9 COMMISSIONING
e) INTERMEDIATE CURVE BETWEEN BREAKPOINT 1 AND BREAKPOINT 2
This procedure tests the intermediate section of the differential characteristic curve that lies between the Breakpoint 1 and
Breakpoint 2 points (points B
1
1.
Inject currents so that the magnitude of I
2; that is:
<
For this example,
2 pu I
WINDING 1
PHASE
SINGLE CURRENT (I
0 A ∠0°
A
1.2 A ∠0°
B
1.2 A ∠–180°
C
2.
The following differential and restraint current should be read from the T60 actual values menu:
PHASE
DIFFERENTIAL CURRENT (I
0 ∠0°
A
1.287 pu ∠–180°
B
1.287 pu ∠0°
C
⁄
The
I
I
ratio is 36.77% and the Differential element does not operate because the actual
d
r
low at
I
3.5 pu
.
=
r
Due to the mathematical complexity involved in shaping the curve between Breakpoint 1 and Breakpoint 2, an
Excel-based simulation tool is available from the GE Multilin website at http://www.GEindustrial.com/multilin.
NOTE
With this tool, the user can see the preset
test currents. The tool graphically indicates differential and restraint current magnitudes and indicates whether
the relay should operate.
3.
In this example, a ratio of
the differential current I
, causing the element to operate.
d
WINDING 1
PHASE
SINGLE CURRENT (I
0 A ∠0°
A
1.1 A ∠0°
B
1.1 A ∠–180°
C
4.
The following differential and restraint current should be read from the T60 actual values menu:
PHASE
DIFFERENTIAL CURRENT (I
0 ∠0°
A
1.471 pu ∠–180°
B
1.471 pu ∠0°
C
GE Multilin
and B
on the Differential Restraint Characteristic diagram).
2
is between the restraint magnitudes defined by Breakpoint 1 and Breakpoint
r
(
) I
<
<
I
at Breakpoint 1
r
r
<
8 pu
. Remember that the maximum current is the restraint current
r
WINDING 2
)
PHASE
SINGLE CURRENT (I
1
0 A ∠0°
A
3.5 A ∠–180°
B
3.5 A ∠0°
C
)
PHASE
RESTRAINT CURRENT (I
d
0 ∠0°
A
3.5 pu ∠–180°
B
3.5 pu ∠0°
C
⁄
I
I
d
r
⁄
>
I
I
38%
causes the element to trip. Decreasing I
d
r
WINDING 2
)
PHASE
SINGLE CURRENT (I
1
0 A ∠0°
A
3.5 A ∠–180°
B
3.5 A ∠0°
C
)
PHASE
RESTRAINT CURRENT (I
d
0 ∠0°
A
3.5 pu ∠–180°
B
3.5 pu ∠0°
C
T60 Transformer Protection System
9.2 DIFFERENTIAL CHARACTERISTIC TEST EXAMPLES
(
)
I
at Breakpoint 2
r
)
2
)
r
curve point ratios and the actual
as shown in the table below increases
1
)
2
)
r
(EQ 9.10)
I
3.5 pu
.
=
r
I
1.287 pu
is still too
=
d
⁄
I
I
ratio as per the entered
d
r
9-7
9