The Short Circuit Decrement Curves represent the alternator’s fault current during Sub-transient, Transient and Sustained periods of the fault condition.
The sub-transient and transient regions, as indicated in Figure 1, deliver fault current resulting from the energy within the magnetic field strength associated with electromagnetic circuits prevailing within the alternator associated with an excitation system operating to maintain rated output voltage, whilst supporting the connected electrical load. When the fault initially occurs, this store of energy begins to be dispersed, initially driving the high levels of current associated with the sub-transient region, then continuing to force a decaying level of fault current through the transient region until eventually the store of energy is drained.
It should be noted that self-excited alternators fitted with an AVR that is powered by the output of the alternator main stator winding and therefore, does not have an independent power supply, are inherently unable to force the alternator to deliver a steady state level of 3 phase fault current. Therefore, the transient region ends with zero fault current, meaning the alternator has effectively stopped working after a period of typically 0.35s. Such alternators do have a certain ability to provide a limited forcing capability when that alternator is subjected to single phase fault conditions.
Sustained Region(Steady State Fault)
With regard to the sustained region level of fault current, this area is often referred to as the steady state or synchronous region. With the current level in this region being multiples of rated output current, the alternator is clearly operating under a condition of forced excitation; therefore, no effort should be made to determine the level of steady state fault current level by a calculation using the alternator’s advised value for the synchronous reactance (Xd).
An alternator’s actual sustained short circuit current level is displayed on the individual Decrement Curve for that alternator design. The generating set industry ‘standard’ of expecting a 300% short circuit current is no more a ‘given’ than it is likely that the connected electrical loads will result in the alternator operating at only 0.8pf lag.
Mechanical Consideration For Short Circuit
Engineers with experience of watching a generating set being subjected to short circuit testing will have witnessed the dynamic forces that cause the generating set to rock about its axis in a violent manner, even lift from the ground on one side if not anchored securely and recall the audible experience of equipment suffering severe levels of imposed stress. In fact, it is the unbalanced short circuit conditions that impose the greatest forces.
Crash synchronization event: Consider alternators’ operating in parallel and a fault occurs within the electrical system they are supporting. If that fault is close to the alternators, therefore very low impedance, furthermore the fault condition is not immediately disconnected by a discriminating circuit breaker; it is very likely that the system voltage will be suppressed to a level which makes it difficult for the alternators to remain in perfect synchronization. When the fault is cleared, the resulting behavior of the individual generating set control functions for speed and excitation may have performance differences. These performance differences result in the generating sets being forced to realign and return to perfect synchronism under what is often termed a ‘rough synchronizing’ event. Such a scenario imposes a doubly stressful event on the equipment.