5.H
parameter, the blocking removal is enabled. The enabling and disabling of
deblocking feature is also done through the
Harmonic deblock 5.H
parameter.
Figure 390: The limits and operation of the fifth harmonic blocking when both
blocking and deblocking features are enabled using the Harmonic deblock 5.H
control parameter.
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
Protection functions
1MRS759142 F
688 REX640
Technical Manual
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