GE Multilin G30 Generator Protection System 5-183
5 SETTINGS 5.6 GROUPED ELEMENTS
5
There are two negative-sequence directional overcurrent protection elements available. The element provides both forward
and reverse fault direction indications through its output operands NEG SEQ DIR OC1 FWD and NEG SEQ DIR OC1 REV,
respectively. The output operand is asserted if the magnitude of the 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 of the element essentially responds to the magnitude of a fundamental frequency phasor of either the
negative-sequence or neutral current as per user selection.
A positive-sequence restraint is applied for better performance: a small user-programmable portion of the positive-
sequence current magnitude is subtracted from the negative or zero-sequence current magnitude, respectively, when form-
ing the element operating quantity.
(EQ 5.43)
The positive-sequence restraint allows for more sensitive settings by counterbalancing spurious negative-sequence and
zero-sequence currents resulting from:
• System unbalances under heavy load conditions.
• Transformation errors of current transformers (CTs).
• Fault inception and switch-off transients.
The positive-sequence restraint must be considered when testing for pick-up accuracy and response time (multiple of
pickup). The positive-sequence restraint is removed for low currents. If the positive-sequence current is less than 0.8 pu,
then the restraint is removed by changing the constant K to zero. This results in better response to high-resistance faults
when the unbalance is very small and there is no danger of excessive CT errors, since the current is low.
The operating quantity depends on the way the test currents are injected into the G30. For single phase injection:
• I
op
= ⅓ × (1 – K)×I
injected
for I_2 mode.
• I
op
= (1 – K)×I
injected
for I_0 mode if I_1 > 0.8 pu.
The directional unit uses the negative-sequence current (I_2) and negative-sequence voltage (V_2).
The following tables define the negative-sequence directional overcurrent element.
Table 5–31: NEGATIVE-SEQUENCE DIRECTIONAL OVERCURRENT UNIT
Table 5–32: NEGATIVE-SEQUENCE DIRECTIONAL UNIT
Z_offset is the offset impedance, for which magnitude is the OFFSET setting and angle is the FWD ECA.
The negative-sequence voltage must be greater than 0.02 pu to be validated for use as a polarizing signal. Additionally,
when offset impedance is applied and negative-sequence current is above 0.2 pu, compensated negative-sequence volt-
age -V_2 + Z_offset x I_2 has to be above 0.02 pu in order to discriminate fault direction; otherwise when negative-
sequence current is less than 0.2 pu, -V_2 is then used as the polarizing signal. If the polarizing signal is not validated nei-
ther forward nor reverse indication is given. The following figure explains the usage of the voltage polarized directional unit
of the element.
The following figure shows the phase angle comparator characteristics for a phase A to ground fault, with settings of:
ECA = 75° (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)
MESSAGE
NEG SEQ DIR OC1
EVENTS: Disabled
Range: Disabled, Enabled
MODE OPERATING CURRENT
Negative-sequence I
op
= |I_2| – K × I_1|
Zero-sequence 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
DIRECTION COMPARED PHASORS
Forward –V_2 + Z_offset × I_2 I_2 × 1∠ECA
Reverse –V_2 + Z_offset × I_2 –(I_2 × 1∠ECA)
I
op
I_2 K I_1×–= or I
op
3I_0K I_1×–()×=
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