P74x/EN OP/N
(OP) 5-
• Bias characteristic ( I
diff
> (I
D
CZ>2)) and Differential current setting are not
exceeded (I
diff
> kCZ I
bias
) for the check zone for at least 1 sample in the 4
following samples
• Bias characteristic ( I
diff
> (I
D
>2)) and Differential current setting are
exceeded (I
diff
> k2 I
bias
) for the zone for all the 7 following samples
• Bias characteristic ( I
diff
> (I
D
CZ>2)) and Differential current setting are
exceeded (I
diff
> kCZ I
bias
) for the check zone for all the 7 following samples
• Signal quality (CT supervision, , AD converter, etc…)
Local Level (Peripheral Unit)
• Local confirmation by an instantaneous Overcurrent element
(enabled/disabled) (I>BB or IN>BB)
1.2.7 Trip duration
The 87BB and 50BF trip orders have been hold 200ms by the Central Unit with a drop-off
timer of 200ms.
The 50BF trip orders have been hold 250ms integrated in the 50BF logic with a drop-off
timer of 250ms.
In case of operation of the 50BF logic of the PU, this CU 200ms drop-off timer is added to
the PU 250ms, thus the trip duration will exceed 450ms.
From software E1.0, model 50, the 200ms drop-off timer in the Central Unit has been
replaced by a 200ms dwell timer.
The fault recorder has been modified to start from the following tripping time and the fault
duration is determined from the:
• 87BB fault for the 87BB protection
• Backtrip signals coming from the PU or opto inputs in the CU for the 50BF protection
1.2.8 Current Circuit Supervision
During normal operation the differential current in the scheme should be zero or negligible.
Any anomaly is detected via a given threshold I
D
>1.
A biased differential element is used to supervise the current circuit. A differential current will
result if the secondary circuit of a CT becomes open circuited, short circuited; the amplitude
of this current is proportional to the load current flowing in the circuit monitored by the faulty
current circuit.
The setting is chosen to be as low as possible (minimum suggested setting is 2% of the
biggest CT primary winding) but also allow for standing differential current for example due
to CT mismatch and varying magnetising current losses. 5 to 20% is a typical application
range.
The element is typically time delayed for 5 seconds (set greater than the maximum
clearance time of an external fault). Instead the time delay allows the relevant protection
element (which should be substantially faster) to clear the fault instead i.e. I
D
>2 in the case
of an internal phase fault.
1.2.8.1 Protection options for the Zones
1.2.8.1.1 Options for a Circuitry Fault
When something happens on the primary equipments that creates a small differential current
in a Zone such as CT short-circuited or a closed isolator “seen” open etc…, it is detected by
the Central Unit. To deal with this:
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