GE C70 Instruction Manual page 353

CHAPTER 5: SETTINGS
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 zero-sequence current (I_0) must be greater than the
setting value and IG must be greater than 0.05 pu to be validated as the operating quantity for directional
CUT-OFF LEVEL
current. The following tables define the neutral directional overcurrent element. V_0 is the zero-sequence voltage, I_0 is
the zero-sequence current, ECA is the element characteristic angle, and IG is the ground current.
Table 5-32: Quantities for "calculated 3I0" configuration
Directional unit
Polarizing mode Direction
Voltage Forward
Reverse
Current Forward
Reverse
Dual, Dual-V, Dual-I Forward
Reverse
Table 5-33: Quantities for "measured IG" configuration
Directional unit
Polarizing mode Direction
Voltage Forward
Reverse
where:
ECA = element characteristic angle
IG = ground current
When
NEUTRAL DIR OC1 POL VOLT
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:
• ECA = 90° (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)
Take the bias into account when using the neutral directional overcurrent element to directionalize other protection
elements.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
Compared phasors
–V_0 + Z_offset × I_0 I_0 × 1∠ECA
–V_0 + Z_offset × I_0 –I_0 × 1∠ECA
IG I_0
IG –I_0
–V_0 + Z_offset × I_0 I_0 × 1∠ECA
or
IG I_0
–V_0 + Z_offset × I_0 –I_0 × 1∠ECA
or
IG –I_0
Compared phasors
–V_0 + Z_offset × IG/3
–V_0 + Z_offset × IG/3
is set to "Measured VX," one-third of this voltage is used in place of V_0. The following
PRODUCT SETUP  DISPLAY PROPERTIES  CURRENT
Overcurrent unit
I = 3 × (|I_0| – K × |I_1|) if |I
op
I = 3 × (|I_0|) if |I
op
Overcurrent unit
IG × 1∠ECA I = |IG|
op
–IG × 1∠ECA
GROUPED ELEMENTS
| > 0.8 pu
1
| ≤ 0.8 pu
1
5-187
5