ABB Relion 670 Series Technical Reference Manual page 134

Section 5
Differential protection
128
Module (LDCM) where they are time coordinated with the local current samples, and
interpolated in order to be comparable with the local samples.
In the Pre-Processing Block, the real and imaginary parts of the fundamental frequency
phase currents and negative sequence currents are derived. Together with the current
samples, they are then forwarded to the differential function block where three
different analyses are carried out.
The first analysis is the classical differential and bias current evaluation with the
characteristic as seen in figure 52. 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 as the
greatest phase current in any line end and it is common for all three phases. The two
slopes (SlopeSection1, SlopeSection2) and breakpoints (EndSection1, EndSection2) can
be set in PCM600 or via the local HMI.
Current values plotted above the characteristic formed by IdMin and the dual slope will
give a pickup in that phase. The level IdMinHigh is a setting value that is used to
temporarily decrease the sensitivity in situations when:
• the line is energized
• when a fault is classified as external
• when a tap transformer is switched in
There is also an unrestrained high differential current setting that can be used for fast
tripping of internal faults with very high currents.
1MRK505222-UUS C
Technical reference manual