Trasformatori elettrici
Stabilizzatori di tensione
Convertitori di Frequenza
UPS, Inverter, Alimentatori
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K-FACTOR SRL unipersonale - Via Giotto 9
–
42014 Castellarano (RE) Italy - C.F. e P.IVA (VAT#) IT 02422010369
R.E.A. di RE n. 286968
–
C.S.
€
10.000,00
–
N. Iscrizione Reg. Imprese di RE: 02422010369
Tel +39-0536261380
–
e-mail: [email protected] - http://www.kfactor.it
01
ISO9001
• SURGE
Overcurrent can be caused by:
o External fault
o Internal fault in the transformer
o Insertion of the transformer
or overloads
❖
Any failure that comes downstream of a line fed by the transformer causes an increase in current in that line that can
reach very high values.
This causes anomalous electrodynamic heat and stress in the machinery and the various components involved, so it is
necessary to prevent the damage involving the transformer and therefore to timely eliminate the cause of the failure.
With respect to the selectivity of the protectors, if the failure occurs on only one of the lines powered by the transformer,
then only the circuit breaker of that line should be opened while the transformer machine switch must open (with a delay
of slightly higher) only in case the fault should occur between it and the circuit breakers of the various power lines or if the
line switch did not intervene.
❖
If the failure occurs instead inside the transformer, the situation would be similar to an overcurrent fault, with the only
variant in this case that the line to be protected would be that upstream of the transformer.
The protections to be installed must therefore prevent the fault from expanding on the upstream network, and therefore
have to act on the primary switch to exclude the transformer. Clearly, the relays of this primary switch will be calibrated
so that they will not open before the corresponding secondary switch opens in case of an external fault.
❖
If the transformer is inserted exactly at the moment when the sinusoidal supply voltage passes for its maximum value,
instant of which it corresponds, in a permanent state, to a null value for the current flow, there is no transient significant
phenomenon and the current magnetizing, starting from the zero value corresponding to the zero flow, follows its normal
permanent curve.
If, conversely, the transformer is inserted in an instant in which the voltage has no maximum value, in particular it has a
null value, the flow at that instant should assume no null value, in particular maximum value.
Not being able, conversely, to change instantaneously the value and having to depart at any rate from the zero value that
it had until the instant of the insertion, there is a transient period with an exponential trend and of a maximum value equal
to and contrary to what it would have permanent stream at the moment of insertion.
The transient phenomenon lasts for a few seconds. After a half-period from the insertion, the flow can reach a theoretically
close to twice (practically only 1.8 times) maximum of the permanent regime. Given the inevitable strong saturation of
iron under these conditions, the magnetizing current can then assume very high values, close to the short circuit current.
The protection to be provided to the primary according to the foregoing considerations must therefore be delayed to allow
venting the first peak of the current at the transformer insertion without having to intervene, while having to intervene for
lower current values if they persist for longer times.
❖
If a transformer is overloaded beyond the expected limit, with ambient temperatures close to the maximum permissible
or, for various reasons, failing to dispose of its leaks at temperatures within the limits specified, the temperature of its
windings may reach values such as to damage the insulation and cause internal failures.
What matters more than the maximum load current or the efficiency of the cooling system is the overheating that can be
produced in the windings.
Small overloads of long duration are more dangerous than strong overloads of short duration. Overload protection must
therefore first limit the secondary loads to the maximum permissible value in real operating situations.
• LIMITATING PROTECTORS OF TRANSFORMERS FAULTS
Protections can be achieved by:
- PT100 or PTC probes placed between the core and the secondary windings
- Electronic control unit
•
LIMITATION PROTECTION AGAINST OVERLOADS
- BT magnetic circuit breakers
