Compensation for charging current requires the voltage at the terminals be supplied to the relays. The algorithm calculates
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 measurement error accounted for by the
Capacitive currents leak from the unit protection zone causing a phase shift for the phase comparison principle. Charging
currents are present both in the balanced pre-fault state (positive-sequence charging current) and during internal and exter-
nal faults (unbalanced charging currents).
The phase shift caused by the capacitive current depends on the X / R ratio of the line and system equivalents. For large
X / R values, the capacitive current affects mostly magnitudes of the terminal currents. This is a concern for the line current
differential, and less of a problem for the phase comparison relays. For smaller X / R values (highly resistive impedances),
the capacitive current has a greater effect on the phase relationship, creating greater problems for phase comparison
relays. As long transmission lines are typically EHV (extra high voltage) lines with a high X / R ratio, the question becomes
critical not for a wrong phase due to charge current, but for providing proper coordination between FDL and FDH at oppo-
site line terminals.
The underlying single phase model for compensation for a two and three terminal system are shown below.
Figure 9–19: TWO-TERMINAL TRANSMISSION LINE SINGLE PHASE MODEL FOR COMPENSATION
If the VTs are connected in wye, the compensation is accurate for both balanced conditions (that is, all positive, negative
and zero-sequence components of the charging current are compensated). If the VTs are connected in delta, the compen-
sation is accurate for positive and negative-sequence components of the charging current. Since the zero-sequence volt-
age is not available, the L60 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 required for the compensation model is the effective positive and
zero-sequence capacitance seen at the line terminals.
Figure 9–20: THREE-TERMINAL TRANSMISSION LINE SINGLE PHASE MODEL FOR COMPENSATION
In some applications the effect of shunt reactors must be taken into account. Shunt reactors may be installed in very long
lines 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 subtract-
ing the shunt inductive reactance from the total capacitive reactance at the power system frequency.
L60 Line Phase Comparison System
9 THEORY OF OPERATION