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7UT51 v3
I
OP
1 . 2
*
I
0
1 . 0
0 . 8
0 . 6
CLASSICAL
TRIP AREA
0 . 4
0 . 2
0 . 0
0
3 0
6 0
EXTENDED TRIP AREA
Figure 7.5
Trip Area for I*o/Io** = 2
I
OP
1.2
*
I
0
1.0
0.8
0.6
CLASSICAL
TRIP AREA
0.4
0.2
0.0
0
30
60
EXTENDED TRIP AREA
Figure 7.6
Trip Area for I*o/Io**=4
Fault Detection
The algorithm detects that a fault has occurred if the
differential current, I
, exceeds a relay setting
D
(indicating that the ground current and zero sequence
current differ too much), or if the restraining current, I
exceeds another relay setting (indicating that the total
amount of current flowing through the transformer is
too high). Once a fault has been detected, further
analysis occurs. As with the classical solution, the
question to be answered is whether the fault is internal
(requiring a trip) or external (not requiring a trip).
PRIM-2330C
*
* *
=
I
I
/
2
0
0
BLOCK
AREA
9 0
1 2 0
1 5 0
1 8 0
ϕ
(degrees)
*
* *
=
I
/
I
4
0
0
BLOCK
AREA
k
0
= 1.03
1.37
2.0
4
4.0
6
90
120
150
180
ϕ
(degrees
)
Ground Differential Protection (87N)
7.2
Trip Decision
In theory, an external fault can be easily recognized
since the calculated quantities I
equal magnitudes and a phase angle difference of ϕ =
90°. In reality, inrush effects or CT-saturation may
distort the measured currents. CT-saturation can
affect both the perceived amplitudes of the
fundamental current vectors and the phase angle
between them.
Classical Trip Area: The algorithm calculates a value called
the "stabilization current," I
Vector analysis can show that the amplitude of the
stabilization current, I
angle ϕ between I
90°. In this case, the fault is internal, so a trip is
appropriate if the amplitude of a calculated "operating
current," I
, is above a minimum level (a relay
OP
setting):
*
=
I
I
OP
0
≥
I
I
Trip if
OP
Trip
Extended Trip Area: The algorithm extends the trip
area to recognize internal faults that the classical
solution will fail to respond to, while still avoiding an
improper trip if the fault is external.
If the phase angle ϕ is in the range 90° ≤ ϕ ≤ 270°
(outside the classical trip area), the magnitude of I
will be positive. In this case, the algorithm still bases
the trip decision on the amplitude of the operating
current, I
, but calculates I
OP
,
R
*
=
−
I
I
k
I
OP
0
0
STAB
(if 90° ≤ ϕ ≤ 270°)
Trip if
where k
, the "stabilization factor," is a relay setting
0
used to adjust the sensitivity of the protection when
90° ≤ ϕ ≤ 270°. Note that when ϕ is in that range, I
a function of four quantities: the amplitudes of the
currents
and
and the stabilization factor, k
* and I
** will have
0
0
:
STAB
, will be negative if the phase
STAB
** is in the range −90° ≤ ϕ ≤
* and I
0
0
(if −90° ≤ ϕ ≤ 90°)
(if −90° ≤ ϕ ≤ 90°)
_
SET
differently:
OP
(if 90° ≤ ϕ ≤ 270°)
, the phase angle between them,
:
0
7
(7.9)
(7.10)
(7.11)
STAB
(7.12)
(7.13)
is
OP
93
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