ABB REL670 2.2 IEC, Applications Manual

Page: 192 / 908

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
69
11 11
< < +
C S
X X X X
EQUATION1898 V1 EN-US
(Equation 69)
in figure 95 , 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
95 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 95: 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 96: 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 70)
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
Section 8
1MRK 506 369-UEN B
Impedance protection
186 Line distance protection REL670 2.2 IEC
Application manual