Instantaneous Differential Currents; Harmonic And Waveform Block Criteria - ABB REG650 Technical Manual

Section 5
Differential protection
5.1.3.10
5.1.3.11
88

Instantaneous differential currents

The instantaneous differential currents are calculated from the instantaneous values
of the input currents in order to perform the harmonic analysis and waveform
analysis upon each one of them (see section
criteria" for more information).

Harmonic and waveform block criteria

The two block criteria are the harmonic restrain and the waveform restrain. These
two criteria have the power to block (that is, to prevent) a trip command by the
traditional differential protection, which produces start signals by applying the
differential currents, and the bias current, to the operate - restrain characteristic.
Harmonic restrain
The harmonic restrain is the classical restrain method traditionally used with power
transformer differential protections. The goal is to prevent an unwanted trip
command due to magnetizing inrush currents at switching operations, or due to
magnetizing currents at over-voltages.
The magnetizing currents of a power transformer flow only on one side of the
power transformer (one or the other) and are therefore always the cause of false
differential currents. The harmonic analysis (the 2
applied to instantaneous differential currents. Typically instantaneous differential
currents during power transformer energizing are shown in figure 37. The
harmonic analysis is only applied in those phases, where start signals have been
set. For example, if the content of the 2
current of phase L1 is above the setting I2/I1Ratio, then a block signal is set for
that phase.
Waveform restrain
The waveform restrain criterion is a good complement to the harmonic analysis.
The waveform restrain is a pattern recognition algorithm, which looks for intervals
within each fundamental power system cycle with low instantaneous differential
current. This interval is often called current gap in protection literature. However,
within differential function this criterion actually searches for long-lasting intervals
with low rate-of-change in instantaneous differential current, which are typical for
the power transformer inrush currents. Block signal BLKWAV is set in those
phases where such behavior is detected. The algorithm do not requires any end user
settings. The waveform algorithm is automatically adapted dependent only on the
power transformer rated data.
"Harmonic and waveform block
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harmonic in the instantaneous differential
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