P54x/EN AP/La4 Application Notes
(AP) 6-76
MiCOM P543, P544, P545 & P546
AP
4.3 Circuit breaker condition monitoring
4.3.1 Setting the ^ threshold
s
Where overhead lines are prone to frequent faults and are protected by oil circuit breakers
(OCB’s), oil changes account for a large proportion of the life cycle cost of the switchgear.
Generally, oil changes are performed at a fixed interval of circuit breaker fault operations.
However, this may result in premature maintenance where fault currents tend to be low, and
hence oil degradation is slower than expected. The ^ counter monitors the cumulative
severity of the duty placed on the interrupter allowing a more accurate assessment of the
circuit breaker condition to be made.
For OCB’s, the dielectric withstand of the oil generally decreases as a function of
2
t. This is where ‘’ is the fault current broken, and ‘t’ is the arcing time within the
interrupter tank (not the interrupting time). As the arcing time cannot be determined
accurately, the relay would normally be set to monitor the sum of the broken current
squared, by setting ‘Broken ^’ = 2.
For other types of circuit breaker, especially those operating on higher voltage systems,
practical evidence suggests that the value of ‘Broken ^’ = 2 may be inappropriate. In such
applications ‘Broken ^’ may be set lower, typically 1.4 or 1.5. An alarm in this instance may
be indicative of the need for gas/vacuum interrupter HV pressure testing, for example. The
setting range for ‘Broken ^’ is variable between 1.0 and 2.0 in 0.1 steps. It is imperative that
any maintenance program must be fully compliant with the switchgear manufacturer’s
instructions.
4.3.2 Setting the number of operations thresholds
Every operation of a
circuit breaker results in some degree of wear for its components.
Therefore, routine maintenance, such as oiling of mechanisms, may be based upon the
number of operations. Suitable setting of the maintenance threshold will allow an alarm to
be raised, indicating when preventative maintenance is due. Should maintenance not be
carried out, the relay can be set to lockout the auto-reclose function on reaching a second
operations threshold. This prevents further reclosure when the circuit breaker has not been
maintained to the standard demanded by the switchgear manufacturer’s maintenance
instructions.
Certain circuit breakers, such as oil circuit breakers (OCB’s) can only perform a certain
number of fault interruptions before requiring maintenance attention. This is because each
fault interruption causes carbonizing of the oil, degrading its dielectric properties.
The maintenance alarm threshold No CB Ops. Maint. may be set to indicate the
requirement for oil sampling for dielectric testing, or for more comprehensive maintenance.
Again, the lockout threshold No CB Ops. Lock may be set to disable auto-reclosure when
repeated further fault interruptions could not be guaranteed. This minimizes the risk of oil
fires or explosion.
4.3.3 Setting the operating time thresholds
Slow CB
operation is also indicative of the need for mechanism maintenance. Therefore,
alarm and lockout thresholds (CB Time Maint./CB Time Lockout) are provided and are
settable in the range of 5 to 500 ms. This time is set in relation to the specified interrupting
time of the circuit breaker.
4.3.4 Setting the excessive fault frequency thresholds
Persi
stent faults will generally cause auto-reclose lockout, with subsequent maintenance
attention. Intermittent faults such as clashing vegetation may repeat outside of any reclaim
time, and the common cause might never be investigated. For this reason it is possible to
set a frequent operations counter on the relay which allows the number of operations
Fault Freq. Count over a set time period Fault Freq. Time to be monitored. A separate
alarm and lockout threshold can be set.
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