Toshiba GR200 Series Instruction Manual page 311

Figure 2.18-3 shows another impedance locus when transient power swing occurs; the OSTZ
function does not detect the out-of-step condition when this impedance locus is observed†.
Figure 2.18-3 Impedance locus during transient power swing
†Note: Distance protection function can detect such impedance locus as the occurrence of
power swing. For more information, see Chapter
protection
Function features
2.18.2
The OSTZ function has the elements of distance protection relays; the out of step condition can
be detected using these elements. The user should notice that the OSTZ function requires a
certain amount of time to decide the out of step condition because the OSTZ function should
observe the impedance locus and its swing. (As a rule, the time is dozens of milliseconds) Thus,
the user should keep in mind that the OSTZ function is not suitable to detect the transient
out-of-step condition. Additionally, the OSTZ function cannot recognize the electrical center on
the system being in complex. Thereby, the OSTZ is operated for star system rather than loop
system.
Operation zones and element characteristics
2.18.3
The OSTZ function has three operation zones: Zone A, Zone B, and Zone C (Figure 2.18-4). The
three zones are made by overlapping two elements: OSTZ-ZM element and OSTZ-ZN element.
The OSTZ-ZM element has a quadrilateral form, which are configured by reactance-lines being
parallel to R-axis and ohm-lines 75° leading of R-axis. On the other hand, the OSTZ-ZN
characteristics can be configured by shifting the OSTZ-ZM element in minus R-axis (the gap
between two elements = OSTZ-R1 − OSTZ-R2). The following equations represent the OSTZ-
ZM element:
(R − R1) × tan 75° ≤ X ≤ (R − R2) × tan75°
Distance relay in MHO characteristic
X
(operation area is represented in hatching.)
Locus seen from Relay A
.
−1 × XB ≤ X ≤ XF
- 290 -
Impedance locus
R
Load Area
Relay application: Distance
6F2S1915 (0.46)
(2.18-1)
(2.18-2)
GRZ200 (Soft 031 & 032)