GE C70 Instruction Manual page 366

GROUPED ELEMENTS
There are two negative-sequence directional overcurrent protection elements available. The element provides both
forward and reverse fault direction indications through its output operands
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
forming the element operating quantity.
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)
5
• Fault inception and switch-off transients
The positive-sequence restraint must be considered when testing for pickup 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 C70. For single phase injection
= ⅓ × (1 – K) × I
• I
op
• I = (1 – K) × I
op
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 5-34: Negative-sequence directional overcurrent unit
Mode
Negative-sequence
Zero-sequence
Table 5-35: Negative-sequence directional unit
Direction Compared phasors
–V_2 + Z_offset × I_2
Forward
–V_2 + Z_offset × I_2
Reverse
–V_2 + Z_offset × I_2
Forward
Reverse –V_2 + Z_offset × I_2
5-200
NEG SEQ DIR OC1 REV

PICKUP: 0.050 pu
NEG SEQ DIR OC1 BLK:

Off
NEG SEQ DIR OC1

TARGET: Self-reset
NEG SEQ DIR OC1

EVENTS: Disabled
I = |I_2| - K x |I_1| or I
op
for I_2 mode
injected
for I_0 mode if I_1 > 0.8 pu
injected
Operating current
= |I_2| – K × I_1|
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
I
op
I_2 × 1∠ECA
–(I_2 × 1∠ECA)
I_2 × 1∠ECA
–(I_2 × 1∠ECA)
Range: 0.015 to 30.000 pu in steps of 0.005
Range: FlexLogic operand
Range: Self-reset, Latched, Disabled
Range: Disabled, Enabled
NEG SEQ DIR OC1 FWD
= 3 x (|I_0| - K x |I_1|)
op
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
and
NEG SEQ DIR OC1 REV,
Eq. 5-20