ABB REL670 2.2 IEC - Page 389

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 365.
=
MOV
p
NC
U
k
U
EQUATION1910 V1 EN-US (Equation 365)
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-US
Figure 207: Equivalent impedance of MOV protected capacitor in dependence
of protection factor K
P
Figure 207 presents three typical cases for series capacitor located at line end (case
LOC=0% in figure 205).
• 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
).
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 383
Application manual