scheider electric MiCOM P746, Technical Manual

Page: 120 / 530

P746/EN OP/G31 Operation
(OP) 5-
6
MiCOM P746
OP
Example:
1 bar, 1 incomer (CT1 = 2000/1) and 2 outfeeds (CT2 = 1000/1 and CT3 = 500/1)
If the setting of the phase comparison threshold is 50%, the phase comparison algorithm will
use:
•
CT1 phase when a current higher than 1000A will flow in CT1,
•
CT2 phase when a current higher than 500A will flow in CT2,
•
CT3 phase when a current higher than 250A will flow in CT3.
The phase comparison algorithm will prevent a maloperation as long as all the phase
currents are not within a 90° area.
Case 1: If 200A is getting in through CT1 and 100A are going out through CT2 and CT3, no
current is taken into account and the phase algorithm is not blocking busbar trip.
Case 2: If 500A is getting in through CT1 and 250A are going out through CT2 and CT3, only
one current (CT3) is taken into account and the phase algorithm is not blocking busbar trip.
Case 3: If 900A is getting in through CT1 and 600A are going out through CT2 and 300A in
CT3, 2 currents (CT2 and CT3) are taken into account, both are within a 90° area and the
phase algorithm is not blocking busbar trip.
Case 4: If 1000A is getting in through CT1 and 500A are going out through CT2 and CT3,
the 3 currents are taken into account, they are not within any 90° area and the phase
algorithm is blocking busbar trip.
Case 5: Case 1 then External fault: Over 1000A is flowing from CT1 through CT2 and:
•
Current 3 decreases below 250A then the 2 currents are not within any 90° area and the
phase algorithm is blocking busbar trip.
•
Current 3 does not decrease below 250A then the 3 currents are not within any 90° area
and the phase algorithm is blocking busbar trip.
Case 6: Case 1 then Internal fault: Over 1000A is flowing from CT1 and:
•
Current 2 or 3 decreases below 500A or 250A then there is only one current thus the
phase algorithm is releasing the busbar trip.
•
Current 2 or 3 does not decrease below 500 or 250A but reverses then they are within a
90° area and the phase algorithm is releasing the busbar trip.
1.2.3 Scheme supervision by "Check Zone” element
For security, the busbar protection will only trip a particular busbar zone if that zone
differential element AND the check zone element are in agreement to trip.
The principal advantage of this element is total insensitivity to topological discrepancies.
Under such circumstances the "check zone" element will see two currents with equal
amplitude but of opposite sign in adjacent zones.
The Check Zone characteristic is determined from the following protection settings:
•
Area above the I
D
CZ>2 threshold check zone differential current threshold setting and
the set slope of the bias characteristic (kCZ × I
bias
) (kCZ is the percentage bias setting
(“slope”) for the Check Zone)
Note: The origin of the bias characteristic slope is 0.