GE 845 Instruction Manual page 304

PROTECTION
NOTE:
REV Operating
Region
-ECA line
-3I_0 line
VCG
4–176
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 emulate an electromechanical
directional device. Larger operating and polarizing signals will result in faster directional
discrimination bringing more security to element operation.
The forward-looking function is designed to be more secure as compared to the reverse-
looking function, and should therefore 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 better protection coordination.
The above bias should be taken into account when using the Neutral Directional
Overcurrent element to directionalize other protection elements.
For 845 relays ordered without voltage inputs, the polarizing signal for the Neutral
Directional OC element is the ground current (Ig) from the bank selected as Signal Input in
the element's menu. In this case the setponts "Polarizing Mode", and "Polarizing Voltage"
are not displayed.
Figure 4-59: Neutral Directional Voltage-polarized Characteristics
REV LA
line
LA
LA
REV LA
line
-3V_0 line
FWD LA
line
VAG
(reference)
LA
ECA
FWD LA
line
3V_0 line
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
FWD Operating
Region
3I_0 line
ECA line
LA
VBG
827805A1.CDR