Siemens siprotec 7SA6, Manual

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Functions
6-72
7SA6 Manual
C53000-G1176-C156-2
6.3 Measures to Be Taken in Case of Power Swings (optional)
Following dynamic events such as load jumps, short-circuits, reclose dead times or
switching actions it is possible that the generators must realign themselves, in an os-
cillatory manner, with the new load balance of the system. The distance protection
registers large transient currents during the power swing and, especially at the electri-
cal centre, small voltages (Figure 6-39). Small voltages with simultaneous large cur-
rents apparently imply small impedances, which again could lead to tripping by the dis-
tance protection. In expansive networks with large transferred power, even the stabil-
ity of the energy transfer could be endangered by such power swings.
Figure 6-39 Power swing
To avoid uncontrolled tripping, the distance protection devices are supplemented with
power swing blocking functions. At particular locations in the system, out-of-step trip-
ping devices are also applied to split the system into islanded networks at selected lo-
cations, when system stability (synchronism) is lost due to severe (unstable) power
swings.
The 7SA6 has an integrated power swing supplement which allows both the blocking
of trips by the distance protection during power swings (power swing blocking) and the
calculated tripping during unstable power swings (out-of-step tripping).
6.3.1 Method of Operation
System power swings are three phase symmetrical processes. Therefore in general a
certain degree of measured value symmetry may be assumed. System power swings
may however also occur during unsymmetrical processes, e.g. during two-phase
short-circuits or during single-pole dead times. The power swing detection in the 7SA6
is therefore based on three measuring systems. For each phase, a dedicated meas-
uring system is available. Even if a power swing has been detected, any subsequent
M
E
2
E
1
M
E
2
E
1
I
U
E
2
E
1
M
U
Z
1
Z
2
I
at the Measuring Location M
I
E
1
E
2
–
Z
1
Z
2
+
-------------------
=
Note:
The power swing supplement operates only in conjunction with the impedance pick-
up mode.