GE D90 Plus Instruction Manual page 258

GROUPED PROTECTION ELEMENTS
248
The positive-sequence restraint is removed for low currents. If the positive-sequence
current is below 0.8 pu, the restraint is removed by changing the constant K to zero. This
facilitates better response to high-resistance faults when the unbalance is very small and
there is no danger of excessive CT errors as the current is low.
The directional unit uses the zero-sequence current (I_0) or ground current (IG) for fault
direction discrimination and can be programmed to use either zero-sequence voltage
("Calculated V0" or "Measured VX"), ground current (IG), or both for polarizing. The phasors
for the neutral directional overcurrent element directional unit are described below, where
V_0 is the zero-sequence voltage, I_0 is the zero-sequence current, ECA is the element
characteristic angle, IG is the ground current, and
Table 14: Phasors for "Calculated 3I_0" configuration
Polarizing mode Direction
Voltage Forward
Reverse
Current Forward
Reverse
Dual Forward
Reverse
The operating current for overcurrent unit for the "Calculated 3I_0" configuration is
Table 15: Phasors for "Measured IG" configuration
Polarizing mode Direction
Voltage Forward
Reverse
The operating current for overcurrent unit for the "Measured IG" configuration is
When the
Polarizing Voltage
voltage is used in place of V_0. The following figure explains the usage of the voltage
polarized directional unit of the element.
The figure shows the voltage-polarized phase angle comparator characteristics for a
phase A to ground fault, with the element characteristic angle equal to 90° (center line of
operating characteristic), forward limit angle equal to 80° (the ± angular limit with the ECA
for operation), and reverse limit angle equal to 80° (the ± angular limit with the ECA for
operation).
Compared phasors
–V_0 + Z_offset × I_0 and I_0 × 1∠ECA
–V_0 + Z_offset × –I_0 and I_0 × 1∠ECA
IG and I_0
IG and –I_0
–V_0 + Z_offset × I_0 and I_0 × 1∠ECA or IG and I_0
–V_0 + Z_offset × I_0 and I_0 × 1∠ECA or IG and –I_0
Compared phasors
–V_0 + Z_offset × IG/3 and IG × 1∠ECA
–V_0 + Z_offset × –IG/3 and –IG × 1∠ECA
setting is programmed as "Measured VX," one-third of this
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION
Eq. 28
Eq. 29
Eq. 30
Eq. 31