ABB RELION REX640 Technical Manual page 808

Protection and control

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Protection functions

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

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

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

1MRS759142 F

) and lead resistance

Technical Manual