258
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
GROUPED PROTECTION ELEMENTS
CHAPTER 7: PROTECTION
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. 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.
Eq. 35
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 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, the restraint is removed by
setting the constant
K
to zero. This facilitates 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 0.02 pu to be validated for use as a
polarizing signal. If the polarizing signal is not validated neither forward nor reverse
indication is given. In addition, the negative-sequence current (or zero-sequence current)
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)
B
B
RU
B
B
,
.
,
,
,
.
,
,
RS
RS
î
²
î
²
î
