ABB REG670 Technical Manual page 139

1MRK502052-UEN B
Technical manual
IEC05000343 V1 EN
Figure 37: Inrush currents to a transformer as seen by a protective IED. Typical
is a high amount of the 2
low rate-of-change of current within each period.
Cross-blocking between phases
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
differential algorithm the user can control the cross-blocking between the phases via
the setting parameter CrossBlockEn=On.
When parameter CrossBlockEn=On 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=Off, any cross blocking between phases will be
disabled. It is recommended to use the value Off with caution in order to avoid the
unwanted tripping during initial energizing of the power transformer.
Switch onto fault feature
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 ensures quick differential protection tripping in cases
where a transformer is energized with an internal fault (for example, forgotten
earthing on transformer LV side). Operation of this feature is based on the fact that a
current gap (term current gap is explained under waveblock feature above) will exist
within the first power system cycle when healthy power transformer is energized. If
this is not the case the waveblock criterion will reset quickly. This quick reset of the
Differential protection
nd
harmonic, and intervals of low current, and
nd th
or 5 harmonic content). In
Section 6
133