GE D90 Plus Instruction Manual page 227

CHAPTER 7: PROTECTION
Negative-sequence directional overcurrent
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
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
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 zero-sequence current
as per user selection. The zero-sequence current should not be mistaken with the neutral
current, as there is a factor of 3 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.
The positive-sequence restraint allows for more sensitive settings by counterbalancing
spurious negative 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 operating quantity depends on the way the test
currents are injected into the relay:
• Single-phase injection: I
• Three-phase pure zero-sequence or negative-sequence injection: I
• The directional unit uses the negative-sequence current and negative-sequence
voltage for fault direction discrimination.
The following tables define the negative-sequence directional overcurrent element.
Table 16: Negative-sequence directional overcurrent unit
Mode
Negative-sequence
Zero-sequence
Table 17: Negative-sequence directional unit
Mode Direction
Negative-sequence Forward
Reverse
Zero-sequence Forward
Reverse
The negative-sequence voltage must be greater than the
validated for use as a polarizing signal. If the polarizing signal is not validated neither
forward nor reverse indication is given. The following figure explains the usage of the
voltage polarized directional unit of the element.
and
NEG SEQ DIR OC1
= ⅓ × (1 – K) × I .
op injected
Operating current
I = |I_2| – K × I_1|
op
I = |I_0| – K × I_1|
op
Compared phasors
–V_2 + Z_offset × I_2 and I_2 × 1∠ECA
–V_2 + Z_offset × I_2 and –(I_2 × 1∠ECA)
–V_0 + Z_offset × I_0 and I_0 × 1∠ECA
–V_0 + Z_offset × I_0 and –(I_0 × 1∠ECA)
GROUPED PROTECTION ELEMENTS
REV, respectively. The output
= I .
op injected
value to be
Voltage Cut-Off Level
Eq. 33
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