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Functions
2.3 Differential Protection
Note
If the protected line section includes a transformer or compensation reactors, the charging current compensa-
tion must not be switched on.
In the following cases, an active protection device is not able to assess the charging current (charging current
compensation is ineffective):
• No voltage measurement (depends on configuration),
• Fuse Failure, or
• Detection of a ΣU measuring error.
In case of a measuring-voltage failure, the charging current cannot be calculated. Therefore, in this case, an
additional stabilization is used. This stabilization is calculated as follows: the rated charging current multiplied
by the parameter 1224 IcSTAB/IcN divided by the number of devices. The recommended basic stabilization
should reach 2 to 3 times the amount of the rated charging current. As you have yet considered the charging
current with the setting I-DIFF> = 1 · I
to 1.5.
Inrush restraint
The inrush restraint of the differential protection is only necessary when the devices are operated on a trans-
former or on lines which end on transformers. The transformer is located inside the differential protection zone.
Inrush restraint can be turned ON or OFF at address 2301 INRUSH REST..
It is based on the evaluation of the second harmonic which exists in the inrush current. Ex-works a ratio of 15 %
of the 2nd HARMONIC I
ponent required for restraint can be parameterized. In order to be able to achieve a higher degree of restraint
in case of exceptionally unfavourable inrush conditions, you may also set a smaller value.
However, if the local measured current exceeds a value set in address 2305 MAX INRUSH PEAK, there will
be no inrush restraint. The peak value is decisive. The set value should be higher than the maximum inrush
current peak value that can be expected. For transformers, you can set the value above √2·I
rule of thumb. If a line ends on a transformer, a smaller value may be selected, considering the damping of the
current by the line impedance.
At address 2303 CROSS BLOCK, the crossblock function can be activated (YES) or deactivated (NO). The time
after exceeding the current threshold for which this crossblock is to be activated is set at address 2310 CROSSB
2HM. With the setting ∞ the crossblock function is always active until the second harmonic content in all phases
has dropped below the set value.
106
, we recommend you to set the parameter 1224 IcSTAB/IcN at least
cN
is set under address 2302, which can normally be taken over. However the com-
/I
2fN
fN
NTransf
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
/u
by
kTransfo
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