Lv-Side Faults; External Ground Faults - GE L90 Instruction Manual

10 APPLICATION OF SETTINGS 10.7 LINES WITH TAPPED TRANSFORMERS

10.7.3 LV-SIDE FAULTS

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 earlier, the distance elements should be set to overreach all line terminals, and at the
same time safely underreach the LV busbars of all the tapped transformers. This may present some challenges, 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, partic-
ularly on parallel lines, and the LV sides are interconnected, detailed short circuit studies may be needed to determine the
distance settings.

10.7.4 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 (   
SETTINGS SYSTEM SETUP L90 POWER SYSTEM ZERO-SEQ CURRENT
= "Enabled"). Removing the zero-sequence component from the phase currents may cause the L90 to overtrip
REMOVAL
healthy phases on internal ground fault. This is not a limitation, as the single-pole tripping is not recommended for lines with
tapped transformers.
10
GE Multilin L90 Line Current Differential System 10-25