ABB RELION 670 SERIES REG670 Applications Manual page 268

Section 8
Impedance protection
262
ordinary fault. However, a good protection system should be able to operate correctly
before and after gap flashing occurs.
jX
X
12
X
11
Z
S
X
C
en06000625.vsd
IEC06000625 V1 EN-US
Figure 124: Cross-polarized
quadrilateral
characteristic
If the distance protection is equipped with a ground-fault measuring unit, the negative
impedance occurs when
× > × +
3 X 2 X X
C 1 _ 11 0 _ 11
EQUATION1919 V1 EN-US
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 124. 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 125).
Negative IED impedance, negative fault current (current inversion)
If equation 119 is valid in Figure
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.
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
jX
R
RV
R
Z
S
IEC06000584-SMALL V1 EN-US
Figure 125:
112 and a fault occurs behind the capacitor, the
1MRK 502 071-UUS A
X
12
X
11
R
R
FW
X
C
en06000584_small.vsd
Quadrilateral
characteristic with
separate impedance
and directional
measurement
(Equation 118)
GUID-F858877A-E736-4B1C-97B4-8A2B5F8EE644 v2
Application manual