Figure 3.44-1 VTS criterion logic diagram
3.44.3 Protection Principle
Each side of the device is furnished with VT line-break alarm element
In case start-up element fails to activate voltage quantity related protection, VT line-break alarm
signal would be given with a delay of 8s once any of the following conditions is satisfied:
Voltage at each phase is less than 30V;
Negative-sequence voltage is greater than 8V.
When voltage switch-on hard strap of some certain side is disabled, VT line-break judgment
function of this side is automatically lifted.
3.45 Faulty Phase Selection (FPS)
3.45.1 Overview
Faulty phase selection is used to discriminate faulty phase for all kinds of fault type. If protection
element operates, faulty phase selection is succeeded and the device output tripping command.
Faulty phase selection is adaptive to both earthed system and unearthed system. For the
unearthed system, earthed protection elements should be disabled, such as earth fault protection,
and only phase-to-phase protection elements are enabled, such as phase overcurrent protection.
Detecting the phase difference between I0 and I2A
When phase overcurrent element operates, pickup phase due to overcurrent is judged as a faulty
phase.
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