Charging Current Compensation - GE L90 Instruction Manual

8 THEORY OF OPERATION
Figure 8–4: CT SATURATION ADAPTIVE RESTRAINT MULTIPLIER
The basic premise for the operation of differential protection schemes in general, and of the L90 line differential element in
particular, is that the sum of the currents entering the protected zone is zero. In the case of a power system transmission
line, this is not entirely true because of the capacitive charging current of the line. For short transmission lines the charging
current is a small factor and can therefore be treated as an unknown error. In this application the L90 can be deployed with-
out voltage sensors and the line charging current is included as a constant term in the total variance, increasing the differ-
ential restraint current. For long transmission lines the charging current is a significant factor, and should be computed to
provide increased sensitivity to fault current.
Compensation for charging current requires the voltage at the terminals be supplied to the relays. The algorithm calculates
× ⁄
C dv dt for each phase, which is then subtracted from the measured currents at both ends of the line. This is a simple
approach that provides adequate compensation of the capacitive current at the fundamental power system frequency. Trav-
elling waves on the transmission line are not compensated for, and contribute to restraint by increasing the measurement of
errors in the data set.
The underlying single phase model for compensation for a two and three terminal system are shown below.
Figure 8–5: 2-TERMINAL TRANSMISSION LINE SINGLE PHASE MODEL FOR COMPENSATION
GE Multilin
arg(I /I )=180 degrees
1 2
(external fault)
MULT=5
MULT=1
arg(I /I )= 0 degrees
1 2
(internall fault)
Is
Vs
L
R
C/2
L90 Line Differential Relay
MULT = abs( arg(I /I )) x 5/180
1 2
MULT=1
831744A2.CDR

8.1.18 CHARGING CURRENT COMPENSATION

Ir
Vr
C/2
L00011a1.vsd
8.1 OVERVIEW
8
8-13