Differential Protection (ANSI 87G/87M/87T)
83
7UM62 Manual
C53000-G1176-C149-3
High-Speed Trip
Stage I
Diff
>>
The high-speed trip stage I
Diff
>> clears high-current internal faults instantaneously.
As soon as the differential current rises above the threshold I
Diff
>> (branch d), a trip
signal is issued regardless of the magnitude of the stabilizing current.
This stage can operate even when, for example, a considerable second harmonic is
present in the differential current, which is caused by current transformer saturation by
a DC component in the short-circuit current, and which could be interpreted by the
inrush stabilization function as an inrush current.
This high-current stage evaluates the fundamental component of the differential
current as well as the instantaneous values. Instantaneous value processing ensures
fast tripping even in case the fundamental component of the current is strongly
reduced by current transformer saturation.
High-current faults in the protected transformer may be cleared instantaneously
without regard of the magnitude of the stabilizing currents when the amplitude of the
differential currents can exclude that it is an external fault. This is always the case
when the short-circuit current is higher than 1/u
sc
⋅
I
N Transf
.
Add-on
Stabilization During
Current
Transformer
Saturation
During an external fault which produces a high through flowing short-circuit current
causing current transformer saturation, a considerable differential current can be
simulated, especially when the degree of saturation is different at the two measuring
points. If the quantities I
diff
/I
stab
result in an operating point which lies in the trip area
of the operating characteristic (Figure 2-37), a trip signal would be the consequence if
no special measures were taken.
Figure 2-37
Operation Characteristics of the Differential Protection with Fault Characteristic
The 7UM62 provides a saturation indicator which detects such phenomena and
initiates add-on stabilization measures. The saturation indicator evaluates the
dynamic behaviour of the differential and stabilizing current.
The dotted line in Figure 2-37 shows the instantaneous development of currents in
case of a external fault with transformer saturation on one side.
1
2
3
4
5
6
7
8
9
10 11 12
13 14 15 16 17 18
1
2
3
4
5
6
7
8
9
10
2044
BASE POINT 2
BASE POINT 1
2042
Trip area
I
diff
I
N
---------
I
stab
I
N
------------
Fault characteristic
A
B
C
Block area
Add-on stabilization
Saturation
D
