ABB RELION REX640 Technical Manual page 729

1MRS759142 C
REX640
Technical Manual
High-impedance principle
The high-impedance principle is stable for all types of faults outside the protection
zone. The stabilization is obtained by a stabilizing resistor in the differential circuit.
This method requires all the CTs to have a similar magnetizing characteristic, same
ratio and a relatively high knee point voltage. The CTs in each phase are connected in
parallel with a relay measuring branch. The measuring branch is a series connection
of the stabilizing resistor and the protection relay.
The stability of the protection is based on the use of the stabilizing resistor (R
fact that the impedance of the CT secondary quickly decreases as the CT 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.
GUID-BCD60482-4C12-48B9-8A85-39A8B6631844 V1 EN
Figure 401: Three-phase differential protection for motors based on high-
impedance 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.
Section 4
Protection functions
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