GE T60 Instruction Manual page 457

CHAPTER 5: SETTINGS
Table 5-41: Negative-sequence directional overcurrent unit
Mode Operating current
Negative-sequence I = |I_2| – K × I_1|
op
Zero-sequence I = 3 × (|I_0| – K × |I_1|) if |I_1| > 0.8 pu
op
I = 3 × |I_0| if |I_1| ≤ 0.8 pu
op
Table 5-42: Negative-sequence directional unit
Direction Compared phasors
Forward –V_2 + Z_offset × I_2
Reverse –V_2 + Z_offset × I_2
Forward –V_2 + Z_offset × I_2
Reverse –V_2 + Z_offset × I_2
The negative-sequence voltage must be greater than the
menu to be validated for use as a polarizing signal. If the polarizing signal is not validated, neither
 DISPLAY PROPERTIES
forward nor reverse indication is given. The following figure explains the usage of the voltage polarized directional unit of
the element. It shows the phase angle comparator characteristics for a phase A to ground fault, with settings of
ECA = 75° (element characteristic angle = centerline of operating characteristic)
FWD LA = 80° (forward limit angle = ± the angular limit with the ECA for operation)
REV LA = 80° (reverse limit angle = ± the angular limit with the ECA for operation)
The element incorporates a current reversal logic: if the reverse direction is indicated for at least 1.25 of a power system
cycle, the prospective forward indication is delayed by 1.5 of a power system cycle. The element emulates an
electromechanical directional device. Larger operating and polarizing signals result in faster directional discrimination,
bringing more security to the element operation.
T60 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
I_2 × 1∠ECA
–(I_2 × 1∠ECA)
I_2 × 1∠ECA
–(I_2 × 1∠ECA)
VOLTAGE CUTOFF LEVEL
GROUPED ELEMENTS
setting specified in the
PRODUCT SETUP
5
5-275