9.6 LINES WITH TAPPED TRANSFORMERS
9 APPLICATION OF SETTINGS
9.6.3 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 of
the transformer(s). As explained in the DISTANCE BACKUP/SUPERVISION section, the distance elements should be set
to 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 the
L90 system retrofits distance relays, there is a good chance that one can set the distance elements to satisfy the imposed
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 FAULTS
External ground faults behind the line terminals will be seen by the overreaching distance elements. At the same time, the
tapped transformer(s), if connected in a grounded wye, will feed the zero-sequence current. This current is going to be
seen 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 (
L90 POWER SYSTEM
= "Enabled"). Removing the zero-sequence component from the phase currents may cause the L90 to overtrip
healthy phases on internal ground fault. This is not a limitation, as the single-pole tripping is not recommended for lines with
L90 Line Differential Relay
GE Power Management