Functions
2.6 Power Swing Detection (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
181
2.6
Power Swing Detection (optional)
The 7SD5 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 tripping during unstable power swings (out-of-
step tripping). To avoid uncontrolled tripping, the distance protection devices are supplemented with power
swing blocking functions. At particular locations in the system, out-of-step tripping devices are also applied to
split the system into islanded networks at selected locations, when system stability (synchronism) is lost due
to severe (unstable) power swings.
2.6.1
General
Following dynamic events such as load jumps, faults, reclose dead times or switching actions it is possible that
the generators must realign themselves, in an oscillatory manner, with the new load balance of the system. The
distance protection registers large transient currents during the power swing and, especially at the electrical
centre, small voltages (Figure 2-76). Small voltages with simultaneous large currents apparently imply small
impedances, which again could lead to tripping by the distance protection. In expansive networks with large
transferred power, even the stability of the energy transfer could be endangered by such power swings.
Figure 2-76
Measured quantities during a power swing
System power swings are three-phase symmetrical processes. Therefore a certain degree of measured value
symmetry may be assumed in general. System power swings may, however, also occur during asymmetrical
processes, e.g. after faults or during a single-pole dead time. Thus the power swing detection in the 7SD5 is
based on three measuring systems. For each phase, there is a measuring system that ensures phase-selective
power swing detection. In case of faults, the detected power swing is terminated in the corresponding phases,
which enables selective tripping of the distance protection.
