ABSTRACT
The heat generation rate and the mechanical force in conductors, both being square functions of the current, are the highest at the first subtransient peak of the asymmetrical fault current. The current then decays to the transient symmetrical value in several cycles and steady-state value in about 1 s, as the generator reactance rises from low subtransient to transient to steady-state synchronous reactance value. The mechanical damage is avoided by bracing the structural parts to withstand the first peak of the mechanical force without exceeding the allowable stress or deflection limits. If that is not possible or practical, the protection system must incorporate a suitable fault current-limiting fuse or circuit breaker, or insert a series reactor in the lines, to limit the current. The thermal damage is avoided by interrupting the fault, typically in several cycles, before it can overheat the equipment and burn the insulation.
