Protection
9
comparison
amongst
changes
of
phase-phase currents,
iAB,
iBC and
iCA.
Symmetrical component phase
selector
During the whole period of fault, the phase
selector checks the angle between
negative sequence current and zero
sequence current vectors to determine
faulty phases. In addition, phase to phase
faults
will
be
discriminated through
impedance characteristic.
Low voltage phase selector
Both current sudden-change phase and
symmetric component phase selector are
not applicable for weak-infeed end of
protected line, so low-voltage phase
selector is employed in this condition
without VT failure. Theoretically, when one,
two or three phase voltages reduce, the
relevant phase(s) is selected as faulty
phase.
Directional elements
Four kinds of directional elements are
employed for reliable determination of
various faults direction. The related
protection modules, such as distance
protection, tele-protection schemes and
overcurrent and earth fault protections,
utilize the output of the directional
elements as one of their operating
condition. All the following directional
elements cooperate with the mentioned
protection functions.
Memory voltage directional
element
The IED uses the memory voltage and
fault current to determine the direction of
the fault. Therefore transient voltage of
short circuit conditions doesn’t influence
the direction detection. Additionally, it
improves the direction detection sensitivity
for symmetrical or asymmetrical close-in
faults with extremely low voltage. But it
should be noted that the memory voltage
cannot be effective for a long time.
Therefore,
the
following
directional
elements work as supplement to detect
direction correctly.
Zero sequence component
directional element
Zero-sequence directional element has
efficient features in the solidly grounded
system. The directional characteristic only
relates to zero sequence impedance angle
of the zero sequence network of power
system, regardless of the quantity of load
current and/or fault resistance throughout
the fault. The characteristic of the zero
sequence directional element is illustrated
in
Figure
1
.
Forward
Φ
0
_Char
Bisector
0_Ref
3
U
0°
-3I
0
3I
0
90°
Figure 1 Characteristic of zero sequence
directional element
where:
Содержание CSC-161
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