Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-19
OP
1.1.2 Capacitive charging current (all models)
The charging current of a line or cable will be seen as differential current. If this current is of
a sufficiently high magnitude, as is the case for cables and long feeders, then relay
maloperation could occur. Two issues are apparent with charging current; the first being
inrush during line energization and the second being steady state charging current.
Inrush charging current is predominately high order harmonics (9th and 11th for example).
The Fourier filtering used by the P54x relays will remove these frequency components and
hence provide stability.
Steady state charging current is nominally at fundamental frequency and hence may cause
relay maloperation.
To overcome this problem the P54x relays include a feature to extract the charging current
from the measured current before the differential quantity is calculated.
IL IR
ZL
IchL IchR
VRVL
P2057ENa
Figure 6 Capacitive charging current
L = Local end line current
r = Remote end line current
VL = Local end voltage
VR = Remote end voltage
ZL = Line impedance
chL = Local end charging current
chR = Remote end charging current
By considering Figure 6 it is evident that the line charging
current at a particular location is
equal to the voltage at that location multiplied by the line positive sequence susceptance. It
is therefore possible for the relays at each line end to calculate the respective line charging
currents and compensate accordingly.
The differential current (d) can be calculated as follows:
d = L + R - (jVLBS/2) - (jVRBS/2)
d = {L - (jVLBS/2)} + {R - (jVRBS/2)}
d = Local relay current + remote relay current
Where BS is the line positive sequence susceptance.
This feature can be selectively enabled or disabled. If selected, the normal phase current
data in the protection message is replaced by { - (jVBS/2)}.
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