GE UR Series C70 Instruction Manual page 256

5.6 GROUPED ELEMENTS
• Transformation errors of current transformers (CTs) during double-line and three-phase faults.
• Switch-off transients during double-line and three-phase faults.
The positive-sequence restraint must be considered when testing for pickup accuracy and response time (multiple of
pickup). The operating quantity depends on the way the test currents are injected into the relay (single-phase injection:
= (1 – K) × I
I ; three-phase pure zero-sequence injection: I
op injected
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 may
be programmed to use either zero-sequence voltage ("Calculated V0" or "Measured VX"), ground current (IG), or both for
polarizing. The following tables define the neutral directional overcurrent element.
Table 5–20: QUANTITIES FOR "CALCULATED 3I0" CONFIGURATION
POLARIZING MODE
Voltage
Current
5
Dual
Table 5–21: QUANTITIES FOR "MEASURED IG" CONFIGURATION
POLARIZING MODE
Voltage
1
(
-- - VAG
where: V_0 =
3
1
-- - IN
I_0 = =
3
ECA = element characteristic angle and IG = ground current
When
NEUTRAL DIR OC1 POL VOLT
figure explains the usage of the voltage polarized directional unit of the element.
The figure below 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)
The above bias should be taken into account when using the neutral directional overcurrent element to directionalize other
protection elements.
5-132
DIRECTIONAL UNIT
DIRECTION COMPARED PHASORS
–V_0 + Z_offset × I_0
Forward
–V_0 + Z_offset × I_0
Reverse
Forward IG
Reverse IG
–V_0 + Z_offset × I_0
Forward
IG
–V_0 + Z_offset × I_0
Reverse
IG
DIRECTIONAL UNIT
DIRECTION
–V_0 + Z_offset × IG/3
Forward
–V_0 + Z_offset × IG/3
Reverse
)
+ VBG + VCG = zero sequence voltage
1
( )
-- - IA
+ IB + IC = zero sequence current
3
is set to "Measured VX", one-third of this voltage is used in place of V_0. The following
C70 Capacitor Bank Protection and Control System
= 3 × I
).
op injected
I_0 × 1∠ECA
–I_0 × 1∠ECA
I_0
–I_0
I
I_0 × 1∠ECA
op
or
I_0
–I_0 × 1∠ECA
or
–I_0
COMPARED PHASORS
IG × 1∠ECA
–IG × 1∠ECA
,
,
5 SETTINGS
OVERCURRENT UNIT
= 3 × (|I_0| – K × |I_1|) if |I
| > 0.8 pu
1
= 3 × (|I_0|) if |I | ≤ 0.8 pu
I
op 1
OVERCURRENT UNIT
I = |IG|
op
GE Multilin