Quantities For "Calculated 3I0" Configuration; Quantities For "Measured Ig" Configuration - GE T60 Instruction Manual

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
op injected
The positive-sequence restraint is removed for low currents. If the positive-sequence current is below 0.8 pu, the restraint is
5
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 zero-sequence current (I_0) must be greater than the
RENT CUT-OFF LEVEL
tional current. The following tables define the neutral directional overcurrent element.
Table 5–31: QUANTITIES FOR "CALCULATED 3I0" CONFIGURATION
POLARIZING MODE
Voltage
Current
Dual, Dual-V, Dual-I
Table 5–32: QUANTITIES FOR "MEASURED IG" CONFIGURATION
POLARIZING MODE
Voltage
where: V_0
=
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.
5-238
; three-phase pure zero-sequence injection: I
setting value and IG must be greater than 0.05 pu to be validated as the operating quantity for direc-
DIRECTIONAL UNIT
DIRECTION
–V_0 + Z_offset  I_0
Forward
–V_0 + Z_offset  I_0
Reverse
Forward
Reverse
–V_0 + Z_offset  I_0
Forward
–V_0 + Z_offset  I_0
Reverse
DIRECTIONAL UNIT
DIRECTION
–V_0 + Z_offset  IG/3
Forward
–V_0 + Z_offset  IG/3
Reverse
1
 
-- - VAG VBG VCG zero sequence voltage
+ + =
3
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
T60 Transformer Protection System
= 3  I
op
PRODUCT SETUP
COMPARED PHASORS
I_0  1ECA
–I_0  1ECA
IG I_0
IG –I_0
I_0  1ECA
or
IG I_0
–I_0  1ECA
or
IG –I_0
COMPARED PHASORS
IG  1ECA
–IG  1ECA
,
,
).
injected

DISPLAY PROPERTIES
OVERCURRENT UNIT
= 3  (|I_0| – K  |I_1|) if |I
I
op
= 3  (|I_0|) if |I
I
op
OVERCURRENT UNIT
I
op
5 SETTINGS

CUR-
| > 0.8 pu
1
|  0.8 pu
1
= |IG|
GE Multilin