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

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:

1. Subtransient (enabling determination of the closing capacity of circuit breakers and electrodynamic contraints), average duration, 10 ms
2. Transient (sets the equipment’s thermal contraints), average duration 250 ms
3. Permanent (this is the value of the short-circuit current in steady state).

Short-circuit current – three characteristic periods

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

I_sc = I_r / X_sc

X_sc - Short-circuit reactance c/c

The most common values for a synchronous generator are:

Example

Calculation method for an alternator or a synchronous motor.

• Alternator 15 MVA
• Voltage U = 10 kV
• X’d = 20%

Reference: Medium voltage design guide – Schneider Electric