44x/EN AP/Hb6
-116 MiCOM P40 Agile
capacity of the generators in that particular subsystem. Industrial plants that are dependent
on utilities to supply part of their loads will experience underfrequency conditions when the
incoming lines are lost.
An underfrequency condition at nominal voltage can result in over-fluxing of generators and
transformers and many types of industrial loads have limited tolerances on the operating
frequency and running speeds e.g. synchronous motors. Sustained underfrequency has
implications on the stability of the system, whereby any subsequent disturbance may lead to
damage to frequency sensitive equipment and even blackouts, if the underfrequency
condition is not corrected sufficiently fast.
To minimize the effects of underfrequency on a system, a multi stage load shedding scheme
may be used with the plant loads prioritized and grouped. During an underfrequency
condition, the load groups are disconnected sequentially depending on the level of
underfrequency, with the highest priority group being the last one to be disconnected.
The effectiveness of each stage of load shedding depends on what proportion of the power
deficiency it represents. If the load shedding stage is too small compared to the prevailing
generation deficiency, then the improvement in frequency may be non-existent. This aspect
should be taken into account when forming the load groups.
Time delays should be sufficient to override any transient dips in frequency, as well as to
provide time for the frequency controls in the system to respond. This should be balanced
against the system survival requirement since excessive time delays may jeopardize system
stability.
The relatively long time delays are intended to provide time for the system controls to
respond and will work well in a situation where the decline of system frequency is slow. For
situations where rapid decline of frequency is expected, the load shedding scheme should
be supplemented by rate of change of frequency protection elements.
4.13.2 Overfrequency protection
Overfrequency running of a generator arises when the mechanical power input to the
machine exceeds the electrical output. This could happen, for instance, when there is a
sudden loss of load due to tripping of an outgoing feeder from the plant to a load center.
Under such over speed conditions, the governor should respond quickly to obtain a balance
between the mechanical input and electrical output, thereby restoring normal frequency.
Over frequency protection is required as a back-up to cater for slow response of frequency
control equipment.
Following faults on the network, or other operational requirements, it is possible that various
subsystems will be formed within the power network and it is likely that each of these
subsystems will suffer from a generation to load imbalance. The “islands” where generation
exceeds the existing load will be subject to overfrequency conditions, the level of frequency
being a function of the percentage of excess generation. Severe over frequency conditions
may be unacceptable to many industrial loads, since running speeds of motors will be
affected.
The relatively long time delays are intended to provide time for the system controls to
respond and will work well in a situation where the increase of system frequency is slow.
For situations where rapid increase of frequency is expected, the protection scheme above
could be supplemented by rate of change of frequency protection elements, possibly utilized
to split the system further.
4.14 Circuit breaker fail protection
Following inception of a fault one or more main protection devices will operate and issue a
trip output to the circuit breaker(s) associated with the faulted circuit. Operation of the circuit
breaker is essential to isolate the fault, and prevent damage / further damage to the power
system. For transmission/sub-transmission systems, slow fault clearance can also threaten
system stability. It is therefore common practice to install circuit breaker failure protection,
which monitors that the circuit breaker has opened within a reasonable time. If the fault
current has not been interrupted following a set time delay from circuit breaker trip initiation,
breaker failure protection (CBF) will operate.
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