Toshiba GR200 Series Instruction Manual page 310

Phenomenon and principle of out-of-step
2.18.1
Occurrence of out-of-step
(i)
Figure 2.18-1 shows simple two power sources (machines) system; it introduces the discussion
of a transient operation. Electrical center is at which the impedances to the opposite terminals
are equal.
Bus G
V
S
Figure 2.18-1 Simplified system and electrical center
During the early stage of out-of-step, it is assumed that the electrical center loses
synchronism, but two terminals do not lost synchronism. Thereby, the OSTZ can disconnect
the power sources from the system, and each power source can keep operating steadily and
independently.
Impedance locus during out-of-step
(ii)
Figure 2.18-2 shows an example of the loci of impedance; it is drawn as an arc during the out-
of-step condition. The locus is observed to swing from right to left; the OSTZ function detects
the out-of-step condition when swinging the locus is observed.
Figure 2.18-2 Impedance locus during out-of-step condition seeing from distance relay
The user should notice that the OSTZ function does not operate except the out-of-step.
Electrical
center
CB
1
Relay A
Distance relay in MHO characteristic
X
(operation area is represented in hatching.)
Locus seen from Relay A
- 289 -
Bus H
CB
2
Relay B
Arc made impedance loci.
R
Load Area
6F2S1915 (0.46)
V
R
GRZ200 (Soft 031 & 032)