used for these calculations. The only difference is that the matrix algorithm is fed by
instantaneous values of currents, that is, samples.
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 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 and are therefore always the cause of false differential currents. The
harmonic analysis (the 2
nd
and the 5
th
harmonic) is applied to the instantaneous
differential currents. Typical instantaneous differential currents during power
transformer energizing are shown in figure
. The harmonic analysis is only applied
in those phases, where start signals have been set. For example, if the content of the 2
nd
harmonic in the instantaneous differential current of phase L1 is above the setting
I2/
I1Ratio
, then a block signal is set for that phase, which can be read as BLK2HL1
output of the differential protection.
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 signals BLKWAVLx are set in those phases where
such behavior is detected. The algorithm does not require any end user settings. The
waveform algorithm is automatically adapted dependent only on the power
transformer rated data.
Section 6
1MRK502052-UEN B
Differential protection
132
Technical manual
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