Figure 314: 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 315: 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 protection, any flashover in the CT secondary circuits or any other part of the
scheme can prevent the correct operation of MHZPDIF.
1MRS757644 H
Protection functions
620 series
Technical Manual
599
