GE T60, Instruction Manual

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5-238 T60 Transformer Protection System GE Multilin
5.6 GROUPED ELEMENTS 5 SETTINGS
5
The neutral directional overcurrent element provides both forward and reverse fault direction indications the
NEUTRAL DIR
OC1 FWD
and
NEUTRAL DIR OC1 REV
operands, respectively. The output operand is asserted if the magnitude of the oper-
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.
(EQ 5.39)
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:
I
op
= (1 – K )
×
I
injected
; three-phase pure zero-sequence injection: I
op
= 3
×
I
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
PRODUCT SETUP

DISPLAY PROPERTIES

CUR-
RENT CUT-OFF LEVEL
setting value and IG must be greater than 0.05 pu to be validated as the operating quantity for direc-
tional current. The following tables define the neutral directional overcurrent element.
Table 5–32: QUANTITIES FOR "CALCULATED 3I0" CONFIGURATION
DIRECTIONAL UNIT
OVERCURRENT UNIT
POLARIZING MODE DIRECTION COMPARED PHASORS
Voltage
Forward –V_0 + Z_offset × I_0 I_0 × 1 ∠ ECA
I
op
= 3
×
(|I_0| – K
×
|I_1|) if |I
1
| > 0.8 pu
I
op
= 3 × (|I_0|) if |I
1
| ≤ 0.8 pu
Reverse –V_0 + Z_offset
×
I_0 –I_0
×
1
∠
ECA
Current
Forward IG I_0
Reverse IG –I_0
Dual, Dual-V, Dual-I
Forward
–V_0 + Z_offset × I_0 I_0 × 1 ∠ ECA
or
IG I_0
Reverse
–V_0 + Z_offset × I_0 –I_0 × 1 ∠ ECA
or
IG –I_0
Table 5–33: QUANTITIES FOR "MEASURED IG" CONFIGURATION
DIRECTIONAL UNIT
OVERCURRENT UNIT
POLARIZING MODE DIRECTION COMPARED PHASORS
Voltage
Forward –V_0 + Z_offset
×
IG/3 IG
×
1
∠
ECA
I
op
= |IG|
Reverse –V_0 + Z_offset × IG/3 –IG × 1 ∠ ECA
I
op
3 I_0 K I_1
×
–
( ) ×
=