GE L90 Instruction Manual page 321

5 SETTINGS
The positive-sequence restraint must be considered when testing for pick-up accuracy and response time (multiple of
pickup). The operating quantity depends on the way the test currents are injected into the relay:
• Single-phase injection: I
op
• Three-phase pure zero- or negative-sequence injection, respectively: I
• The directional unit uses the negative-sequence current and voltage for fault direction discrimination.
The following table defines the negative-sequence directional overcurrent element.
OVERCURRENT UNIT
MODE
Negative-sequence
Zero-sequence
The negative-sequence voltage must be greater than 0.02 pu to be validated for use as a polarizing signal. If the polarizing
signal is not validated neither forward nor reverse indication is given. The following figure explains the usage of the voltage
polarized directional unit of the element.
The figure below 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 will be delayed by 1.5 of a power system cycle. The element is designed to emu-
late an electromechanical directional device. Larger operating and polarizing signals will result in faster directional discrimi-
nation bringing more security to the element operation.
REV Operating
Region
Figure 5–100: NEGATIVE-SEQUENCE DIRECTIONAL CHARACTERISTIC
The forward-looking function is designed to be more secure as compared to the reverse-looking function, and therefore
should be used for the tripping direction. The reverse-looking function is designed to be faster as compared to the forward-
looking function and should be used for the blocking direction. This allows for better protection coordination. The above
GE Multilin
= 1/3 × (1 – K) × I
.
injected
OPERATING CURRENT
= |I_2| – K × I_1|
I
op
= 3 × |I_0| – K × |I_1|
I
op
–V_2 line
REV FWD
LA LA
LA
–I_2 line
–ECA line
LA
VCG
V_2 line
L90 Line Current Differential System
= I .
op injected
DIRECTIONAL UNIT
DIRECTION COMPARED PHASORS
–V_2 + Z_offset × I_2
Forward
–V_2 + Z_offset × I_2
Reverse
–V_2 + Z_offset × I_2
Forward
–V_2 + Z_offset × I_2
Reverse
VAG (reference)
LA
ECA line
ECA
I_2 line
LA
FWD Operating
VBG
REV FWD
LA LA
5.6 GROUPED ELEMENTS
I_2 × 1∠ECA
–(I_2 × 1∠ECA)
I_2 × 1∠ECA
–(I_2 × 1∠ECA)
Region
827806A2.CDR
5-185
5