10-14 L30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALOVERVIEW CHAPTER 10: THEORY OF OPERATION10• For all the terminal currents that are above the 3 pu level, the relative angle difference is calculated. If all threeterminals see significant current, then all three pairs (1, 2), (2, 3), and (1, 3) are considered and the maximum angledifference is used in further calculations.• Depending on the angle difference between the terminal currents, the value of sigma used for the adaptive restraintcurrent is increased by the multiple factor of 1, 5, or 2.5 to 5 as shown in the figure. As shown, a factor of 1 is used forinternal faults, and a factor of 2.5 to 5 is used for external faults. This allows the relay to be sensitive simultaneously forinternal faults and robust for external faults with a possible CT saturation.If more than one CT is connected to the relay (breaker-and-the half applications), the CT saturation mechanism is executedbetween the maximum local current against the sum of all others, then between the maximum local and remote currentsto select the secure multiplier MULT. A maximum of two (local and remote) is selected and then applied to adaptiverestraint.Figure 10-4: CT saturation adaptive restraint multiplier10.1.18 Charging current compensationThe basic premise for the operation of differential protection schemes in general, and of the L30 line differential element inparticular, is that the sum of the currents entering the protected zone is zero. In the case of a power system transmissionline, this is not entirely true because of the capacitive charging current of the line. For short transmission lines the chargingcurrent is a small factor and can therefore be treated as an unknown error. In this application, the L30 can be deployedwithout voltage sensors and the line charging current is included as a constant term in the total variance, increasing thedifferential restraint current. For long transmission lines, the charging current is a significant factor and needs to becomputed to provide increased sensitivity to fault current.Compensation for charging current requires the voltage at the terminals be supplied to the relays. The algorithmcalculates Iq = C x dv/dt for each phase, which is then subtracted from the measured currents at both ends of the line. Thisis a simple approach that provides adequate compensation of the capacitive current at the fundamental power systemfrequency. Travelling waves on the transmission line are not compensated for, and contribute to restraint by increasing themeasurement of errors in the data set.The figures show underlying single phase models for compensation for two and three-terminal systems.
