scheider electric MiCOM P746, Техническое руководство

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P746/EN OP/G31 Operation
(OP) 5-
8
MiCOM P746
1.3.2 Circuit Breaker Fail (CBF)
The detailed logic of the circuit breaker failure element follows.
The P746 hardware corresponds to up to 6 circuit breaker and can accommodate 1 or 2 trip
coils per breaker (depending on relay number, defined by P746 model number, and PSL(CB
trip signal connection to the relay output)).:
•
1 main trip coil
•
1 back-up trip coil
Furthermore these can be either 3 single-phase trip coils or 1 three-phase trip coil. These
can be combined for example 3 single-phase trip coils on the main system and 1 three-
phase trip coil for the back-up system.
1.3.2.1 Circuit Breaker Fail Reset Criteria
1.3.2.1.1 Overcurrent Criterion
One of the most common causes of busbar mal-tripping is error introduced in the back
tripping of adjacent sections. To prevent such an error it is possible to condition the
operation of 50BF protection only when there is presence of a significant current i.e. a short-
circuit on the concerned feeder. This confirmation is provided by the
I
> threshold which is set
by default at 1.2 times the nominal rated current of the CT.
OP
1.3.2.1.2 Undercurrent reset Criterion
The criterion normally used for the detection of a circuit breaker pole opening is the
disappearance of the current i.e. undercurrent element. This function is generally preferred
above other elements due to its very fast response time. In P746, this method of detection is
used and has the threshold
I <.
See figure 4, part for phase A.
Note: The same algorithm is used for the other phases.
This Undercurrent elements have an I< threshold, which is used to supervise that each
circuit breaker has opened correctly, when commanded to do so. By use of the I< threshold,
it is possible to ensure that all load and fault current has been ruptured, ensuring that no
arcing remains across the circuit breaker primary contacts. Optionally, the user can decide to
include 52a supervision in the breaker fail logic (see chapters 1.3.2.1.3 and 1.3.2.1.4).
Note : – 52a is the setting name, it means CB closed.
– The CB closed position is created in the PSL either using 52b
reversed or a combination of (52a and 52b).
The first function is to compare the current sample to the I< threshold and check for the
following sequence:
•
positive value of the current
•
no current (below the threshold)
•
negative value of the current
•
no current (below the threshold)
•
positive value of the current
•
…
The output signal is pl(t), it changes between 0 and 1.
Internal overcurrent signals are available per phase and neutral to confirm that the CB failure
algorithm has started to count down.