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Another OSTV element (OSTV-2) is provided when the protection is carried out by three
IEDs. The characteristic of the OSTV-2 element is the same as the characteristic of the OSTV-
1 element.
Note that to detect the state of out-of-step, phasor V
B
should stay at each quadrant for
the setting time (1.5 cycles). This is because that the influence of any VT transient should be
cancelled. The voltages in positive-sequence are used for the calculation of the OSTV function;
and when amplitudes are larger than 1V, the operation of the OSTV function is valid.
Figure 2.27-3 Voltage phase comparison between terminal A and terminal C (OSTV-2)
A
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