Looking at Transformer-Based along with Transformerless Uninterruptible Power Supplies

Choosing between transformer-based or transformerless uninterruptible power supplies may not be an easy ‘either/or’ decision, particularly above 10kVA. Both technologies have their devote today’s power protection scenarios but the important thing differences between them are: physical size, efficiency, noise output and the degrees of input harmonic distortion that they generate.

Transformer-based Uninterruptible Power Supplies: before the early 1990s, the only design of online uninterruptible power was transformer-based. Nowadays, the style remains available but generally in larger sizes for UPS from eight to 800kVA. The most common applications with this are large industrial sites.

The inverter generates an ac supply from its dc power source, which will be fed in to a step-up transformer. The primary function of the transformer is to improve the inverter ac voltage to that particular required by the load. The transformer also protects the inverter from load disruption, whilst also providing Galvanic isolation (a approach to isolating input and output).

Modern inverter designs use IGBTs (Insulated Gate Bipolar Transistors) in the place of more traditional switching components (such as power transistors and thyristors). IGBTs combine the fast-acting and high power capability of the Bipolar Transistor with the voltage control features of a MOSFET gate to form a versatile, high frequency switching device. This in turn has given rise to more powerful, efficient and reliable inverters.

Transformer-based UPS will also be given a twin input option as standard, which is often selected at installation simply by removing a linking connector from its input terminal. This enables it to be powered from two separate ac supply sources thus adding further resilience. A transformerless UPS may be installed with dual input capability, with supplies derived from the same source, but that is typically a factory-fit option.

Transformerless Uninterruptible Power Supplies: transformerless UPS is a newer design, commonly available from 700VA to 120kVA. The primary purpose behind the introduction of transformerless units was to cut back the entire physical size and weight thus making an uninterruptible  CN505 power station power unit more ideal for smaller installations and/or computer room/office type environments, where space might be limited. Additionally it generates much less noise and heat than its transformer-based cousin and has far lower input harmonic distortion levels rendering it suitable for environments where electronic equipment (such as computers) may become more sensitive to this type of distortion.

Instead of the step-up transformer, a transformerless UPS works on the staged process of voltage conversion. The initial stage combines a rectifier and booster-converter to generate a dc supply for the inverter. An uncontrolled, three-phase bridge rectifier converts the ac supply in to a dc voltage. This is passed by way of a mid-point booster circuit to step the dc voltage around typically 700-800Vdc from which a battery charger and inverter are powered. In the second stage, the inverter takes the supply from the booster-converter and inverts it back again to an ac voltage to provide the load.

An added good thing about this process is that the rectifier can operate from whether three or single-phase input supply. This is configured at installation for systems around 20kVA. A get a grip on system ensures a reliable, regulated dc voltage comes to the inverter all the time and the inverter can operate no matter UPS output load variations or mains power fluctuations or disturbances.

Choosing between Transformer-based or Transformerless Uninterruptible Power Systems: in lots of applications the option between both might be clear. It is where both ranges overlap, when it comes to power rating, that your choice is more complicated. Consideration needs to be provided with then to: initial purchase cost, physical size, running costs, the installation environment, and in particular, the degrees of input harmonic distortion they generate. Both designs may be operated in parallel to reach higher degrees of availability and resilience.

Over the last decade, the gap between these two uninterruptible power technologies has reduced as manufacturers have applied common techniques and research & development efforts to both designs. The driving force behind it’s been cost and size, alongside demands to boost operating efficiency and reduce harmonic generation. With regards to online performance, both designs provide the same amount of performance and are classified as VFI systems (voltage and frequency independent – in respect with EN/IEC 62040-3). Their principal differences are their effects on upstream supplies and the operating environment.

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