Choosing between transformer-based or transformerless uninterruptible power supplies might not be a straightforward ‘either/or’ decision, particularly above 10kVA. Both technologies have their place in 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 they generate.

Transformer-based Uninterruptible Power Supplies: before early 1990s, the sole design of online uninterruptible power supply was transformer-based. Nowadays, the look 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 is fed into a step-up transformer. The principal function of the transformer is to boost 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 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 options that come with a MOSFET gate to form a versatile, high frequency switching device. As a result has given rise to more powerful, efficient and reliable inverters.

Transformer-based UPS may also be supplied with a dual input option as standard, which can be selected at installation 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. FFPOWER  A transformerless UPS may be installed with dual input capability, with supplies derived from exactly the same source, but this really is typically a factory-fit option.

Transformerless Uninterruptible Power Supplies: transformerless UPS is really a newer design, commonly available from 700VA to 120kVA. The principal purpose behind the introduction of transformerless units was to reduce 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. It also generates less noise and heat than its transformer-based cousin and has far lower input harmonic distortion levels which makes it suitable for environments where electronic equipment (such as computers) may be more sensitive to this sort of distortion.

In place of the step-up transformer, a transformerless UPS uses a staged procedure for voltage conversion. The very 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. That is passed through a mid-point booster circuit to step the dc voltage up to 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 to an ac voltage to produce the load.

An added benefit of this approach is that the rectifier can operate from whether three or single-phase input supply. This can be configured at installation for systems up to 20kVA. A control system ensures a stable, regulated dc voltage is supplied to the inverter all the time and the inverter can operate irrespective of UPS output load variations or mains power supply fluctuations or disturbances.

Choosing between Transformer-based or Transformerless Uninterruptible Power Systems: in lots of applications the choice between the 2 might be clear. It’s where the 2 ranges overlap, when it comes to power rating, that your choice is more complicated. Consideration needs to be given then to: initial purchase cost, physical size, running costs, the installation environment, and specifically, the degrees of input harmonic distortion they generate. Both designs may be operated in parallel to accomplish higher degrees of availability and resilience.

Over the last decade, the gap between those two uninterruptible power supply 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. In terms of 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.

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