ABB RELION 620 Series Technical Manual page 522

Section 4
Protection functions
516
consecutive fulfillments of the condition. When the condition is not fulfilled, the
counter is decreased (if >0).
Also the fifth harmonic deblocking has a hysteresis and a counter which counts the
required consecutive fulfillments of the condition. When the condition is not fulfilled,
the counter is decreased (if >0).
Waveform blocking
The biased low stage can always be blocked with waveform blocking. The stage can
not be disabled with the Restraint mode parameter. This algorithm has two parts. The
first part is intended for external faults while the second is intended for inrush
situations. The algorithm has criteria for a low current period during inrush where also
the differential current (not derivative) is checked.
Biased low stage
The current differential protection needs to be biased because the possible appearance
of a differential current can be due to something else than an actual fault in the
transformer (or generator).
In the case of transformer protection, a false differential current can be caused by:
• CT errors
• Varying tap changer positions (if not automatically compensated)
• Transformer no-load current
• Transformer inrush currents
• Transformer overexcitation in overvoltage
• Underfrequency situations
• CT saturation at high currents passing through the transformer
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
1MRS757644 F
620 series
Technical Manual