Siemens SIPROTEC 4 7UT6 Series Manual page 120

Functions
2.2 Differential Protection
The harmonic content intended for blocking the differential protection is set at address 1276 n. HARMONIC.
For example, if the 5th harmonic restraint is used to avoid trip during overexcitation, 30 % (default setting) are
convenient.
Harmonic restraint with the n-th harmonic operates individually per phase. However, it is also possible – as it
is for the inrush restraint – to set the protection in such manner that not only the phase with harmonic
content overshoot but also the other phases of the differential stage I-DIFF> are blocked (so-called cross-
block function). The duration for which the crossblock function is active can be limited at address 1277
CROSSB. n.HARM. Setting is in multiple of the AC-cycle. This parameter can only be set with DIGSI at Addi-
tional Settings. If set to 0 (presetting is 3) the protection can trip when the transformer is switched on a
single-phase fault even while the other phases carry inrush current. If set to ∞ the crossblock function remains
effective for as long as high-order harmonics are detected in any phase.
If the differential current exceeds the magnitude set at address 1278 IDIFFmax n.HM no n-th harmonic
restraint takes place. This parameter can only be altered in DIGSI at Additional Settings.
Current Waveform Analysis Restraint (CWA)
When switching unloaded transformers or shunt reactors to a live busbar, high magnetizing (inrush) currents
may occur. These inrush currents produce a differential current as they seem like internal fault currents. Also
during paralleling switching of transformers, or an overexicitation of a power transformer, a differential
current may occur due to magnetizing currents cause by increased voltage and/or decreased frequency.
Inrush current may result in a mal-operation of the differential protection.
Typical for a transformer energizing process is the occurrence of flat areas in the current (see following
figure). If these flat areas occur in all 3 phases at the same time, this is a typical characteristic of this process.
The CWA-method works as a supplement to the 2nd harmonic and covers cases that are not controlled
through the 2nd harmonic. This function is based on analyzing the wave form. The physical behavior of an
inrush current is asymmetric, additional pronounced flat areas exists. Based on the method of measurement,
the blocking always affects all phases imultaneously. The method has internal thresholds available.
[scinrush-120120-01.tif, 1, en_GB]
Figure 2-50
Restraint with current waveform analysis is available only when the device is used as transformer protection,
i.e. the PROT. OBJECT (address 105) is set to 3 phase transf. or 1 phase transf. or Autotransf.
or Autotr. node.
This function is also used to shunt reactors if current transformers are installed at both sides of the connection
points.
The inrush restraint function with Current Waveform Analysis can be enabled in address 1300 Blocking with
CWA. OFF and ON. It is based on the evaluation of the asymmetry of the differential current. The restraint with
harmonics does not influence the stage I-DIFF>>.
In address 1300 Blocked w. CWA, the inrush restraint function with Current Waveform Analysis can be
switched ON or OFF.
120
Inrush Current with Pronounced Flat Areas
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016