ABB RELION REX640 Technical Manual page 643

1MRS759142 C
REX640
Technical Manual
Biased low stage
Biasing of the current differential protection is needed since a possible appearance of
a differential current can also be due to something else than an actual fault in the
transformer (or generator).
In the case of transformer protection, there can be several reasons for the false
differential current.
• CT errors
• Varying tap changer positions (if not automatically compensated)
• Transformer no-load current
• Transformer inrush currents
• Transformer overexcitation during overvoltage situations
• Transformer overexcitation during underfrequency situations
• CT saturation at high currents passing through the transformer
The differential current caused by CT errors and tap changer position increases at the
same percentage ratio as the load current.
In the protection of generators, the false differential current can be caused by several
reasons.
• CT errors
• CT saturation at high currents flowing through the generator
Section 4
Protection functions
637