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Calculating the short-circuit current across the terminals of a synchronous generator

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Calculating the short-circuit current across the terminals of asynchronous generator

Alternators and motorsCalculating the short-circuit current across the terminals of a synchronous generator is very complicated becausethe internal impedance of the latter varies according to time.

When the power gradually increases, the current reduces passing through three characteristic periods:

1. Subtransient (enabling determination of the closing capacity of circuit breakers and electrodynamiccontraints), average duration, 10 ms

2. Transient (sets the equipments thermal contraints), average duration 250 ms3. Permanent (this is the value of the short-circuit current in steady state).

The short-circuit current is calculated in the same way as for transformers but the different states must be takenaccount of.

The short-circuit current is given by the following equation:

Isc = Ir / Xsc

Xsc - Short-circuit reactance c/c

The most common values for a synchronous generator are:

Short-circuit current three characteristic periods

State Subtransient Xd Transient Xd Permanent XdXsc 10 20% 15 25% 200 350%

ExampleCalculation method for analternator or a synchronous motor.

Alternator 15 MVAVoltage U = 10 kVXd = 20%

All electrical installations have to beprotected against short-circuits,without exception, whenever there isan electrical discontinuity; whichmore generally corresponds to achange in conductor cross-section.

The short-circuit current shall becalculated at each stage in theinstallation for the various configurations that arepossible within the network, in order to determine thecharacteristics of the equipment that has to withstand orbreak this fault current.

Reference: Medium voltage design guide Schneider Electric

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Calculating the short-circuit current across the terminals of a synchronous generatorAlternators and motorsExample