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 112.
EQUATION1910 V1 EN (Equation 112)
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 118: Equivalent impedance of MOV protected capacitor in dependence of
protection factor K
P
Figure 118 presents three typical cases for series capacitor located at line end (case
LOC=0% in figure 116).
• 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 earth fault measurement as well as for phase to phase measurement.
• Series capacitor becomes nearly completely bridged by MOV when the line
current becomes higher than 10-times the protective current level (I £ 10· k
p
·
I
NC
).
Section 8 1MRK 502 071-UEN -
Impedance protection
238 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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