ABB RELION RET670, Applications Manual

Page: 436 / 1016

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 217: 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 218: Quadrilateral
characteristic with
separate impedance
and directional
measurement
If the distance protection is equipped with a ground-fault measuring unit, the negative
impedance occurs when
1_11 0 _11
3 2
× > × +
C
X X X
EQUATION1919 V1 EN-US
(Equation 346)
Cross-polarized distance protection (either with mho or quadrilateral characteristic)
will normally handle ground-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
217 . Distance IEDs with separate impedance and directional
measurement offer additional setting and operational flexibility when it comes to
measurement of negative apparent impedance (as shown in figure
218 ).
Negative IED impedance, negative fault current (current inversion)
GUID-F858877A-E736-4B1C-97B4-8A2B5F8EE644 v2
If equation 347 is valid in Figure 205 and a fault occurs behind the capacitor, the
resultant reactance becomes negative and the fault current will have an opposite
direction compared with fault current in a power line without a capacitor (current
inversion). The negative direction of the fault current will persist until the spark gap
has flashed. Sometimes there will be no flashover at all, because the fault current is
less than the setting value of the spark gap. The negative fault current will cause a high
voltage on the network. The situation will be the same even if a MOV is used.
Section 8
1MRK 504 163-UUS A
Impedance protection
430 Transformer protection RET670 2.2 ANSI
Application manual