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
a non-linear equivalent circuit with series connected capacitor and resistor. Their value
depends on complete line (fault) current and protection factor k
p
. The later is defined
by equation
335.
ANSI13000293 V1 EN-US (Equation 335)
Where
V
MOV
is the maximum instantaneous voltage expected between the capacitor immediately before the
MOV has conducted or during operation of the MOV, divaded by
√2
V
NC
is the rated voltage in RMS of the series capacitor
en06000615.vsd
R
jX
1
I
£
R
jX
2
I
=
R
jX
10
I
=
Kp
×
In Kp
×
In
Kp
×
In
IEC06000615 V1 EN-US
Figure 211: Equivalent impedance of MOV protected capacitor in dependence of
protection factor K
P
Figure
211 presents three typical cases for series capacitor located at line end (case
LOC=0% in figure 209).
• Series capacitor prevails the scheme as long as the line current remains lower or
equal to its protective current level (I £
k
p
· I
NC
). Line apparent impedance is in
this case reduced for the complete reactance of a series capacitor.
• 50% of capacitor reactance appears in series with resistance, which corresponds to
approximately 36% of capacitor reactance when the line current equals two times
the protective current level (I £ 2· k
p
· I
NC
). This information has high importance
for setting of distance protection IED reach in resistive direction, for phase to
ground fault measurement as well as for phase to phase measurement.
1MRK 504 163-UUS A Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 423
Application manual
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