
Protection
16
Each stage can be set individually for
inrush restraint
Settable maximum inrush current
VT secondary circuit supervision for
directional protection function. Once
VT failure happens, the directional
stage can be set to be blocked
CT secondary circuit supervision for
earth fault protection. Once CT failure
happens, all stages will be blocked
Zero-sequence current is measured
from earth phase CT
Directional element
The earth fault protection adopts zero
sequence
directional
element
which
compares the zero sequence system
quantities:
3I
0
, current is measured from earth
phase CT
3U
0
, the voltage is used as reference
voltage. It is calculated from the sum of
the three phase voltages
Forward
Φ
0
_Char
Bisector
0_Ref
3
U
0°
-3 I
0
3 I
0
90°
Figure 9 Direction detection characteristic of
zero sequence directional element
where:
Ф
0_
Char: The settable characteristic angle
For earth fault protection, users can
choose negative sequence directional
element as the complement of zero
sequence directional element. It can be
used in case of too low zero sequence
voltage due to some fault condition e.g. the
unfavorable zero-sequence voltage. The
negative sequence directional element
characteristic is shown in Figure 10.
Forward
Φ
2
_Char
I
3
2
I
-3
2
3
Ref
U
2_
0°
90°
Bisector
Figure 10 Direction detection characteristic of
negative sequence directional element
where:
Ф
2_
Char: The settable characteristic angle
Furthermore, under the VT failure situation,
it can be set to block directional earth fault
protection.
Inrush restraint function
The protection relay may detect large
magnetizing
inrush
currents
during
transformer energizing. In addition to
considerable
unbalance
fundamental
current, Inrush current comprises large
second harmonic current which doesn’t
appear in short circuit current. Therefore,
Содержание CSC-161
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