GE C70 Instruction Manual page 352

GROUPED ELEMENTS
5
The neutral directional overcurrent element provides both forward and reverse fault direction indications for the
and
DIR OC1 FWD NEUTRAL DIR OC1 REV
operating 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 either the neutral current
calculated 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
performance: 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
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.
5-186
NEUTRAL DIR OC1

SOURCE: SRC 1
NEUTRAL DIR OC1

POLARIZING: Voltage
NEUTRAL DIR OC1 POL

VOLT: Calculated V0
NEUTRAL DIR OC1 OP

CURR: Calculated 3I0
NEUTRAL DIR OC1 POS-

SEQ RESTRAINT: 0.063
NEUTRAL DIR OC1

OFFSET: 0.00 Ω
NEUTRAL DIR OC1 FWD

ECA: 75° Lag
NEUTRAL DIR OC1 FWD

LIMIT ANGLE: 90°
NEUTRAL DIR OC1 FWD

PICKUP: 0.050 pu
NEUTRAL DIR OC1 REV

LIMIT ANGLE: 90°
NEUTRAL DIR OC1 REV

PICKUP: 0.050 pu
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
I = 3 x (|I_0| - K x |I_1|)
op
; three-phase pure zero-sequence injection: I
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
Range: SRC 1, SRC 2, SRC 3, SRC 4, SRC 5, SRC 6
Range: Voltage, Current, Dual, Dual-V, Dual-I
Range: Calculated V0, Measured VX
Range: Calculated 3I0, Measured IG
Range: 0.000 to 0.500 in steps of 0.001
Range: 0.00 to 250.00 Ω in steps of 0.01
Range: –90 to 90° in steps of 1
Range: 40 to 90° in steps of 1
Range: 0.006 to 30.000 pu in steps of 0.001
Range: 40 to 90° in steps of 1
Range: 0.006 to 30.000 pu in steps of 0.001
Range: FlexLogic operand
Range: Self-reset, Latched, Disabled
Range: Disabled, Enabled
= 3 × I
.
op injected
CHAPTER 5: SETTINGS
NEUTRAL
Eq. 5-18