CHAPTER 7: PROTECTION GROUPED PROTECTION ELEMENTS
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL 261
Negative-sequence directional overcurrent
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. Do not mistake the zero-sequence current with the neutral current, as there
is a factor of three difference.
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.
Eq. 35
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 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 setting 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, as the
current is low.
The operating quantity depends on the way the test currents are injected into the D90
Plus
.
For single-phase injection,
I
op
= ⅓ × (1 – K) × I
injected
for I_2 mode and
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 16: Negative-sequence directional overcurrent unit
Table 17: Negative-sequence directional unit
The negative-sequence voltage must be greater than the Voltage Cutoff Level of the
general voltage inputs (found under the Protection > Power System > AC Inputs - Voltage
settings) to be validated for use as a polarizing signal. If the polarizing signal is not
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
Mode Direction Compared phasors
Negative-sequence Forward –V_2 + Z_offset × I_2 and I_2 × 1∠ECA
Reverse –V_2 + Z_offset × I_2 and –(I_2 × 1∠ECA)
Zero-sequence Forward –V_2 + Z_offset × I_2 and I_2 × 1∠ECA
Reverse –V_2 + Z_offset × I_2 and –(I_2 × 1∠ECA)
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