The key to UPS’s uninterrupted power supply is the introduction of energy storage batteries. When the mains power is normal, the poor quality AC power of the mains power is rectified into DC power through the rectifier. On the one hand, this DC power is transformed into constant voltage and constant frequency high-quality AC power through the inverter to supply power to the load. On the other hand, it also charges the battery to store energy: when the mains fails or is interrupted, the control circuit immediately The mains power supply is converted to the high-quality AC power with constant voltage and constant frequency from the storage battery through the inverter to continue supplying power to the load to ensure that the load receives uninterrupted high-quality AC power.
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The future development direction of UPS is to replace the third-generation UPS with SCR multi-phase phase-controlled rectifier boosted by output transformer with high-frequency UPS.
The currently widely used third-generation UPS has two shortcomings: one is large in size and quality; second, the harmonic content of the mains input current is large, and the mains input power factor is low.
First of all, because the third-generation UPS uses SCR multi-phase phase-controlled rectifiers, this rectifier is a step-down converter, and the SPWM inverter is also a step-down converter, so the mains power supply passes twice After the step-down conversion, if you want to have the same output voltage level as the mains voltage, you must use the mains input multi-phase rectifier transformer or the UPS output isolation transformer to boost the voltage, which increases the volume and quality of the UPS. Secondly, the SCR multi-phase phase-controlled rectifier is a rectifier with a time delay link with a displacement factor. Its mains input current has a large harmonic content, and the harmonic content is also greatly affected by the size of the load. Increasing the number of phases of the input rectifier transformer or adding a passive filter at the input can reduce the harmonic content of the mains input current, but it cannot solve the problem of the harmonic content of the mains input current and the displacement factor caused by phase control. The issue of impact. The extent of the impact is shown in Table 1. Taking the 6-pulse rectification method as an example, although the method of increasing the filter can reduce the THDi of the mains input current to 5% ~ 10%, and increase the mains input power factor to 0.99, but due to the load on the THDi and displacement The influence of the factor can only be achieved when the load rate is greater than 60%, and it is impossible to do it when the load rate is less than 40%. Furthermore, SCR phase-controlled rectification is a time-delay link, which is also greatly affected by the displacement factor. As for the fourth-generation high-frequency UPS, due to the use of IGBT SPWM high-frequency Boost PFC rectifier, it is a high-speed rectifier that eliminates the influence of the displacement factor, so it can maintain the mains even in the load range of 10% to 100% The THDi of the input current is 5%~10%, and the mains input power factor is greater than or equal to 0.99.
|Rectification method ↓ /Load factor→||No load||25% load||50% load||75% load|
|6-pulse rectification||86%||50 %||40 %||30 %|
|6 pulse + 5 times filter||50 %||30 %||15 % ~20 %||10 % ~12 %|
|12-pulse rectification||20 %||15 %||12 %||9.5|
|12 pulse + 11 times filter||12 % ~15 %||8 % ~12 %||5 % ~8 %||4.5 %|
It can be seen from the above introduction that the fourth-generation UPS, that is, high-frequency UPS, uses IGBT SPWM high-frequency Boost PFC rectifier to replace the SCR multi-phase phase-controlled rectifier with input transformer. The advantage brought by this is: no need for boosting Transformer; reduces the volume and quality of the UPS: eliminates the influence of the displacement factor; can reduce the THDi of the mains input current to less than 5% without being affected by the load rate: the input power factor of the mains can be increased to 0.99 or more. Therefore, the fourth-generation UPS is considered to be the development direction of today’s UPS.