ABB RET670 - Page 238

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
Figure 100: 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
Figure 101: 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
(Equation 147)
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 100. 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
101).
Negative IED impedance, negative fault current (current inversion)
If equation
148
S l1
XC X X> +
EQUATION2036 V1 EN (Equation 148)
Section 3 1MRK504116-UUS C
IED application
232
Application manual