Cross-Block Logic Scheme - ABB RELION 670 Series Technical Manual

Section 6
Differential protection
1. Output OPENCT provides instant information to indicate that an open CT circuit has been
detected.
2. Output OPENCTAL provides a time-delayed alarm that the open CT circuit has been detected.
Time delay is defined by the parameter
3. Integer output OPENCTIN provides information on the local HMI regarding which open CT
circuit has been detected (1=CT input No 1; 2=CT input No 2).
4. Integer output OPENCTPH provides information on the local HMI regarding in which phase an
open CT circuit has been detected (1=Phase A; 2= Phase B; 3= Phase C).
Once the open CT condition is declared, the algorithm stops to search for further open CT circuits.
It waits until the first open CT circuit has been corrected. Note that once 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.
6.3.7.6

Cross-block logic scheme

According to the cross-block logic, to issue a common trip command, the harmonic contents in all
phases with a pickup signal set (pickup = 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
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 (87G) is shown in figure 57.
162
tOCTAlarmDelay .
OpCrossBlock = Yes . By always using the cross-block
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