ABB REG670 - Page 243

en06000614.vsd
MOV protected series capacitor
M OV
i
L
i
C
i
M
u
C
-jX
C
0 10 20 30 40 50 60
20
10
10
20
0 10 20 30 40 50 60
100
50
50
100
0 10 20 30 40 50 60
20
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0 10 20 30 40 50 60
20
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Line current as a function of time Capacitor voltage as a function of time
Capacitor current as a function of time MOV current as a function of time
IEC06000614 V1 EN
Figure 117: 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 116. 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
116 for 50%
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
116, 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
117.
Extensive studies at Bonneville Power Administration in USA ( ref. Goldsworthy,
D,L “A Linearized Model for MOV-Protected series capacitors” Paper 86SM357–8
IEEE/PES summer meeting in Mexico City July 1986) have resulted in construction of
1MRK 502 071-UEN - Section 8
Impedance protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 237
Application manual