ABB RELION RET670 Applications Manual page 238

Section 8
Impedance protection
232
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 108: 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 108. 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 109).
Negative IED impedance, negative fault current (current inversion)
If equation 79
X X X
> +
C S 11
EQUATION2036 V2 EN-US
in figure 94 and a fault occurs behind the capacitor, the resultant reactance becomes
negative and the fault current will have an opposite direction compared with fault
jX
R
RV
R
Z
S
IEC06000584-SMALL V1 EN-US
Figure 109:
1MRK 504 163-UUS A
X
12
X
11
R
R
FW
X
C
en06000584_small.vsd
Quadrilateral
characteristic with
separate impedance
and directional
measurement
(Equation 78)
SEMOD168320-408 v2
(Equation 79)
Transformer protection RET670 2.2 ANSI
Application manual