Non Protection Functions; Circuit Breaker Condition Monitoring - GE MiCOM P40 Agile Technical Manual

Application Notes
MiCOM P40 Agile P442, P444
5

NON PROTECTION FUNCTIONS

5.1

Circuit breaker condition monitoring

Periodic maintenance of circuit breakers is necessary to ensure that the trip circuit and
mechanism operate correctly and also that the interrupting capability has not been
compromised due to previous fault interruptions. Generally, such maintenance is based on a
fixed time interval, or a fixed number of fault current interruptions. These methods of
monitoring circuit breaker condition give a rough guide only and can lead to excessive
maintenance.
The relays record various statistics related to each circuit breaker trip operation, allowing a
more accurate assessment of the circuit breaker condition to be determined. These
monitoring features are discussed in the following section.
5.1.1 Circuit Breaker Condition Monitoring Features
For each circuit breaker trip and autoreclose operation the relay, records statistics. The 'CB
condition' and 'CB monitor setup' menu cells are counter values only. The Min/Max values
show the range of the counter values. These cells can be disabled.
The counters may be reset to zero, for example, following a maintenance inspection and
overhaul.
The circuit breaker condition monitoring counters will be updated every time the relay issues
a trip command. The number of operation is displayed phase by phase.
These operating values are compared to two thresholds (setting with 'CB Monitor Setup'
menu): Maintenance Alarm or LockOut Alarm can be generated. Counters can be re-initiated
with the command Reset all values.
When the breaker is tripped by an external protection device, it is also possible to update the
CB condition monitoring. This is achieved by allocating one of the relays opto-isolated inputs
(via the programmable scheme logic) to accept a trigger from an external device. The DDB
signal that is mapped to the opto is called 'External TripA or B or C'.
Note:
5.1.2 CB condition monitoring
Setting the Σ I^ thresholds guidelines
5.1.2.1
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 Σ I^ 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 Σ I
'I' 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 I^' = 2
(I² or "I^2").
For other types of circuit breaker, especially those operating on higher voltage systems,
practical evidence suggests that the value of 'Broken I^' = 2 may be inappropriate. In such
applications 'Broken I^' 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 I^' 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.
When the relay is in Commissioning test mode, the CB condition monitoring counters
will not be updated.
P44x/EN AP/Hb6
(AP) 5-121
2
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