Reviewing Transformer-Based and even Transformerless Uninterruptible Power Supplies

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

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

The inverter generates an ac supply from its dc power source, that is fed into a step-up transformer. The principal function of the transformer is to increase the inverter ac voltage compared to that required by the load. The transformer also protects the inverter from load disruption, whilst also providing Galvanic isolation (a way of isolating input and output).

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

Transformer-based UPS will also be provided with a dual input option as standard, which may be selected at installation by simply 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 based on exactly the same source, but that is typically a factory-fit option.

Transformerless Uninterruptible Power Supplies: transformerless UPS is just a newer design, commonly available from 700VA to 120kVA. The principal purpose behind the introduction of transformerless units was to cut back the entire physical size and weight thus making an uninterruptible power unit more suitable for smaller installations and/or computer room/office type environments, where space may 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 appropriate for environments where electronic equipment (such as computers) may be much more sensitive to this sort of distortion.

As opposed to the step-up transformer, a transformerless UPS uses a staged procedure for 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 into a P2001 power station  dc voltage. That is passed through a mid-point booster circuit to step the dc voltage around typically 700-800Vdc that 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 supply the load.

An extra good thing about this approach is that the rectifier can operate from either a three or single-phase input supply. This can be configured at installation for systems around 20kVA. A control system ensures a stable, regulated dc voltage is supplied to the inverter constantly and the inverter can operate regardless of UPS output load variations or mains power fluctuations or disturbances.

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

Throughout the last decade, the gap between those 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 enhance operating efficiency and reduce harmonic generation. With regards to online performance, both designs provide exactly the same degree 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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