Office Building Solution

BU I LDI NG SOLUT IONS

17

— Static UPS

Non-linear loads can be among others: electronic and ICT equipement.

The fundamental characteristic of loads consisting of computer equipment, and more generally of all loads equipped with switching power supplies, is the waveform and phase of the current. Since these power supplies absorb current only near the voltage maximum, the typical waveform, far from being sinusoidal, has a rather small base and a vertex at the voltage peak. With the same rms value, this waveform has the peculiarity of having a crest factor much higher than that of a sine wave. The UPS must be able to provide this peak current value, which is normally declared in the technical characteristics of the product as crest factor. Regarding the current phase, it should be noted that the power factor of the loads in question is capacitive and therefore some special precautions may have to be taken in sizing. The most modern computer loads have an input power factor of up to 0.9 capacitive. It should be noted, however, that there are static uninterruptible power supply systems on the market that are capable of supplying this type of load without derating. In the specific case of printers, and in particular laser printers, it is necessary to oversize the static unit. In fact, in order to maintain the temperature of some internal components, at relatively long intervals and without any particular relation to the operating status, the current periodically exceeds the nominal value.

Sometimes the power rating of an uninterruptible power supply is confused with terms such as “switching power”, “computer power”, “effective power”.

The origin of these terms is probably inherent in the attempt to find a parameter able to model the power of the group even in conditions of deformation of current and voltage waveforms, however it should be borne in mind that these parameters have no official definition at the regulatory level and therefore can not have any correlation with the apparent power and the rated active power of the static group.

Therefore, they cannot be used for a correct dimensioning of the UPS.

A

B

Typical current waveform of computer loads (A) compared with a sinusoidal waveform (B)

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