(2160908 EPS II)Laxmi Institute of Technology, Sarigam,
Gujarat
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Outline2
Symmetrical faults : That fault which gives rise to symmetrical
fault currents (i.e. equal faults currents with 120o displacement)
is called a symmetrical fault. Example: when all the three
conductors of a 3-phase line are brought together simultaneously
into a short-circuit condition.Unsymmetrical faults: Those faults
which give rise to unsymmetrical currents (i.e. unequal line
currents with unequal displacement) are called unsymmetrical
faults.Single line-to-ground fault Line-to-line fault Double
line-to-ground fault
Common: a short-circuit from one line to ground
INTRODUCTION
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INTRODUCTION OF UNSYMMETRICAL FAULTS Most Of the faults that
occur on power systems are unsymmetrical faults, which may consist
of unsymmetrical short circuits, unsymmetrical faults through
impedances, or open conductors. Unsymmetrical Faults occur as
single line--to--ground faults, line--to--line faults, or double
line--to--ground faults. The Path of the fault current from line to
line or line to ground may or may not contain impedance. One Or two
open conductors result in unsymmetrical faults, through either the
breaking of one or two conductors or the achon of fuses and other
devices that may not open the three phases simultaneously
Analysis of unsymmetrical faults using bus impedance matrix
Sequence ImpedancesSlide 10 of 11Each element of power system
will offer impedance to different phase sequence components of
current which may not be the same. Therefore, in unsymmetrical
fault calculations, each piece of equipment will have three values
of impedanceone corresponding to each sequence current viz.(i)
Positive sequence impedance (Z1)(ii) Negative sequence impedance
(Z2)(iii) Zero sequence impedance (Z0)
