Symmetrical and Asymmetrical are terms used to describe the
symmetry of the short-circuit current waveform around the zero axis. If a
short-circuit occurs in an inductive reactive circuit at the peak of the
voltage waveform, the resulting short-circuit current will be totally
symmetrical.
The amount of offset or asymmetry depends on the point when
the fault occurs. In circuits containing both resistance and inductive
reactance, the amount of asymmetry will vary between the same limits as before.
However, the X/R ratio (ratio of inductive reactance to
resistance looking upstream from the fault point) will determine the rate of
decay of the DC component.
As X/R increases, the rate of decay decreases. Interrupting
current ratings may have to be derated for high X/R values. Practically
speaking, most all short-circuit currents are partially asymmetrical during the
first few cycles after a short-circuit occurs.
Modern personal computers can now be used to easily
calculate symmetrical and asymmetrical current values at various times after a
fault. Low-voltage protective switchgear has a momentary and an interrupting
rating.
The momentary rating is the short-circuit duty during the
first cycle after a fault, and defines the equipment's ability to close and
latch against worst-case mechanical stresses. The interrupting rating is the
short-circuit duty as the equipment contacts part, and is expressed in
symmetrical amperes or MVA.
Medium-voltage fuses have interrupting ratings expressed in
symmetrical amperes.
No comments:
Post a Comment