GE M60 Instruction Manual page 295

5 SETTINGS
MESSAGE
MESSAGE
MESSAGE
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
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 5–13: QUANTITIES FOR "CALCULATED 3I0" CONFIGURATION
POLARIZING MODE DIRECTION
Forward
Voltage
Reverse
Forward
Current
Reverse
Forward
Dual
Reverse
GE Multilin
NEUTRAL DIR OC1 BLK:
Off
NEUTRAL DIR OC1
TARGET: Self-reset
NEUTRAL DIR OC1
EVENTS: Disabled
operands, respectively. The output operand is asserted if the magnitude of the oper-
× (
I 3 I_0
= –
op
DIRECTIONAL UNIT
COMPARED PHASORS
–V_0 + Z_offset × I_0
–V_0 + Z_offset × I_0
IG
IG
–V_0 + Z_offset × I_0
or
IG
–V_0 + Z_offset × I_0
or
IG
M60 Motor Protection System
Range: FlexLogic™ operand
Range: Self-reset, Latched, Disabled
Range: Disabled, Enabled
× )
K I_1
= 3 × I
).
op injected
I_0 × 1∠ECA
–I_0 × 1∠ECA
I_0
–I_0
= 3 × (|I_0| – K × |I_1|) if |I
I
I_0 × 1∠ECA
op
I
op
I_0
–I_0 × 1∠ECA
–I_0
5.6 GROUPED ELEMENTS
NEUTRAL DIR
(EQ 5.31)
OVERCURRENT UNIT
| > 0.8 pu
1
= 3 × (|I_0|) if |I | ≤ 0.8 pu
1
5-157
5