ABB RELION 620 Series Technical Manual page 597

1MRS757644 F
620 series
Technical Manual
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Figure 304: 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.
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Figure 305: Secondary waveform of a saturated CT
At an internal fault, the secondary circuit voltage can easily exceed the isolation
voltage of the CTs, connection wires and the protection relay. To limit this voltage, a
voltage-dependent resistor (VDR) is used.
The whole scheme, that is, the stabilizing resistor, voltage-dependent resistor and
wiring, must be adequately maintained (operation and insulation tested regularly) to
be able to withstand the high-voltage pulses that appear during an internal fault
throughout the lifetime of the equipment. Otherwise, during a fault within the zone of
Section 4
Protection functions
591