CHAPTER 10: THEORY OF OPERATION OVERVIEWL90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 10-1510Figure 10-5: Terminal transmission line single phase model for compensationFigure 10-6: Terminal transmission line single phase model for compensationApportioning the total capacitance among the terminals is not critical for compensating the fundamental power systemfrequency charging current as long as the total capacitance is correct. Compensation at other frequencies is approximate.If the VTs are connected in wye, the compensation is accurate for both balanced conditions (that is. all positive, negativeand zero sequence components of the charging current are compensated). If the VTs are connected in delta, thecompensation is accurate for positive and negative sequence components of the charging current. Since the zerosequence voltage is not available, the L90 cannot compensate for the zero sequence current.The compensation scheme continues to work with the breakers open, provided that the voltages are measured on the lineside of the breakers.For very long lines, the distributed nature of the line leads to the classical transmission line equations that can be solvedfor voltage and current profiles along the line. What is needed for the compensation model is the effective positive andzero 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 shuntreactors can be installed to provide some of the charging current required by the line. This reduces the amount of chargingcurrent flowing into the line. In this application, the setting for the line capacitance is the residual capacitance remainingafter subtracting the shunt inductive reactance from the total capacitive reactance at the power system frequency.10.1.19 Differential element characteristicsThe differential element is dependent completely on receiving data from the relay at the remote end of the line, therefore,upon startup, the differential element is disabled until the time synchronization system has aligned both relays to acommon time base. After synchronization is achieved, the differential is enabled. If the communications channel delaytime increases, such as caused by path switching in a SONET system or failure of the communications power supply, therelay acts as outlined in the next section.
