Alstom MiCOM P50 Agile P15D, Техническое руководство

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P50 Agile P15D 6 Protection Functions
P15D/EN TM/B 6-9
CLP operation occurs when the circuit breaker remains open for a time greater than tcold and is
subsequently closed. CLP operation is applied after tcold and remains for a set time delay of tclp
following closure of the circuit breaker. The status of the circuit breaker is provided by means of the
CB auxiliary contacts. Whilst CLP operation is in force, the CLP settings are enabled. After the time
delay
tclp has elapsed, the normal overcurrent settings are applied and the CLP settings are disabled.
If desired, instead of applying different current setting thresholds for the cold load time, it is also
possible to completely block the overcurrent operation during this time, for any of the overcurrent
stages.
2.4.1 Cold Load Pickup logic
The CLP logic is in operation when CLP is enabled AND CLP is initiated from a CB open condition
after the
tcold period has elapsed. The CLP logic becomes inactive when CLP is disabled or when
there is a CB closed condition. tcold and tclp are initiated using the CB status signal produced by
connecting CB NO auxiliary contact from the circuit breaker. The CLP logic gets driven from a single
breaker contact.
2.4.2 Cold Load Pickup for SWITCH ONTO FAULT condition
In some feeder applications, fast tripping may be required if a fault is already present on the feeder
when it is energized. Such faults may be due to a fault condition not having been removed from the
feeder, or due to earthing clamps having been left on following maintenance. In either case, it is
desirable to clear the fault condition quickly, rather than waiting for the time delay imposed by IDMT
overcurrent protection. The CLP logic can cater for this situation. Selected overcurrent/earth fault
stages could be set to instantaneous operation for a defined period following circuit breaker closure
(typically 200 ms). Therefore, instantaneous fault clearance would be achieved for a switch onto fault
(SOTF) condition
2.5 Inrush Current Blocking function (2ND Harm Blocking)
When a transformer is initially connected to a source of AC voltage, there may be a substantial surge
of current through the primary winding called inrush current. This is analogous to the inrush current
exhibited by an electric motor that is started up by sudden connection to a power source, although
transformer inrush is caused by a different phenomenon.
In an ideal transformer, the magnetizing current would rise to approximately twice its normal peak
value as well, generating the necessary MMF to create this higher-than-normal flux. However, most
transformers are not designed with enough of a margin between normal flux peaks and the saturation
limits to avoid saturating in a condition like this, and so the core will almost certainly saturate during
this first half-cycle of voltage. During saturation, disproportionate amounts of MMF are needed to
generate magnetic flux. This means that winding current, which creates the MMF to cause flux in the
core, could rise to a value way in excess of its steady state peak value. Furthermore, if the transformer
happens to have some residual magnetism in its core at the moment of connection to the source, the
problem could be further exacerbated.
We can see that inrush current is a regularly occurring phenomenon and should not be considered a
fault, as we do not wish the protection device to issue a trip command whenever a transformer or
machine is switched on. This presents a problem to the protection device, because it should always
trip on an internal fault. The problem is that typical internal transformer faults may produce
overcurrents which are not necessarily greater than the inrush current. Furthermore faults tend to
manifest themselves on switch on, due to the high inrush currents. For this reason, we need to find a
mechanism that can distinguish between fault current and inrush current. Fortunately this is possible
due to the different natures of the respective currents. An inrush current waveform is rich in harmonics,
whereas an internal fault current consists only of the fundamental. We can thus develop a restraining
method based on the harmonic content of the inrush current. The mechanism by which this is
achieved is called second harmonic blocking.