In the power frequency UPS, the commonly used SCR polyphase phase-controlled rectifier is a 12-phase two-phase superimposed rectifier, and the 18-phase three-phase superimposed rectifier and the 24-phase four-phase superimposed rectifier may be used. Their characteristic values are compared in Table 1. It can be seen from this table that when the UPS is working at full load, the mains input power factor of the SCR polyphase phase-controlled rectifier can also reach more than 0.99, but a bad feature of this type of power frequency rectifier is the mains input power factor. It is greatly affected by the displacement factor caused by phase control and the UPS load rate. This influence limits the application of SCR polyphase phase-controlled rectifiers and becomes a major disadvantage of this rectifier. The fundamental reason is that it does not have the PFC function.
From this table, it can be seen that their mains input power factor is related to the following two factors.
①It is related to the number of phases of the rectifier. The more phases of the rectifier, the higher the input power factor of the mains. Mains input power factor PF=0.9971cosα.
②It is related to the displacement factor kφ=cosα, and the phase control angle a in kφ=cosα is related to the size of the DC output voltage Ud of the rectifier, and it is the load rate of the UPS that affects the size of the DC output voltage. Because the UPS circuit itself has internal resistance, in order to maintain the stability of the output voltage of the UPS, when the load rate of the UPS is large (such as full load), the output DC voltage of the rectifier must be increased; when the load rate of the UPS is small (eg no load), the DC output voltage of the rectifier must be adjusted down. The high or low DC output voltage of the rectifier is achieved by adjusting the phase control angle α of the SCR. If the value of α is reduced, the DC output voltage Ud will increase; on the contrary, if the value of α is increased, the DC output voltage will decrease. Because the mains input power factor PF is proportional to cosα, the smaller the α, the higher the PF; the larger the α, the lower the PF. Therefore, the load rate of the UPS is different, and the input power factor of the mains is also different. The mains input power factor is the highest at full load, and the lowest at light load or no-load.
Figure1 shows the relationship between the DC output voltage Ud/Udo of the three-phase bridge fully-controlled rectifier and the phase control angle α. When the rectifier is fully loaded, the rectifier is generally set at point A, that is, α=0°, cosα= 1. When the UPS is switched from full load to light load, the DC output voltage of the rectifier will rise. In order to keep the DC output voltage stable and unchanged, the method of increasing the control angle α will be used to reduce the DC output voltage.
For example, for a 12-phase two-superimposed rectifier, its full-load operating point is A. When the UPS changes from full-load to light-load or no-load, the operating point will be changed from point A to point B when light-loaded, so that you can Restore the DC output voltage to the DC voltage value before load shedding. At this time, the value of cosα will change from cos0° to cosα’=cos36°=0.809, and the mains input power factor will drop from PF=0.9886·cos0°=0.9886 to PF= 0.9886·cos36°=0.7998.
The control characteristics represented by the relationship between the DC output voltage of the three-phase bridge fully controlled rectifier and the phase control angle a cannot be changed. In order to improve the mains input power factor of the SCR polyphase phase-controlled rectifier, there are two methods: one is to increase the number of phases of the rectifier, but this method has two disadvantages. One is to increase the difficulty of manufacturing the mains input transformer. , the second is that it cannot overcome the shortcomings of large size and quality; another method is to use a three-phase Boost single-switch SPWMPFC rectifier, this method has two advantages, one is to improve the power factor of the mains input, and the other is to remove the mains input. Transformer, reducing the size and mass of the rectifier. This rectifier is described in the next section.
Read more: UPS applications, load characteristics and current crest factor