Waveform restrain
M13039-341 v4
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 BLKWAV_A(B,C) 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.
I_A
I_B
I_C
en05000343_ansi.vsd
Time [cycles]
400kV Currents
C
u
r
r
e
n
t
[
%
]
ANSI05000343 V1 EN-US
Figure 40: Inrush currents to a transformer as seen by a protective IED. Typical is a high
amount of the 2
nd
harmonic, and intervals of low current, and low rate-of-change
of current within each period.
Cross-blocking between phases
M13039-384 v8
The basic definition of the cross-blocking is that one of the three phases can block operation (that
is, tripping) of the other two phases due to the harmonic pollution of the differential current in
that phase (that is, waveform, 2
nd
or 5
th
harmonic content). In differential algorithm the user can
control the cross-blocking between the phases via the setting parameter
CrossBlockEn
=
Enabled
.
When parameter
CrossBlockEn
=
Enabled
cross blocking between phases is introduced. There is no
time settings involved, but the phase with the operating point above the set bias characteristic (in
the operate region) will be able to cross-block the other two phases if it is itself blocked by any of
the previously explained restrained criteria. As soon as the operating point for this phase is below
the set bias characteristic (that is, in the restrain region) cross blocking from that phase will be
inhibited. In this way cross-blocking of the temporary nature is achieved. It should be noted that
this is the default setting value for this parameter.
When parameter
CrossBlockEn
=
Disabled
, any cross blocking between phases will be disabled. It is
recommended to use the value
Disabled
with caution in order to avoid the unwanted tripping
during initial energizing of the power transformer.
Switch onto fault feature
M13039-499 v7
The transformer differential function has a built-in, advanced switch onto fault feature. This
feature can be enabled or disabled by a setting parameter
SOTFMode
. When enabled this feature
Section 6 1MRK 502 066-UUS B
Differential protection
136
Technical manual
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