ABB 650 series, Technical Manual

Page: 115 / 778

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 pickup signals have been set. For example, if the content
of the 2
nd
harmonic in the instantaneous differential current of phase A 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.
1MRK 502 048-UUS A
Section 6
Differential protection
109
Technical manual