ABB RED650, Applications Manual

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IEC05000435-2-en.vsdx
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Protected zone in instantaneous function
IEC05000435 V2 EN-US
Figure 31: Line tap with a small power transformer, the currents of which are
not measured, and consequently contribute to a (false) differential
current
6.1.2.3 Charging current compensation
M12022-58 v6
There are capacitances between the line phase conductors and between phase
conductors and earth. These capacitances give rise to line charging currents which
are seen by the differential protection as “false” differential currents, as shown in
figure
32 .
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Communication
I
c1
I
c2
I
diff, false
= I
c1
+ I
c2
IEC05000436-2-en.vsdx
IED
IEC05000436 V2 EN-US
Figure 32: Charging currents
The magnitude of the charging current is dependent of the line capacitance and the
system voltage. For earth cables and long overhead lines, the magnitude can be
such that it affects the possibility to achieve the wanted sensitivity of the
differential protection. To overcome this, a charging current compensation is
available in line differential protection. When enabled, this algorithm will measure
the fundamental frequency differential current under steady state undisturbed
conditions and then subtract it, making the resulting differential current zero (or
Section 6
1MRK 505 393-UEN B
Differential protection
80 Line differential protection RED650 2.2 IEC
Application manual