| Over
the past several years there has been dramatic growth in the use
of equipment incorporating switching type power supplies.
Examples are personal computers, video display terminals, fax
machines, copiers, electronic high efficiency ballasts, UPS systems,
variable speed drives and various medical electronic monitors.
The nature of all these loads is non-linear; they only demand
current during part of the cycle and/or change their impedance
during the voltage cycle. This type of load creates harmonic
currents, which in turn generate heat in the distribution equipment,
neutral conductors and distribution transformers.
Underwriters
Laboratories (UL) has designated K-factor as a means of rating
a transformers ability to handle loads which generate harmonic
currents, UL recognizes K-factor values of 4, 9, 13, 20, 30,
40 and 50. The K-factor ratings are based on information
contained in ANSI/IEEE C57.110-1986, Recommended Practice for
Establishing Capability When Supplying Nonsinusoidal Load Currents.
The K-factor number tells us how much a transformer must be
de-rated to handle a definite non-linear load or, conversely,
how much it must be oversized to handle the same load.
The
following rules will generally result in an acceptable choice
of K-factor value:
1.
Follow successful past practice in sizing the transformer.
2.
Where the harmonic current producing equipment is less than
15 per cent, use a standard transformer.
3.
Where the electronic equipment represents up to 35 per cent
of the load, use a K-4 rated transformer.
4.
Where the electronic equipment represents up to 75 per cent
of the load, use a K-13 rated transformer.
5.
Where 100 per cent of the load is electronic equipment, use
a K-20 rated transformer.
6.
Higher K-factor ratings are generally reserved for specific
pieces of equipment where the harmonic spectrum of the load
is known.
 
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