Control and protection technology of on line ups

As the main power supply equipment of modern communication system and computer network, UPS has more than ten output electrical indexes. This paper discusses output power factor (PF) in detail, and introduces the test method of this index.

The output power factor of UPS is one of the technical indicators that most users pay more attention to, because the high and low output power factor of ups will directly affect the driving ability of various loads (such as inductive, capacitive and rectifier nonlinear loads). The output capacity of AC power supply equipment is expressed in volt ampere (VA), that is, the product of the effective value of output AC voltage and the effective value of current of the power supply equipment, which is what we call the apparent power PS.

The output capacity of UPS is expressed by apparent power va. all UPS indicate output power factor as well as output capacity. At present, the output power factor of imported or domestic UPS sold in domestic market is generally 0. 6～0。 8. For UPS output power factor, there are some incomplete understanding or improper evaluation among some users and UPS vendors. Some ups users or sales personnel think that the product of output capacity PS and power factor PF is the actual output power or output active power P of UPS, that is, P = PS × PF. Although there is no error in understanding and interpreting the output power factor in this way, it is not comprehensive, and it ignores the output capacity of reactive power PQ, another aspect of UPS output capacity. In modern computer network system and automatic control system, most of the AC power load is nonlinear load, among which the rectifier nonlinear load is the first. In the automatic control system, there are inductive nonlinear loads with iron core, such as transformer, AC motor, etc. When these electrical loads work normally, not only active power P but also reactive power PQ must be provided by UPS when the output voltage waveform has no obvious distortion. The reactive power PQ provided by UPS to the load is provided by harmonic currents other than fundamental current.

According to the different impedance characteristics of each AC load, the expression of power factor is different. There are two ways to express power factor: phase shifted power factor cos φ and distortion power factor PFD.

Phase shifted power factor is usually generated on linear loads, such as capacitive or non core inductive loads. Due to the phase difference between the sinusoidal voltage and the sinusoidal current on the load, the phase shifted power factor is generated. The cosine value of the phase angle φ is the phase shifted power factor, as shown in Fig. 1. It can be seen from the figure that although there is a phase difference between voltage U and current I, they are both sine waves, and there is no additional harmonic current caused by the load in the current waveform.