Researching Transformer-Based and also Transformerless Uninterruptible Power Supplies

Choosing between transformer-based or transformerless uninterruptible power supplies may possibly 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 quantities of input harmonic distortion that they generate.

Transformer-based Uninterruptible Power Supplies: before the early 1990s, the only real design of online uninterruptible power was transformer-based. Nowadays, the design remains available but generally in larger sizes for UPS from eight to 800kVA. The most frequent 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 principal function of the transformer is to boost 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) in place 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 options that come with a MOSFET gate to make a versatile, high frequency switching device. As a result has given rise to stronger, efficient and reliable inverters.

Transformer-based UPS may 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 permits it to be powered from two separate ac supply sources thus adding further resilience. A transformerless UPS can be installed with dual input capability, with supplies produced 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 principal purpose behind  FFPOWER the introduction of transformerless units was to reduce the general physical size and weight thus making an uninterruptible power unit more ideal for smaller installations and/or computer room/office type environments, where space may be limited. In addition it generates far 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 become more sensitive to this kind of distortion.

Rather than the step-up transformer, a transformerless UPS runs on the staged means of 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 through a mid-point booster circuit to step the dc voltage up to 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 to an ac voltage to produce the load.

An added advantageous asset of this method is that the rectifier can operate from the 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 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 the 2 may be clear. It is where the 2 ranges overlap, when it comes to power rating, that the decision is more complicated. Consideration needs to get then to: initial purchase cost, physical size, running costs, the installation environment, and in particular, the quantities of input harmonic distortion they generate. Both designs can be operated in parallel to reach higher quantities of availability and resilience.

During the last decade, the gap between both of these uninterruptible power technologies has reduced as manufacturers have applied common techniques and research & development efforts to both designs. The driving force behind this has been cost and size, alongside demands to boost operating efficiency and reduce harmonic generation. When it comes to online performance, both designs provide the same degree 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.