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 72 is valid
EQUATION1898 V1 EN (Equation 376)
in figure 97, 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 97 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.
IEC06000625 V1 EN
Figure 215: Cross-polarized
quadrilateral
characteristic
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 216: 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 377)
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
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 417
Application manual
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