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

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

The inverter generates an ac supply from its dc power source, which is fed into a step-up transformer. The primary function of the transformer is to improve the inverter ac voltage to that 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 top features of a MOSFET gate to form a versatile, high frequency switching device. Therefore has given rise to more powerful, efficient and reliable inverters.

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

Transformerless Uninterruptible Power Supplies: transformerless UPS is really a newer design, commonly available from 700VA to 120kVA. The primary purpose behind the introduction of transformerless units was to cut back the general physical size and weight thus making an uninterruptible power supply unit more ideal for smaller installations and/or computer room/office type environments, where space might be limited. Additionally, it generates far 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 be much more sensitive to this type of distortion.

As opposed to the step-up transformer, a transf FFPOWER ormerless UPS runs on the staged procedure for voltage conversion. The first 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 into a dc voltage. This is passed via a mid-point booster circuit to step the dc voltage up to typically 700-800Vdc where a battery charger and inverter are powered. In the second stage, the inverter takes the supply from the booster-converter and inverts it back once again to an ac voltage to produce the load.

An added advantageous asset of this technique is that the rectifier can operate from whether three or single-phase input supply. This is often configured at installation for systems up to 20kVA. A get a handle on system ensures a well balanced, regulated dc voltage is supplied to the inverter constantly and the inverter can operate no matter UPS output load variations or mains power supply fluctuations or disturbances.

Choosing between Transformer-based or Transformerless Uninterruptible Power Systems: in several applications the choice between the two might be clear. It’s where the two ranges overlap, in terms of power rating, that your choice is more complicated. Consideration must get 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 could be operated in parallel to accomplish higher degrees of availability and resilience.

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