Cross-Block Logic Scheme - ABB REG670 Technical Manual

1MRK502052-UEN B
6.3.7.6
Technical manual
the open CT condition has been detected, it can be reset automatically within the
differential function. It is not possible to externally reset an open CT condition. To
reset the open CT circuit alarm automatically, the following conditions must be
fulfilled:
• Bias current is for at least one minute smaller than 110%
• The open CT condition in the defective CT circuit has been corrected (for
example, current asymmetry disappears)
• The above two conditions are fulfilled for a longer time than defined by the
setting parameter tOCTResetDelay
If an open CT has been detected in a separate group of three CTs, the algorithm is reset
either when the missing current returns to the normal value, or when all three currents
become zero. After the reset, the open CT detection algorithm starts again to search for
open CT circuits within the protected zone.

Cross-block logic scheme

According to the cross-block logic, to issue a common trip command, the harmonic
contents in all phases with a start signal set (start = TRUE) must be below the limit
defined with the setting HarmDistLimit. In the opposite case, no trip command is
issued.
The cross-block logic is active if the setting OpCrossBlock = Yes. By always using the
cross-block logic, the false trips can be prevented for external faults in cases where the
internal or external fault discriminator should for some reason fail to declare an
external fault. For internal faults, the higher frequency components of an
instantaneous differential current are most often relatively low, compared to the
fundamental frequency component. While for an external (heavy) fault, they can be
relatively high. For external faults with moderate fault currents, there can be little or
no current transformer saturation and only small false differential currents.
The principle design of the Generator differential protection GENPDIF is shown in
figure 54.
Section 6
Differential protection
161