Areva MiCOM P441, Техническое руководство

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 159/220
Maintenance Alarm or Lock Out Alarm can be generated.
A pre-lock out Alarm is generated at value n-1.
All counters can be re-initiated with the command Reset all values (by HMI)
In cases where 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 signal that is mapped to the opto is called ‘External TripA or B or C’.
Note that when in Commissioning test mode the CB condition monitoring counters will not be
updated.
4.7.2 Setting guidelines
Setting the
Σ
I^ Thresholds
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
2
t. This
is where ‘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.
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 programme must be fully compliant with the switchgear
manufacturer’s instructions.
4.7.3 Setting the Number of Operations Thresholds
Every operation of a circuit breaker results in some degree of wear for its components.
Thus, 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 autoreclose 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.