ABB RED615 Technical Manual page 398

Section 4
Protection functions
392
The fifth harmonic blocking has a hysteresis to avoid rapid fluctuation between
"TRUE" and "FALSE". The blocking also has a counter, which counts the required
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:
• CT errors
• CT saturation at high currents passing through the generator.
1YHT530004D05 D
615 series
Technical Manual