Short circuit

What it does

A short circuit study calculates fault currents. You use these currents for two purposes:

Maximum currents show that the equipment ratings are sufficient.
Minimum currents show that the protection can detect a fault and has correct reach and grading.

Phasor uses the IEC 60909 method. This method does not put a fault on one specific power flow state. Instead, it puts an equivalent voltage source at the fault location. The magnitude of this source is c · U_n / √3. The voltage factor c represents the highest credible voltage before the fault. Because of this method, the result is reproducible. It does not depend on the operating point. That is what a rating calculation needs.

The voltage factor c

The standard sets the value of c by voltage level and by the purpose of the calculation:

Calculation	Low voltage	Above 1 kV
Maximum fault current	1.05 or 1.10	1.10
Minimum fault current	0.95	1.00

Phasor selects the factor from the voltage level. You can override it for each case.

The four current values

The calculation gives four current values. Each value has a different use:

I″_k — initial symmetrical short-circuit current (RMS). Use it for thermal withstand checks and for protection grading.
i_p — peak current. The value is κ · √2 · I″_k. The factor κ comes from the X/R ratio at the fault location (κ = 1.02 + 0.98 · e^(−3R/X)). Use i_p for the mechanical forces that switchgear and busbar supports must resist.
I_b — symmetrical breaking current at the time when the breaker contacts separate. Compare it with the rated breaking capacity of the breaker. Near generators, I_b is smaller than I″_k, because the AC component decays.
I_k — steady-state current. The value after the transients have decayed. Use it for long-duration effects.

Fault types

Phasor calculates balanced and unbalanced faults from the sequence networks:

Fault	Sequence networks used	Typical result
Three-phase	Positive only	The maximum current on most networks.
Line-to-line	Positive and negative	About 0.87 × the three-phase value.
Line-to-earth	Positive, negative and zero	The earthing arrangement controls this value. Near a solidly earthed transformer, it can be larger than the three-phase value.
Line-to-line-to-earth	Positive, negative and zero	Check this fault where earth-fault relays must coordinate with phase relays.

What is a sequence network? The method of symmetrical components divides an unbalanced three-phase system into three balanced systems: positive, negative and zero sequence. Each element has an impedance in each sequence. Balanced faults use only the positive sequence. Unbalanced faults use two or three sequences.

LEC Monrovia Interconnected System IEC 60909 results for a three-phase fault at Bushrod Island 66 kV, including initial symmetrical, peak, breaking and thermal duties. — LEC IEC 60909 three-phase fault duty at Bushrod Island 66 kV

How to run a short circuit study

Complete the sequence data. The positive-sequence impedance is not sufficient. The zero-sequence impedance and the earthing arrangement of each transformer control the earth-fault current fully.
Set the source fault level. For a grid supply point, enter the three-phase and single-phase fault levels that the utility declares. As an alternative, enter the equivalent impedance and the X/R ratio.
Select maximum or minimum. Use maximum-current cases to check equipment ratings. Use minimum-current cases to show that the protection detects a fault. Run both. They use different voltage factors and different network configurations.
Select the fault locations. You can select each bus, or a specific list of buses. Also calculate mid-line faults on long feeders. On a long feeder, the current at the far end can be below the pickup of a relay.
Include the motor contribution. Enter the locked-rotor impedance and the rated current of each motor above the inclusion threshold.
Compare the results with the ratings. Phasor flags each switchgear item whose rated breaking or making capacity is below the calculated value.

Inputs and outputs

Inputs

Input	Notes
Source fault level	MVA or kA, plus the X/R or R/X ratio.
Sequence impedances	Z₁, Z₂ and Z₀ for each branch.
Transformer earthing	Vector group, neutral earthing, and any earthing resistor or reactor.
Motor contribution	Locked-rotor impedance and rated current, for motors above the inclusion threshold.
Voltage factor	Set by voltage level, per IEC 60909 Table 1. You can override it for each case.
Network configuration	The case to calculate: the normal running arrangement, or a switched alternative.

Outputs

Output	Notes
I″_k, i_p, I_b, I_k	For each fault location and each fault type.
Sequence currents	The positive, negative and zero components, plus the current in each phase.
Contribution breakdown	The current that arrives from each branch. A busbar rating check needs this.
X/R and κ	At the fault location.
Rating comparison	The calculated value against the equipment rating, with the margin.

Notes

The motor contribution changes the peak current more than the steady-state current. Induction motors feed current into a fault for the first few cycles. Then the contribution decays. When you include the motors, i_p and I_b increase. When you exclude them, a switchgear check can pass when it must fail.

The minimum fault level is the safety case. A high fault level is a risk to equipment. A low fault level is a risk to detection. If the minimum single-phase current at the far end of a feeder is below the relay pickup, no device clears the fault. For the low-voltage equivalent, see earth fault loop impedance.

The earthing arrangement controls the earth-fault current

On a network with an impedance-earthed or unearthed neutral, the earthing arrangement sets the earth-fault current almost fully. The network impedance has a much smaller effect. Make the transformer vector groups and the neutral treatment correct first. Examine the cable data after that.