GE P741, Technical Manual

Page: 126 / 560

P74x/EN OP/Na7 Operation
(OP) 5-
10
MiCOM P741, P742, P743
OP
Table below shows operation depending on the thresholds' status.
I
D
>1 k1.
I
bias
I
D
>2 Status Operation
0 0 0 Normal No operation
1 0 0 Normal No operation
0 1 0 Normal No operation
1 0 1 External fault or
circuitry fault External fault with CT saturation or
block circuitry fault alarm after t
CF
1 1 0 Circuitry fault Block and circuitry fault alarm after t
CF
1 1 1 Internal fault Trip
TABLE 1: OPERATION CONDITIONS
1.2.5 Signal Quality
An additional check is carried out to confirm that the signals used to determine the previous
criteria are satisfactory.
This includes checking for CT saturation conditions (information from peripheral unit, refer to
Section 2.1), that no plant discrepancies exist (via check zone as discussed earlier), and that
a change (increase or loss) in current flow has been detected by at least two peripheral units
(

I detection). The latter condition is used, as internal or external faults will cause a change
in levels in at least two circuits whereas, a CT fail only affect a single circuit’s level (faulty
CT).
Note: This condition is used only when there is no dead bus condition.
When a trip is issued for a bus zone by the central unit a signal is sent to all peripheral units
associated (or not) with the faulted bus zone. The peripheral units associated with the faulted
zone can carry out a further local confirmation via local Overcurrent protection,
I >BB or
I
N
>BB, before allowing a trip to take place.
1.2.6 Tripping Criteria
Before a trip signal is issued 4 trip criterions at the top level, i.e. the Central Unit, and 1
(optional) at the local level, i.e. the Peripheral Units, must be met.
These criterions are:
Top level (Central Unit)
Instantaneous criteria:
•
Bias characteristic ( I
diff
> (I
D
>2)) and Differential current setting are
exceeded (I
diff
> k2 I
bias
) for the zone for 2 consecutive 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 2 consecutive samples
•
No CT saturation
•
Signal quality (CT supervision, , AD converter, etc…)
Delayed criteria:
•
Bias characteristic ( I
diff
> (I
D
>2)) and Differential current setting are
exceeded (I
diff
> k2 I
bias
) for the zone for at least 1 sample
•
Bias characteristic ( I
diff
> (I
D
CZ>2)) and Differential current setting are
exceeded (I
diff
> kCZ I
bias
) for the check zone for at least 1 sample
•
Bias characteristic ( I
diff
> (I
D
>2)) and Differential current setting are not
exceeded (I
diff
> k2 I
bias
) for the zone for at least 1 sample in the 4 following
samples