ABB REG650 ANSI, Technical Manual

Page: 108 / 728

6.1.3.10 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
"Harmonic and waveform block criteria" for more
information).
6.1.3.11 Harmonic and waveform block criteria
The two blocking criteria are the harmonic restrain and the waveform restrain. These
two criteria have the power to block a trip command by the restrained differential
protection and sensitive negative sequence based turn-to-turn fault protection.
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
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
36 . 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.
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. The block signal BLKWAV is 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
1MRK 502 048-UEN A
Differential protection
102
Technical manual