5.5 GROUPED ELEMENTS
The neutral directional overcurrent element provides both forward and reverse fault direction indications the
and
OC1 FWD
NEUTRAL DIR OC1 REV
ating current is above a pickup level (overcurrent unit) and the fault direction is seen as forward or reverse, respectively
(directional unit).
The overcurrent unit responds to the magnitude of a fundamental frequency phasor of the either the neutral current calcu-
lated from the phase currents or the ground current. There are separate pickup settings for the forward-looking and
reverse-looking functions. If set to use the calculated 3I_0, the element applies a positive-sequence restraint for better per-
formance: a small user-programmable portion of the positive-sequence current magnitude is subtracted from the zero-
sequence current magnitude when forming the operating quantity.
The positive-sequence restraint allows for more sensitive settings by counterbalancing spurious zero-sequence currents
resulting from:
•
System unbalances under heavy load conditions.
•
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
5
pickup). The operating quantity depends on the way the test currents are injected into the relay (single-phase injection:
I
= (1 – K) × 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 zero-sequence current (I_0) must be greater than the
setting value and IG must be greater than 0.5 pu to be validated as the operating quantity for direc-
RENT CUT-OFF LEVEL
tional current. The following tables define the neutral directional overcurrent element.
Table 5–23: QUANTITIES FOR "CALCULATED 3I0" CONFIGURATION
POLARIZING MODE
Voltage
Current
Dual
5-190
NEUTRAL DIR OC1
MESSAGE
TARGET: Self-reset
NEUTRAL DIR OC1
MESSAGE
EVENTS: Disabled
operands, respectively. The output operand is asserted if the magnitude of the oper-
I
3
×
=
op
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
L60 Line Phase Comparison System
Range: Self-reset, Latched, Disabled
Range: Disabled, Enabled
(
I_0
K
×
I_1
)
–
= 3 × I
).
op
injected
PRODUCT SETUP
I_0 × 1∠ECA
–I_0 × 1∠ECA
I_0
–I_0
I_0 × 1∠ECA
or
I_0
–I_0 × 1∠ECA
or
–I_0
5 SETTINGS
NEUTRAL DIR
DISPLAY PROPERTIES
OVERCURRENT UNIT
= 3 × (|I_0| – K × |I_1|) if |I
I
| > 0.8 pu
op
1
= 3 × (|I_0|) if |I
| ≤ 0.8 pu
I
op
1
GE Multilin
(EQ 5.19)
CUR-