Page 81 - Offshore Electrical Engineering Manual

68 CHAPTER 5 Generation and Distribution Switchgear and Transformers

THE MECHANISM OF SHORT CIRCUIT CURRENT
INTERRUPTION

Fig. 2.5.1 shows the dc or asymmetrical component that will exist in the current flow-
ing in at least one of the phases unless the short circuit appears when circuit voltage
is at a maximum. This voltage decays after a few cycles and the resulting decrement
curve can be seen. On an offshore platform, where power is more than likely to be
locally generated, there will also be an ac or symmetrical decrement associated with

(A)
DC component

I

Time

Short-circuit current

(B)
Circuit voltage
ARC current Restriking ARC current
voltage
ARC voltage ARC
peak voltage ARC voltage
peak

ARC ARC voltage
voltage System recovery
Restriking peak voltage
voltage

Restriking voltage
transient

a b c
ARC restrikes ARC restrikes ARC interrupted
FIGURE 2.5.1
Interruption of asymmetrical short circuit current: (A) short circuit current (B) circuit
breaker current interruption.