line-to-ground current to a value set by the grounding device.

Decrement curve

Symmetrical rms current: I_ac(t) = Ibase × [(e_t/X″d − e_t/X′d)e^−t/Td″ + (e_t/X′d − i_d)e^−t/Td′ + i_d]
i_d = (e_t / Xd) × (I_f / I_fg)
Total asymmetrical rms current: I_dc(t) = √2 × I″k × e^−t/Tdc
I_total(t) = √(I_ac(t)² + I_dc(t)²)

Study base conversion

Change of base: Z_new (pu) = Z_old (pu) × (kVA_new / kVA_old) × (V_old / V_new)²

kVA_old is the reactance base from box 2 and V_old is the rated voltage. X/R ratios and Tdc do not change with base.

V_LL is rated line-to-line voltage in volts, f is frequency in hertz. The base kVA is three-phase apparent power. Resistance and reactance must be on the same base. Changing the reactance base kVA does not convert the entered reactances, so enter the manufacturer's published values on their stated base. Use the study base conversion to express them on your system base.

This page calculates parameters, fault currents and a constant-excitation decrement estimate. It does not calculate overload curves, withstand limits, incident energy, or equipment duty.

Manufacturer data sheets specify the rating and voltage bases for per-unit reactances. See the CAT 3.0 MW generator data sheet used for the calculator example. Its 9.0E-4 Ω stator resistance is entered as 0.0009 Ω/phase for the SERIES STAR connection.

Evaluate your generator source contributions

Power System Calcs provides short circuit studies, protective device coordination studies, and arc flash studies for commercial, industrial, and mission-critical facilities.

Request a quote