GE G30 Instruction Manual page 451

CHAPTER 5: SETTINGS
The negative-sequence voltage must be greater than 0.02 pu to be validated for use as a polarizing signal. Additionally,
when offset impedance is applied and negative-sequence current is above 0.2 pu, compensated negative-sequence
voltage -V_2 + Z_offset x I_2 has to be above 0.02 pu in order to discriminate fault direction; otherwise when negative-
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
G30 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 5-121: Negative-sequence directional characteristic
PRODUCT SETUP  DISPLAY PROPERTIES CURRENT
GROUPED ELEMENTS
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