8 THEORY OF OPERATION
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 8–8: 2-TERMINAL TRANSMISSION LINE SINGLE PHASE MODEL FOR COMPENSATION
Figure 8–9: 3-TERMINAL TRANSMISSION LINE SINGLE PHASE MODEL FOR COMPENSATION
Apportioning the total capacitance among the terminals is not critical for compensation of the fundamental power system
frequency charging current as long as the total capacitance is correct. Compensation at other frequencies will be approxi-
If the VTs are connected in wye, the compensation is accurate for both balanced conditions (i.e. all positive, negative and
zero sequence components of the charging current are compensated). If the VTs are connected in delta, the compensation
is accurate for positive and negative sequence components of the charging current. Since the zero sequence voltage is not
available, the L90 cannot compensate for the zero sequence current.
The compensation scheme continues to work with the breakers open, provided the voltages are measured on the line side
of the breakers.
For very long lines, the distributed nature of the line leads to the classical transmission line equations which can be solved
for voltage and current profiles along the line. What is needed for the compensation model is the effective positive and zero
sequence capacitance seen at the line terminals.
Finally, in some applications the effect of shunt reactors needs to be taken into account. With very long lines shunt reactors
may be installed to provide some of the charging current required by the line. This reduces the amount of charging current
flowing into the line. In this application, the setting for the line capacitance should be the residual capacitance remaining
after subtracting the shunt inductive reactance from the total capacitive reactance at the power system frequency.
GE Power Management
L90 Line Differential Relay