Parameter:
Gn 87T-BD > HB2 Element
•
Default setting:
Disabled
When a transformer is energized transient magnetizing inrush currents flow in each phase of the ener-
gized winding. Inrush currents only flow into 1 transformer winding and the resulting unbalance can be
sufficient to cause mal-operation of the biased differential elements. To prevent the relay operating for
this non-fault condition, the presence of even harmonics in the current is used to distinguish between
inrush currents and short circuit faults.
Harmonic bias is used to prevent relay operation during magnetizing inrush current into 1 winding when
the transformer is first energized. Always activate inrush-current detection with transformer applications.
Harmonic bias is used to prevent relay operation during magnetizing inrush current into 1 winding when
the transformer is first energized.
The recommended setting is
Enabled
.
Parameter:
Gn 87T-BD > HB2 Bias
•
Default setting:
Cross
This select the operating mode of the differential inrush blocking function element.
–
Cross
Each phase is monitored and if the even harmonic present in any phase exceeds the setting then all
3 phases are blocked. This method is used for the majority of applications of the relay to power
transformers.
–
Phase
The even harmonic content in each phase is measured independently and compared to the total
operate current in its own phase i.e. each phase of the biased differential elements is blocked by
even harmonic content in its own phase only.
This method is used with large transformers having 3 separate phase tanks each containing a phase
core. This construction facilitates transportation. Each of the phase cores is not magnetically
affected by the flux in the other phase cores. These large single phase transformers are often auto-
transformers used on EHV transmission systems.
Both ICT settings are selected to Yy e.g. reactor, motor, generator applications.
–
Sum
The level of even harmonic current (2nd and 4th) in the differential signal for each phase is meas-
ured and squared. The squared value of each of the phases is summed and then the square root of
this value is taken. This single current level is compared with the differential current in each phase
and in the phases that the ratio is larger than the setting then the differential protection of that
phase is inhibited.
If the operate current in any phase is greater than this harmonic sum then its differential element
will operate.
The advantage of this method is it allows fast operation of the biased differential element if the
transformer is switched onto an internal phase to earth fault. The cross method may suffer from
slowed operation for this situation, as healthy phase inrush may block all 3 phases (including the 1
feeding the fault current) from operating. The sum method is not slowed down when switching
onto an in zone earth fault, as the harmonic sum is reduced by the presence of the fault current and
therefore allows relay operation.
This setting is recommended if
REF
is not used to protect the windings for earth faults on effectively
earthed power systems.
Protection and Automation Functions
5.22 87T Transformer Differential Protection
Reyrolle 7SR5, Transformer Protection Device, Device Manual
315
C53000-G7040-C015-1, Edition 11.2019
