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ABB REL 356 Current Differential Protection
Backup Protection Disable
An opto isolated dc voltage input has been provided on the rear panel of the relay system to disable the distance backup
system. This input has no effect on the primary current differential protection scheme.
Sensitive Directional Ground Overcurrent Unit
FDOG might be used for detecting high fround resistance faults that may not be detected by the ground distance units.
The timer, TOG, may be blocked (no FDOG trip) or given a definite time delay for operation once FDOG has operated
and IOM has picked up.
Out of Step Trip and Out of Step Block Logic
Out of step detection in REL 356 is achieved by the use of blinders. A two blinder scheme is used. The two blinders are
called BO (outer blinder, setting RU) and BI (inner blinder, setting RT) and are parallel to the line impedance setting, i.e.
they are tilted by the PANG (phase angle) setting.
Figure 4-11. Blinders for the Out of Step Logic
The RU and RT settings are the distance perpendicular to the line positive sequence impedance that the blinders are
displaced from the latter. The RT setting is also for load restriction and if any three phase fault occurs, the inner blinder,
BI has to pickup for tripping in addition to the impedance Zone 2 or 3.
The duration of time it takes BI to operate after BO operates is the indication of an out of step condition. Timer OST1
controls this time. When the timer times out, and out of step condition has been detected. And OSB signal is immedi-
ately sent to block the operation of Zone 2 and/or Zone 3 distance units, as determined by the OSB setting (NONE/Z2/
Z3/BOTH).
Timer OST2 timer the trip after an OST condition has been detected. When OST2 times out, a trip signal is sent if OST
is set to trip on the way in, WAYI.
4-12
Measuring Elements and Operational Logic
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