ABB RELION REX640 Technical Manual page 809

Protection and control

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1MRS759142 F

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.

Flux-balancing principle

In a measuring configuration for the three-phase differential currents according to

the flux-balancing principle, no stabilizing resistors are needed. The configuration,

however, requires the use of core balance current transformers. The compared

currents, the one at the line end and the other at the neutral end, are both measured

by the same core balance current transformer.

In this scheme, the currents flowing through one core balance transformer cancel

each other out when there is no fault within the protected zone. When a fault

occurs within the protected zone, the currents flowing through the core balance

transformer amplify each other and the differential protection operates.

Figure 462: Three-phase differential protection for motors based on fluxbalancing

The advantage of this scheme is that the CT rated primary current can be selected

smaller than the rated current of the machine.

Technical Manual

If six current transformers are used, the flux-balancing principle, that

is, summing two CTs in each phase, cannot be used. Instead, the high-

Protection functions