GE C70 Instruction Manual page 367

CHAPTER 5: SETTINGS
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.
The forward-looking function is designed to be more secure compared to the reverse-looking function, so use the forward-
looking function for the tripping direction. The reverse-looking function is faster compared to the forward-looking function,
so use the reverse-looking function for the blocking direction. This allows for better protection coordination. Take this bias
into account when using the negative-sequence directional overcurrent element to directionalize other protection
elements. The negative-sequence directional pickup must be greater than the
setting value.
 CURRENT CUT-OFF LEVEL
Settings
— Specifies the offset impedance used by this protection. The primary application for the offset
NEG SEQ DIR OC1 OFFSET
impedance is to guarantee correct identification of fault direction on series compensated lines (see the Application of
Settings chapter for information on how to calculate this setting). In regular applications, the offset impedance ensures
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
VOLTAGE CUTOFF LEVEL
Figure 5-102: Negative-sequence directional characteristic
GROUPED ELEMENTS
setting specified in the
PRODUCT SETUP
PRODUCT SETUP  DISPLAY PROPERTIES
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