Siemens SIPROTEC 4 7UT6 Series Manual page 96

Functions
2.2 Differential Protection
nents are transformed into unequal secondary DC components due to different time constants of the secon-
dary circuits. This produces a DC component in the differential current which increases the pickup values of
the differential stage for a short period.
Identification of DC Components
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 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 increases the pickup values of the differential
stage for a short period. In this case, characteristic 1 is increased by factor 2.
Harmonic Stabilisation
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 magnet-
ising 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 differen-
tial current threshold.
Apart from the second harmonic, another harmonic can be selected in 7UT6x to cause blocking. The 3rd or the
5th harmonic are selectable.
Besides detecting an inrush currents with 2nd Harmonics, it exist an another method called Current Waveform
Analysis (CWA). If the CWA function is enabled and this algorithm detects a asymmetric current waveform,
then the tripping is blocked by the differential current threshold.
Steady-state overexcitation is characterised by odd harmonics. The 3rd or 5th harmonic 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 respective phase evaluation is started. The filter algorithms are optimised for
transient behaviour such that additional measures for stabilisation during dynamic conditions are not neces-
sary.
Since the harmonic restraint operates individually per phase, the protection is fully operative 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 crossblock can be limited to a selectable
duration.
Fast Unrestrained Trip with High-Current Faults
High-current faults in the protected zone may be cleared instantaneously without regard to the restraint
currents when the current amplitude excludes an external fault. If the protected object has a high direct impe-
dance (transformers, generators, series reactors), a threshold can be found which can never be exceeded by a
through-fault current. This threshold (primary) is, for example, for a power transformer.
[diff-stromstfehler-021026-rei, 1, en_GB]
96 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016