ABB RET670 Applications Manual page 231

1MRK504116-UUS C
Application manual
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
by equation 136.
V
=
MOV
k
p
U
NC
EQUATION2001-ANSI V1 EN
Where
U is the maximum instantaneous voltage expected between the capacitor immediately before the
MOV
MOV has conducted or during operation of the MOV, divaded by √2
U is the rated voltage in RMS of the series capacitor
NC
I
I
=
£
1
× ×
Kp In Kp In
R
IEC06000615 V1 EN
Figure 94: Equivalent impedance of MOV protected capacitor in dependence of
protection factor K
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
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
for setting of distance protection IED reach in resistive direction, for phase to
ground fault measurement as well as for phase to phase measurement.
2
I
=
10
×
Kp In
R R
en06000615.vsd
P
· I
p
· I ). This information has high importance
p NC
IED application
. The later is defined
p
). Line apparent impedance is in
NC
Section 3
(Equation 136)
225