ABB RELION 670 Series Applications Manual page 177

1MRK 505 343-UEN B
IEC06000614 V1 EN-US
Figure 103: 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 102. 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
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 23, 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 103.
Extensive studies at Bonneville Power Administration in USA (
Linearized Model for MOV-Protected series capacitors" Paper 86SM357–8 IEEE/PES summer
meeting in Mexico City July 1986 ) have resulted in construction of a non-linear equivalent
circuit with series connected capacitor and resistor. Their value depends on complete line
(fault) current and protection factor k
Application manual
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
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
. The later is defined by equation 80.
p
Impedance protection
10 20 30 40
10 20 30 40
en06000614.vsd
102 for 50% compensation at
ref. Goldsworthy, D,L "A
Section 7
50 60
50 60
171