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adjacent line may be lower than the capacitor reactance and voltage inversion phenomenon
may occur also on remote end of adjacent lines. Distance protection of such line must have
built-in functionality which applies normally to protection of series compensated lines.
It usually takes a bit of a time before the spark gap flashes, and sometimes the fault current
will be of such a magnitude that there will not be any flashover and the negative impedance
will be sustained. If equation
11
11
<
<
+
C
S
X
X
X
X
EQUATION1898 V1 EN-US
(Equation 398)
in figure
, the fault current will have the same direction as when the capacitor is bypassed.
So, the directional measurement is correct but the impedance measured is negative and if the
characteristic crosses the origin shown in figure
the IED cannot operate. However, if there
is a memory circuit designed so it covers the negative impedance, a three phase fault can be
successfully cleared by the distance protection. As soon as the spark gap has flashed the
situation for protection will be as for an ordinary fault. However, a good protection system
should be able to operate correctly before and after gap flashing occurs.
en06000625.vsd
jX
R
X
11
X
12
X
C
Z
S
IEC06000625 V1 EN-US
Figure 229: Cross-polarized
quadrilateral
characteristic
en06000584_small.vsd
jX
R
X
11
X
12
X
C
Z
S
X
F
W
X
R
V
R
FW
R
RV
IEC06000584-SMALL V1 EN-US
Figure 230: Quadrilateral
characteristic with
separate impedance and
directional measurement
If the distance protection is equipped with an earth-fault measuring unit, the negative
impedance occurs when
1_11
0 _11
3
2
×
> ×
+
C
X
X
X
EQUATION1919 V1 EN-US
(Equation 399)
Cross-polarized distance protection (either with mho or quadrilateral characteristic) will
normally handle earth-faults satisfactory if the negative impedance occurs inside the
characteristic. The operating area for negative impedance depends upon the magnitude of the
source impedance and calculations must be made on a case by case basis, as shown in figure
. Distance IEDs with separate impedance and directional measurement offer additional
setting and operational flexibility when it comes to measurement of negative apparent
Negative IED impedance, negative fault current (current inversion)
GUID-F858877A-E736-4B1C-97B4-8A2B5F8EE644 v2
If equation
is valid in Figure
and a fault occurs behind the capacitor, the resultant
reactance becomes negative and the fault current will have an opposite direction compared
Section 7
1MRK 505 343-UEN B
Impedance protection
350
Application manual
Содержание Relion 670 series
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