Circuit Breaker Fail (Cbf); Circuit Breaker Fail Reset Criteria; Overcurrent Criterion; Undercurrent Reset Criterion - GE P741 Technical Manual

P74x/EN OP/Na7
(OP) 5-16
1.3.3

Circuit Breaker Fail (CBF)

The detailed logic of the circuit breaker failure element follows.
1.3.3.1 Distributed Tripping, Control and Indication Elements (Peripheral Units)
As the P74x scheme has been designed for use as either a centralised or distributed
scheme, the hardware corresponds to one circuit breaker and can accommodate 1 or 2 trip
coils:
•
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.
OP
1.3.3.2

Circuit Breaker Fail Reset Criteria

1.3.3.2.1 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 and/or by the threshold setting of
residual current I

1.3.3.2.2 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 P74x this method of detection is
utilised and has the threshold I<.
See figure 8, part
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 1.3.3.2.3 and 1.3.3.2.4).
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
•
...
Note: - The backtrip order (coming from the Central Unit) is always given to
the relay 1, 2 and 3 even if the CBF is disabled in the Peripheral
Units.
- The retrip has to be done using the PSL.
> set by default to 0.2 times the rated current.
N
Note: The same algorithm is used for the other phases.
MiCOM P741, P742, P743
for phase A.
Operation