In the preprocessing block, the real and imaginary parts of the fundamental
frequency phase currents and negative sequence currents are derived by means of
fundamental frequency numerical Fourier filters. Together with the current
samples, which are required to internally estimate the 2
nd
and the 5
th
harmonic
contents in the instantaneous differential currents, they are then forwarded to the
differential function block where three kinds of analyses are carried out.
The first analysis is the classical differential and bias current evaluation with the
characteristic as seen in Figure
34. Line differential protection is phase-segregated
where the differential current is the vectorial sum of all measured currents taken
separately for each phase. The bias current, on the other hand, is considered the
greatest phase current in any line end and it is common for all the tree phases. The
two slopes (SlopeSection1, SlopeSection2) and breakpoints (EndSection1,
EndSection2) can be set in PCM600 or via the local human-machine interface
(LHMI).
Current values found to be above the characteristic formed by IdMin and the dual
slope will give a start in that phase. The level IdMinHigh is a setting value that is
used to temporarily decrease the sensitivity in situations when the protected line
circuit is just energized, that is, connected to the power system at one end.
There is also an unrestrained high differential current setting that can be used for
fast tripping of internal faults with very high currents. This unrestrained protection
is phase-segregated, that is, it is known which phase(s) require a trip command.
1MRK 505 394-UEN A Section 6
Differential protection
Line differential protection RED650 2.2 IEC 95
Technical manual
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