9-10L90 Line Differential RelayGE Power Management9.6 LINES WITH TAPPED TRANSFORMERS 9 APPLICATION OF SETTINGS99.6.3 FAULTS AT THE LV SIDE OF THE TRANSFORMER(S)Distance supervision should be used to prevent maloperation of the L90 protection system during faults on the LV side ofthe transformer(s). As explained in the DISTANCE BACKUP/SUPERVISION section, the distance elements should be setto overreach all the line terminals and at the same time safely underreach the LV busbars of all the tapped transformers.This may present some challenge particularly for long lines and large transformer tapped close to the substations. If theL90 system retrofits distance relays, there is a good chance that one can set the distance elements to satisfy the imposedrequirements.If more than one transformer is tapped, particularly on parallel lines, and the LV sides are interconnected, detailed short cir-cuit studies may be needed to determine the distance settings.9.6.4 EXTERNAL GROUND FAULTSExternal ground faults behind the line terminals will be seen by the overreaching distance elements. At the same time, thetapped transformer(s), if connected in a grounded wye, will feed the zero-sequence current. This current is going to beseen at one L90 terminal only, will cause a spurious differential signal, and consequently, may cause maloperation.The L90 ensures stability in such a case by removing the zero-sequence current from the phase cur-rents prior to calculat-ing the operating and restraining signals (SETTINGS ÖØ SYSTEM SETUP ÖØ L90 POWER SYSTEM ÖØ ZERO-SEQ CURRENTREMOVAL = "Enabled"). Removing the zero-sequence component from the phase currents may cause the L90 to overtriphealthy phases on internal ground fault. This is not a limitation, as the single-pole tripping is not recommended for lines withtapped transformers.
