Siemens SIPROTEC 7UT613 series Manual page 108

2 Functions
Identification of DC
Components
Harmonic Stabilisa-
tion
108
The add-on restraint acts individually per phase. It can be determined by a setting pa-
rameter whether only the phase with detected external fault is blocked when this re-
straint criterion is fulfilled or also the other phases of the differential stage.
A further stabilisation (restraint) comes into effect when differential secondary currents
are simulated by different transient behaviour of the current transformer sets. This dif-
ferential current is caused by different DC time constants in the secondary circuits
during through-current conditions, i.e. the equal primary DC components are trans-
formed into unequal secondary DC components due to different time constants of the
secondary circuits. This produces a DC component in the differential current which in-
creases the pickup values of the differential stage for a short period.
A further restraint comes into effect when differential secondary currents are simulated
by different transient behaviour of the current transformer sets. This differential current
is caused by different DC time constants in the secondary circuits during through-
current conditions, i.e. the equal primary DC components are transformed into
unequal secondary DC components due to different time constants of the secondary
circuits. This produces a DC component in the differential current which increases the
pickup values of the differential stage for a short period. In this case, characteristic 1
is increased by factor 2.
In transformers and shunt reactors in particular, high short-time magnetising currents
may be present during power-up (inrush currents). These currents enter the protected
zone but do not exit it again. They thus produce differential quantities, as they seem
like single-end fed fault currents. Also during parallel connection of transformers, or an
overexcitation of a power transformer, differential quantities may occur due to magne-
tising currents caused by increased voltage and/or decreased frequency.
The inrush current can amount to a multiple of the rated current and is characterised
by a considerable 2nd harmonic content (double rated frequency), which is practically
absent during a short-circuit. If the second harmonic content in the differential current
exceeds a selectable threshold, tripping is blocked by the differential current thresh-
old.
Apart from the second harmonic, another harmonic can be selected in 7UT613/63x to
cause blocking. The 3rd or the 5th harmonic are selectable.
Steady-state overexcitation is characterised by odd harmonics. The 3rd or 5th har-
monic is suitable to detect overexcitation. As the third harmonic is often eliminated in
transformers (e.g. in a delta winding), the fifth harmonic is more commonly used.
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 thresholds, a restraint of the respec-
tive phase evaluation is started. The filter algorithms are optimised for transient behav-
iour such that additional measures for stabilisation during dynamic conditions are not
necessary.
Since the harmonic restraint operates individually per phase, the protection is fully op-
erative even when the transformer is switched onto a single-phase fault, whereby
inrush currents may possibly be present in one of the healthy phases. It is, however,
possible to set the protection in a way that when the permissible harmonic content in
the current of only one single phase is exceeded, not only the phase with the inrush
current but also the remaining phases of the differential stage are blocked. This cross-
block can be limited to a selectable duration.
7UT613/63x Manual
C53000-G1176-C160-2