ABB RELION REX640 Technical Manual page 689

1MRS759142 F
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
Figure 391: 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 high currents fed by the transformer in
motor start-up 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.
The operating characteristic of the biased low stage is determined by
value , Slope section 2 and the setting of the second turning point of the operating
characteristic curve,
REX640
Technical Manual
End section 2 (the first turning point is fixed). The settings are
Protection functions
Low operate
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