ABB RELION REX640, Техническое руководство, Страница: 808 / 1960

saturates. The magnetization reactance of a fully saturated CT drops to zero and the
impedance is formed only by the resistance of the winding (R
in
) and lead resistance
(R
m
).
The CT saturation causes a differential current which can flow through the
saturated CT, because of the near-zero magnetizing reactance, or through the
measuring branch. The stabilizing resistor is selected so that the current in the
measuring branch is below the protection relay's operating current during out-of-
zone faults. As a result, the operation is stable during the saturation and can still be
sensitive at the undistorted parts of the current waveform.
Figure 460: Three-phase differential protection for motors based on highimpedance
principle
In case of an internal fault, the fault current cannot circulate through the CTs. It
flows through the measuring branch, and the protection operates. A partial CT
saturation can occur in case of an internal fault, but the undistorted part of the
current waveform causes the protection to operate.
Figure 461: Secondary waveform of a saturated CT
Protection functions 1MRS759142 F
808 REX640
Technical Manual