ABB REG650 ANSI, Technical Manual

Page: 109 / 728

IEC05000343 V1 EN
Figure 36: Inrush currents to a transformer as seen by a protection 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
With the cross-blocking function, one of the three phases can block operation 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 the differential algorithm the user
can control the cross-blocking between the phases via the setting parameter
CrossBlockEn .
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. If the
start signal in this phase is removed, that is, reset from TRUE to FALSE, 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 (recommended) 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.
6.1.3.12 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 a more severe (minor faults cannot be
discovered) internal fault. For example, a forgotten earthing on the transformer LV
side. The feature is based on the waveform check. If a severe internal fault exists, then,
during energization the magnetic density in the iron core will be low and high
1MRK 502 048-UEN A
Section 6
Differential protection
103
Technical manual