Alstom MiCOM P546 - 2.13.1 Setting guidelines; 2.14 Circuit breaker fail protection (CBF); 2.14.1 Breaker fail timer settings; 2.14.2 Breaker fail undercurrent settings

P54x/EN AP/La4 Application Notes
(AP) 6-44 MiCOM P543, P544, P545 & P546
AP
2.13.1 Setting guidelines
The voltage setting applied to the elements is dependent upon the magnitude of residual
voltage that is expected to occur during the earth fault condition. This in turn is dependent
upon the method of system earthing employed and may be calculated by using the formulae
previously given in Figure 12 and Figure 13. It must also be ensured that the relay is set
above any st
anding level of residual voltage that is present on the system.
Note: IDMT characteristics are selectable on the first stage of NVD in order
that elements located at various points on the system may be time
graded with one another.
2.14 Circuit breaker fail protection (CBF)
2.14.1
Breaker fail timer settings
Typic
al timer settings to use are as follows:
CB fail reset
mechanism
tBF time delay
Typical delay for 2 ½
cycle circuit breaker
Initiating element reset
CB interrupting time + element
reset time (max.) + error in tBF
timer + safety margin
50 + 45 + 10 + 50 = 155 ms
CB open
CB auxiliary contacts
opening/closing time (max.) +
error in tBF timer + safety
margin
50 + 10 + 50 = 110 ms
Undercurrent
elements
CB interrupting time +
undercurrent element (max.) +
safety margin
50 + 25 + 50 = 125 ms
Note: All CB Fail resetting involves the operation of the undercurrent
elements. Where element reset or CB open resetting is used the
undercurrent time setting should still be used if this proves to be the
worst case.
The examples above consider direct tripping of a 2½ cycle circuit breaker.
Note: Where auxiliary tripping relays are used, an additional 10-15 ms must
be added to allow for trip relay operation.
2.14.2 Breaker fail undercurrent settings
The pha
se undercurrent settings (I<) must be set less than load current, to ensure that I<
operation indicates that the circuit breaker pole is open. A typical setting for overhead line or
cable circuits is 20% In, reduced to 10% or 5% where the infeed has a high SIR ratio (e.g. at
a spur terminal with embedded generation infeed).
The sensitive earth fault protection (SEF) undercurrent element must be set less than the
respective trip setting, typically as follows:
SEF< = (SEF> trip) / 2