ABB RELION 670 Series Applications Manual page 331

1MRK 504 152-UEN B
KC = 0%
R
IEC06000613 V1 EN-US
Figure 196: Apparent impedances seen by distance IED for different SC locations and
IEC06000614 V1 EN-US
Figure 197: MOV protected capacitor with examples of capacitor voltage and
The impedance apparent to distance IED is always reduced for the amount of capacitive
reactance included between the fault and IED point, when the spark gap does not flash over, as
presented for typical cases in figure 196. Here it is necessary to distinguish between two
typical cases:
• Series capacitor only reduces the apparent impedance, but it does not cause wrong
directional measurement. Such cases are presented in figure
at 50% of line length and 33% compensation located on 33% and 66% of line length. The
remote end compensation has the same effect.
• The voltage inversion occurs in cases when the capacitor reactance between the IED point
and fault appears bigger than the corresponding line reactance, Figure 196, 80%
compensation at local end. A voltage inversion occurs in IED point and the distance IED
will see wrong direction towards the fault, if no special measures have been introduced in
its design.
The situation differs when metal oxide varistors (MOV) are used for capacitor overvoltage
protection. MOVs conduct current, for the difference of spark gaps, only when the
Application manual
KC = 80% KC = 50% KC = 2 x 33%
LOC = 0% LOC = 50% LOC = 33%, 66%
R R
spark gaps used for overvoltage protection
20
10
0 10 20 30
10
20
Line current as a function of time
20
10
0 10 20 30
10
20
Capacitor current as a function of time
corresponding currents
KC = 80%
LOC = 100%
R R
en06000613.vsd
M OV
i
M
-jX
C
i i
L C
u
C
MOV protected series capacitor
100
50
0
40 50 60
50
100
Capacitor voltage as a function of time
20
10
0
40 50 60
10
20
MOV current as a function of time
Impedance protection
10 20 30 40
10 20 30 40
en06000614.vsd
196 for 50% compensation
Section 7
50 60
50 60
325