ABB RELION RET670 Applications Manual page 428

Section 8
Impedance protection
422
20
10
0 10 20
10
20
Line current as a function of time
20
10
0 10 20
10
20
Capacitor current as a function of time
ANSI06000614 V1 EN-US
Figure 210: MOV protected capacitor with examples of capacitor voltage and
corresponding currents
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 209. 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
compensation 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
209, 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 instantaneous voltage drop over the capacitor becomes higher than the
protective voltage level in each half-cycle separately, see figure 210.
MOV
i
M
-jX
C
i i
L C
V
C
MOV protected series capacitor
100
50
0
30 40 50 60
50
100
Capacitor voltage as a function of time
20
10
0
30 40 50 60
10
20
MOV current as a function of time
1MRK 504 163-UUS A
10 20 30 40
10 20 30 40
en06000614_ansi.vsd
209 for 50%
Transformer protection RET670 2.2 ANSI
Application manual
50 60
50 60