Calculating the Short Circuit Current Accros the Terminals of a Synchronous Generator

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Subject 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 a synchronous generator

Alternators and motors

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

because the internal impedance of the latter varies according to time.

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

periods:

Subtransient (enabling determination of the closing capacity of circuit breakers and electrodynamic

contraints), average duration, 10 ms

1.

Transient(sets the equipments thermal contraints), average duration 250 ms2.

Permanent(this is the value of the short-circuit current in steady state).3.

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

taken account of.

Short-circuit current three characteristic periods

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:

State Subtransient Xd Transient Xd Permanent Xd

Xsc 10 20% 15 25% 200 350%

Example

Calculation method f or an alternator or a synchronous motor.

Alternator 15 MVA


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Voltage U = 10 kV

Xd = 20%

All electrical installations have to be protected against short-circuits, without exception, whenever there is an

electrical discontinuity; which more generally corresponds to a change in conductor cross-section.

The short-circuit current shall be calculated at each stage in the installation for the various configurations that

are possible within the network, in order to determine the characteristics of the equipment that has to

withstand or break this fault current.

Reference:Medium voltage design guide Schneider Electric

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Category & Tags: Energy and Power, Motor, alternator, short circuit current, short-circuit, short-circuit

reactance, subtransient, synchronous generator, synchronous motor

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Calculating the Short Circuit Current Accros the Terminals of a Synchronous Generator

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