Generator Fault Current Calculator

Parameters

Every field on the form, what it does to the answer, and what it accepts. The six generator fields are required; everything else is optional and has a stated default.

Generator ratingkVA, 1 to 1,000,000

Nameplate apparent power. Sets the rated current and the base impedance, which is the rated voltage squared divided by the rating. Every reactance is a percentage of that base, so a larger set with the same percentages has proportionally more fault current.

Rated voltagevolts line to line, 100 to 33,000

Taken as both the generator rated voltage and the system nominal voltage, so the IEC ratio Un / UrG is 1.

Rated power factor0.1 to 1

Used only in the correction factor KG, through sin(phi). The correction assumes the set was running at rated load and this power factor before the fault. At 1.0, KG equals the voltage factor c.

Sub-transient reactance Xd''percent, 1 to 50

From the alternator datasheet. Sets the initial symmetrical current, the peak current and the 10 ms MCB check. Must not exceed the transient reactance.

Transient reactance Xd'percent, 1 to 100

Sets the current at about 250 ms and the 250 ms MCB check. Must not exceed the synchronous reactance.

Synchronous reactance Xdpercent, 20 to 1,000

Sets the steady state current when no excitation support is entered. At 250 percent the steady state is about 0.4 times rated current, which is why it rarely trips anything.

MCB checkrating 1 to 125 A, curve B, C or D, fault type

When ticked, the calculator compares the fault current at 10 ms and at 250 ms with the magnetic trip current of the MCB: 4, 7.5 or 12.5 times the rating for curves B, C and D, the multiples behind AS/NZS 3000 Table 8.1. The fault type picks which current is compared. Use phase-to-earth for a final subcircuit, because that is the fault automatic disconnection relies on.

Cable length, size, material and runsmetres, mm2, copper or aluminium, 1 to 12

The cable from the generator to the fault point. Zero length puts the fault at the terminals. Resistance comes from AS/NZS 3008.1.1 Table 35 (multicore, 75 degree column, copper or aluminium) and reactance from the multicore PVC column of Table 30, the same tables the cable sizing calculator uses. Parallel runs divide the active and earth impedance.

Earth conductor sizemm2, optional

Return path for the phase-to-earth fault, one per run. Defaults to the active size. A reduced earth adds 3 times its impedance to the zero sequence path and is often what fails the earth fault check.

Advanced generator data

Apply the correction factor KGon by default

KG = c / (1 + x''d sin(phi)) multiplies the generator impedances, as IEC 60909-0 Clause 6.6.1 sets out. Turn it off and set c to 1.0 to get the simple per unit method, rated current divided by the per unit impedance.

Voltage factor c1.0 to 1.1, default 1.05

IEC 60909-0 Table 1 gives c_max 1.05 for low voltage systems with a plus 6 percent tolerance and 1.10 otherwise. With KG on and no cable, c cancels out of the terminal current; it matters once a cable is in the loop.

X/R ratio1 to 100, optional

Sets the generator resistance as X''d divided by this ratio. Blank uses the IEC fictitious resistance: 0.15 X''d at 1 kV and below, 0.07 X''d above 1 kV under 100 MVA, 0.05 X''d at 100 MVA and over. The resistance drives the peak factor kappa.

Armature resistance Rapercent, 0.01 to 20, optional

The real stator resistance from the datasheet. Takes precedence over the X/R ratio when both are entered, with a warning.

Zero-sequence ratio X0 / Xd''0.05 to 3, default 0.5

Zero sequence reactance as a fraction of X''d. Alternators sit at about 0.1 to 0.7, so 0.5 is indicative. R0 is taken as the generator resistance. A lower ratio raises the phase-to-earth current at the terminals.

Sustained current supportmultiple of rated current, 0.5 to 10, optional

For a set with a PMG, AREP or current booster. Replaces the steady state value with this multiple of rated current, capped at the transient value for each fault type. The support duration field (default 10 s) only labels the working.

Assumptions and limits

What the tool assumes

• Sub-transient current follows IEC 60909-0: I''k = c Un / (sqrt(3) |KG ZG + Zcable|), with ZG = RG + jX''d. The generator is the only source; there is no grid or motor contribution.
• The transient current reuses the same form with X'd and a correction K' = c / (1 + x'd sin(phi)). IEC 60909 does not define this; it is an extension using the same rated load assumption as KG, and it is indicative.
• Without excitation support, the steady state uses no-load excitation, 1 per unit behind Xd. A shunt excited set can collapse below this, and a set with a large field forcing ceiling can hold more. Enter the datasheet support multiple where there is one.
• Phase-to-phase uses c Un / |Z1 + Z2|, and phase-to-earth uses 3 c U0 / |Z1 + Z2 + Z0|. The negative sequence impedance stays at the sub-transient value at every time step, so the phase-to-phase current decays more slowly than the three-phase current.
• The phase-to-earth figure assumes a solidly earthed star point at the generator, an X0 / X''d ratio of 0.5 unless entered, and the earth conductor as the only return path. It is indicative.
• The MCB trip current is the Table 8.1 basis (4, 7.5 or 12.5 times In). IEC 60898-1 allows a curve C MCB to need up to 10 times In, so the tool warns when the current clears the Table 8.1 figure but not the upper limit.
• Cable resistance and reactance are AS/NZS 3008.1.1:2009 Table 35 and Table 30 values (same as 2017), shared with the cable sizing calculator, at the 75 degree column, not end of fault temperature.

What it does not do

Treat the output as an indicative fault level. Check it against IEC 60909-0, AS/NZS 3000 Clause 7.3 and the alternator data, and have it signed off by whoever is responsible for the installation.

Worked examples

Three examples on the same arithmetic the calculator runs, with the default voltage factor c = 1.05 and KG applied. Currents are rounded at the end; intermediate figures carry more places.

Generator fault current, the short version

A transformer behaves like a fixed impedance, so its fault current holds steady until the protection clears it. A generator does not. Its internal reactance rises over the first second of a fault, from the sub-transient value to the transient value and then to the synchronous value, so the current falls from several times rated to, often, less than rated. A circuit that is easily protected on mains supply can be unprotected on generator supply.

That is why the MCB check is the headline here. The fault current at 10 ms tells you whether the breaker trips at all. The current at 250 ms tells you whether it still trips if the fault develops slowly. The steady state tells you what happens if it does not: usually nothing magnetic, which leaves the thermal element and a disconnection time far beyond AS/NZS 3000 limits.