ABB RELION REX640 Technical Manual page 794

Protection and control

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Protection functions

Figure 451: Operating characteristic for the stabilized stage of the generator

differential protection function

The differential protection works on the principle of calculating the differential

current at the two ends of the winding, that is, the current entering the winding

is compared to the current exiting the winding. In case of any internal fault,

the currents entering and exiting the winding are different, which results in a

differential current, which is then used as a base for generating the operating

signal. Due to this principle, the differential protection does not trip during external

faults. However, it should be noted that interturn faults in the same phase are

usually not detected unless they developed into some other kind of fault.

The short circuit between the phases of the stator windings normally causes

high fault currents. The short circuit creates a risk of damages to the insulation,

windings and stator core. The high short circuit currents cause strong current forces

which can damage other components in the machine. The short circuit can also

initiate explosion and fire. When a short circuit occurs in a machine, there is a

damage that has to be repaired. The severity and the repair time depend on the

degree of damage, which is highly dependent on the fault time. The fast fault

clearance of this fault type is of greatest importance to limit the damages and the

economic loss.

To limit the damages in connection to the stator winding short circuits, the fault

clearance time must be as short as possible (instantaneous). The fault current

contributions from both the external power system (via the machine or the block

circuit breaker) and from the machine itself must be disconnected as fast as

1MRS759142 F

Technical Manual