ABB RELION REX640 Technical Manual page 607

1MRS759142 C
REX640
Technical Manual
The differential current caused by CT errors or tap changer positions increases at the
same percent ratio as the load current.
In the protection of generators, the false differential current can be caused by various
factors.
• CT errors
• CT saturation at high currents passing through the generator
GUID-4B50EEB0-EF56-4277-9C48-8A284CC223CA V1 EN
Figure 332: Operation logic of the biased low stage
The high currents passing through a protected object can be caused by the short
circuits outside the protected area, the large currents fed by the transformer in motor
startup or the transformer inrush situations. Therefore, the operation of the differential
protection is biased in respect to the load current. In biased differential protection, the
higher the differential current required for the protection to operate, the higher the load
current.
Section 4
Protection functions
601