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 key differences between them are: physical size, efficiency, noise output and the levels 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 design is still available but generally in larger sizes for UPS from eight to 800kVA. The most typical applications because of this are large industrial sites.
The inverter generates an ac supply from its dc power source, which can be fed in to a step-up transformer. The primary function of the transformer is to increase 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 method of isolating input and output).
Modern inverter designs use IGBTs (Insulated Gate Bipolar Transistors) rather than 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. P2001 power station This in turn has given rise to stronger, 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 simply removing a linking connector from its input terminal. This allows 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 exact same source, but this really 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 general physical size and weight thus making an uninterruptible power supply 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 making 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 works on the staged process of 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 in to a dc voltage. This really is passed via 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 next stage, the inverter takes the supply from the booster-converter and inverts it back to an ac voltage to provide the load.
An added good thing about this method is that the rectifier can operate from either a three or single-phase input supply. This is configured at installation for systems up to 20kVA. A control system ensures a stable, regulated dc voltage is supplied to the inverter at all times and the inverter can operate aside from UPS output load variations or mains power supply fluctuations or disturbances.
Choosing between Transformer-based or Transformerless Uninterruptible Power Systems: in several applications the option between both may be clear. It’s where both ranges overlap, when it comes to power rating, that the decision is more complicated. Consideration must be provided with then to: initial purchase cost, physical size, running costs, the installation environment, and specifically, the levels of input harmonic distortion they generate. Both designs can be operated in parallel to attain higher levels 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 has been cost and size, alongside demands to boost operating efficiency and reduce harmonic generation. With regards to online performance, both designs provide the exact same amount of performance and are classified as VFI systems (voltage and frequency independent – in accordance with EN/IEC 62040-3). Their principal differences are their effects on upstream supplies and the operating environment.