GE C70 Instruction Manual page 427

CHAPTER 5: SETTINGS
sequence current is less than 0.2 pu, -V_2 is then used as the 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. 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 current must be greater than the
setting value.
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
proper operation even if the negative-sequence voltage at the relaying point is very small. If this is the intent, the offset
impedance shall not be larger than the negative-sequence impedance of the protected circuit. Practically, it is several
times smaller. Enter the offset impedance in secondary ohms.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
Figure 5-114: Negative-sequence directional characteristic
PRODUCT SETUP  DISPLAY PROPERTIES CURRENT
GROUPED ELEMENTS
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