Page 35 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 25

i a

i b

i c

i f

Time
Fig. 1.19 Short-circuit currents of a synchronous generator (# 1995 IEEE).

being present during the first few cycles after the disturbance has occurred. This can
be appreciated from Figure 1.19, where a three-phase, short-circuit at the terminals
of a synchronous generator give rise to currents that clearly show the transient and
steady (sustained) states. The figure shows the currents in phases a, b and c,as wellas
the field current. The source of this oscillogram is (Kimbark, 1995).
Faults are unpredictable events that may occur anywhere in the power network.
Given that faults are unforeseen events, strategies for dealing with them must be
decided well in advance (Anderson, 1973). Faults can be divided into those involving
a single (nodal) point in the network, i.e. shunt faults, and those involving two points
in one or more phases in a given plant component, i.e. series faults. Simultaneous
faults involve any combination of the above two kinds of faults in one or more
locations in the network. The following are examples of shunt faults:
. three-phase-to-ground short-circuit
. one-phase-to-ground short-circuit
. two-phase short-circuit
. two-phase-to-ground short-circuit.
The following are examples of series faults:
. one-phase conductor open
. two-phase conductors open
. three-phase conductors open.