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Functions
2.14 Differential Protection and Its Protected Objects
Besides the second harmonic, another harmonic can be selected in the 7UM62 to cause stabilization. A choice
can be made between the third and fifth harmonic for harmonic stabilization.
Steady-state overexcitation is characterized by odd harmonics. The 3rd or 5th harmonic is suitable to detect
overexcitation. Because the third harmonic is often eliminated in power transformers (e.g. in a delta winding),
the use of the fifth is more common.
Converter transformers also produce odd harmonics which are practically absent in the case of an internal
short-circuit.
The differential currents are analysed for harmonic content. For frequency analysis digital filters are used which
perform a Fourier analysis of the differential currents. As soon as the harmonics content exceeds the set thresh-
olds, a stabilization of the respective phase evaluation is started. The filter algorithms are optimized for tran-
sient behaviour such that additional measures for stabilization during dynamic conditions are not necessary.
Harmonic stabilization is maintained for two cycles after decrease of the differential current. This prevents un-
wanted under-stabilization when external faults are cleared and higher-order harmonics disappear.
Since inrush restraint operates individually for each phase, the protection is fully operative even when the trans-
former is switched onto a single-phase fault, in which case it is possible for an inrush current to flow through
one of the undisturbed phases.
In "modern type" transformers in particular, the 2nd harmonics content may not exceed the threshold value in
all three phases on switch-on. To avoid spurious tripping, the so-called "crossblock" function must be activated.
As soon as an inrush current is detected in one phase, the other phases of the differential protection stage I-
DIFF> are blocked.
The "crossblock" function can be limited to a selectable duration. After this cross-block time has elapsed, no
further cross-block is possible for as long as a current fault condition lasts, i.e. cross-blocking is possible only
once after a fault has occurred, and only for the set cross-block time period.
The other harmonic stabilizations also operate individually for each phase. However, it is also possible —as it
is for inrush stabilization— to set protection such that on overshooting of the admissible harmonic content in
the current of only one phase, the other phases of the differential stage I-DIFF> are blocked. This cross-block
feature with 3rd or 5th harmonic works in the same way as with the 2nd harmonic.
Pickup Value Increase on Startup
An increase of the pickup value on startup provides additional security against overfunctioning when a non-
energized protection object is switched in. As soon as the stabilizing current of one phase has undershot a set-
table value I-REST. STARTUP, the increase of the pickup value is activated for the I-DIFF> stage. As the
stabilizing current is twice the through-flowing current in normal operation, its undershooting of that threshold
is a criterion for detecting that the protected object is not energized. The pickup value I-DIFF is now increased
by a settable factor (see following figure); the other branches of the I
This is done by dividing the DIFF current of the respective phase by the factor START-FACTOR before the char-
acteristic monitoring. The differential current for fault recording, tripping current etc. is not affected by this.
The return of the stabilizing current indicates the startup. After a settable time T START MAX the increase of
the characteristic is retracted.
108
> stage are displaced proportionally.
diff
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
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