ABB 615 series Technical Manual page 512

Section 4
Protection functions
506
The position value is available through the Monitored data view on LHMI or through other
communication tools in the tap position indication function. When the quality of the
TAP_POS value is not good, the position information in TAP_POS is not used but the last
value with the good quality information is used instead. In addition, the minimum
sensitivity of the biased stage, set by the Low trip value setting, is automatically
desensitized with the total range of the tap position correction. The new acting low trip
value is
Desensitized Low trip value = Lowtrip value ABS MaxWinding t
GUID-2E5AD399-D8DD-4F64-A194-7540D55DB8ED-ANSI V1 EN
Second harmonic blocking
The transformer magnetizing inrush currents occur when energizing the transformer after
a period of de-energization. The inrush current can be many times the rated current and the
halving time can be up to several seconds. To the differential protection, the inrush current
represents a differential current, which would cause the differential protection to trip
almost always when the transformer is connected to the network. Typically, the inrush
current contains a large amount of second harmonics.
Blocking the operation of the 87T biased low stage at a magnetizing inrush current is
based on the ratio of the amplitudes of the second harmonic digitally filtered from the
differential current and the fundamental frequency (Id2f /Id1f).
The blocking also prevents unwanted operation at the recovery and sympathetic
magnetizing inrushes. At the recovery inrush, the magnetizing current of the transformer
to be protected increases momentarily when the voltage returns to normal after the
clearance of a fault outside the protected area. The sympathetic inrush is caused by the
energization of another transformer running in parallel with the protected transformer
already connected to the network.
The ratio of the second harmonic to a fundamental component can vary considerably
between the phases. Especially when the delta compensation is done for a Ynd1 connected
transformer and the two phases of the inrush currents are otherwise equal but opposite in
phase angle, the subtraction of the phases in a delta compensation results in a very small
second harmonic component.
Some measures have to be taken in order to avoid the false tripping of a phase having too
low a ratio of the second harmonic to the fundamental component. One way could be to
always block all the phases when the second harmonic blocking conditions are fulfilled in
at least one phase. The other way is to calculate the weighted ratios of the second harmonic
to the fundamental component for each phase using the original ratios of the phases. The
latter option is used here. The second harmonic ratios I_2H_RAT_x are given in
Monitored data.
The ratio to be used for second harmonic blocking is, therefore, calculated as a weighted
average on the basis of the ratios calculated from the differential currents of the three
( a a p Min winding tap
+ −
1MAC059074-MB A
× ) Step of tap
(Equation 86)
615 series ANSI
Technical Manual