Compensation For Charging current
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 differential 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
for each phase, which is then subtracted from the measured dt
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. Travelling 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 1: 2-TERMINAL TRANSMISSION LINE SINGLE PHASE MODEL FOR COMPENSATION 两端系统
←Figure 2: 3-TERMINAL TRANSMISSION LINE SINGLE PHASE MODEL FOR COMPENSATION三端系统
Apportioning the total capacitance among the terminals is not critical for compensating the
fundamental power system frequency charging current as long as the total capacitance is correct. Compensation at other frequencies will be approximate.
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.
