In short. A generator’s fault current starts high, set by its subtransient reactance, and decays over a few cycles toward a lower steady-state value set by synchronous reactance and excitation. Without excitation support the current can fall below the rated current, so protection must be designed for that decay.
The three reactances
| Reactance | Symbol | Period | Typical role |
|---|---|---|---|
| Subtransient | X″d | First few cycles | Peak and breaking duty of breakers |
| Transient | X′d | Tenths of a second | Relay timing around instantaneous and short-time elements |
| Synchronous | Xd | Steady state | Sustained fault current if excitation holds up |
Approximate current levels in per unit: subtransient current is the rated current divided by X″d; transient is rated divided by X′d; steady-state is rated divided by Xd (if excitation is maintained). Typical values are a few multiples of rated for subtransient and transient, and close to rated or less for steady-state.
Excitation support
When the terminal voltage collapses in a fault, a simple shunt-excited AVR loses its supply, and the field collapses, so the fault current decays to a very low value. Excitation systems with auxiliary windings or a permanent magnet generator (PMG) keep supplying the field and sustain current at around three times rated for several seconds, long enough for protection. See PMG excitation and reactance and short circuit.
Fault types
- Three-phase: highest initial current, symmetrical.
- Phase-to-phase: lower current, and negative-sequence effects on the rotor.
- Phase-to-earth: depends on neutral earthing and zero-sequence reactance, can be higher or lower than three-phase.
Implications
- Breaker breaking capacity must cover the initial value.
- Overcurrent elements need to pick up on the decayed value, or use voltage-restrained or voltage-controlled elements. See overcurrent protection.
- Coordination curves must be checked at generator-only fault levels.
- Parallel generators add their contributions.
Study steps
- Obtain X″d, X′d, Xd, X2, X0 and time constants from the data sheet.
- Model the generator with these values, using saturated or unsaturated values as the method requires.
- Calculate initial, breaking and steady-state currents.
- Plot the decay curve against protection curves.
- Check the effect of the excitation system.
Illustrative example
A 1,000 kVA, 415 V generator has X″d of 14 percent. Its rated current is about 1,391 A, so the initial fault current is about ten times rated, around 13,900 A. With a PMG, the sustained current is about three times rated, around 4,200 A. Without it, the current would fall toward rated or less within a second and a feeder breaker set above that would fail to trip.
Three reactances, three time zones
| Reactance | Time range | Typical value | Used for |
|---|---|---|---|
| Subtransient Xd” | First few cycles | 10 to 20 percent | Breaker making and breaking duty |
| Transient Xd’ | Up to about a second | 20 to 35 percent | Protection grading |
| Synchronous Xd | Steady state | 150 to 250 percent | Sustained fault current |
Initial fault current = rated current ÷ Xd” (per unit)
Worked example (illustrative)
A 500 kVA, 415 V set has rated current 696 A. With Xd” of 15 percent, initial fault current is 696 ÷ 0.15 ≈ 4.6 kA. With Xd of 200 percent, the steady fault current would be about 348 A, which is half of full load, unless the excitation system forces the field.
Why excitation type changes the answer
An alternator with a shunt-excited AVR loses its excitation as voltage collapses, so fault current can decay below overcurrent pickup. A PMG or auxiliary-winding system sustains current, commonly around three times rated for several seconds. See PMG excitation.
Consequences
- Breaker duty is set by the subtransient value.
- Protection must be tested against the decayed current, not the initial one.
- Earth faults depend on neutral earthing as well as reactance.
Common mistakes
- Assuming the generator behaves like the utility.
- Using data for another frame size.
- Overlooking excitation type.
What to send us
Send the generator data sheet. Send your application details. See also short-circuit study architecture.
Questions engineers ask
Why does generator fault current decay?
The flux linking the rotor and stator changes as the fault begins, reducing reactance effect from subtransient to synchronous over time.
What does a PMG do?
It supplies the field from permanent magnets, independent of terminal voltage, so fault current is sustained.
Which value do I use for breaker rating?
The initial and breaking values, not the steady-state one.
Do all generators have the same behaviour?
No. Reactances and excitation types differ. Use manufacturer data.
Values are illustrative. Use the manufacturer’s data.