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
136.
EQUATION2001-ANSI V1 EN (Equation 136)
Where
U
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
U
NC
is the rated voltage in RMS of the series capacitor
en06000615.vsd
R
j
X
1
I
£
R
j
X
2
I
=
R
j
X
10
I
=
Kp
×
In Kp
×
In
Kp
×
In
IEC06000615 V1 EN
Figure 94: Equivalent impedance of MOV protected capacitor in dependence of
protection factor K
P
Figure 94 presents three typical cases for series capacitor located at line end (case
LOC=0% in figure
92).
• 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.
1MRK504116-UUS C Section 3
IED application
225
Application manual
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