CHAPTER 10: THEORY OF OPERATION OVERVIEWL30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 10-51010.1.7 Ground differential elementThe line ground differential function allows sensitive ground protection for single-line-to-ground faults, allowing the phasedifferential element to be set higher (above load) to provide protection for multi-phase faults. The L30 ground differentialfunction calculates ground differential current from all terminal phase currents. The maximum phase current is used forthe restraint. The L30 is applied in dual-breaker applications to cope with significant through current at remote terminalsthat can cause CT errors or saturation.The line ground differential function uses the same CT matched and time-aligned phasors as the phase-segregatedcurrent differential function. The operate signal is calculated for both real and imaginary parts as follows:Eq. 10-12Eq. 10-13The terms for the second remote terminal are omitted in two-terminal applications.The maximum through current is available locally and re-constructed from the received remote restraint based on themaximum remote restraint current shown in the previous section and as indicated as follows.For two-terminal applications:Eq. 10-14For three-terminal applications:Eq. 10-15The 87G restraining signal is calculated as follows:Eq. 10-16The terms for the second remote terminal are omitted in two-terminal applications.The operate signal for the ground differential function (IOP_87G )2 is then calculated as:Eq. 10-17The restraint signal (I87G )2 is calculated as follows for two-terminal applications:Eq. 10-18
