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What Are The Differences Between Transformer-Based And Transformerless UPS?

Also referred to as transformer-free, transformerless UPS systems were first developed in the 1990s and offered a number of benefits over traditional transformer-based systems in terms of higher efficiency, reduced size and weight, and cost savings. 

Transformerless uninterruptible power supplies are now common in data centre environments and with smaller installations. They are the typical technology for the smallest power ratings (below 10 kVA) and are available up to around 300 kVA at the higher end of the spectrum. Riello UPS's range of transformer-free solutions includes the Sentryum, Multi Sentry, and NextEnergy series. 

Available from 10 kVA and above, transformer-based UPS are still a popular choice in industrial process environments or installations requiring galvanic isolation

A transformer is a wound component consisting of windings around a core, with iron sheet laminates that can be used to change voltage levels and provide galvanic isolation. 


How Do Transformer-Based And Transformer-Free UPS Work?

In a traditional transformer-based UPS, the power flows via the rectifier, inverter and transformer to the output, with the transformer used to step up the AC voltage levels, protect the UPS from load disruptions and provide galvanic isolation. 


Transformerless UPS or transformer-free UPS operate in the same way, apart from one key difference. It uses insulated-gate bipolar transistors (IGBTs) that are capable of dealing with high voltages, eliminating the need for a step-up transformer after the inverter. This improves the energy efficiency of transformer-free uninterruptible power supplies.  


Thanks to R&D and technological improvements, the latest transformer-based UPS can achieve similar levels of efficiency as many transformerless systems (95-96%), although the latter still has the efficiency edge when carrying lower loads as it delivers a flatter efficiency curve. 


What Are The Advantages Of Transformer-Based UPS? 

There are two main benefits of a transformer-based UPS. Firstly, it is generally accepted that they are more robust – there are less points of failure. Secondly, the transformer provides galvanic isolation, a separation of the input and output supplies, which protects the load from any spikes, surges, or electrical noise. 

Transformer-based UPS are the typical technology for 100 kVA and above and the choice to achieve large kW sizes or provide redundancy.  

Main benefits of Transformer-based UPS: 

  • Galvanic isolation 
  • Independent mains power supplies 
  • Dual load protection from DC voltage 
  • Providing a higher phase-neutral inverter short circuit current than a phase-phase short circuit current 
  • Superior power protection when presented with power quality problems 
  • Greater robustness with respect to back feed protection 


What Are The Advantages Of Transformer-Free UPS? 

The obvious benefit of a transformerless UPS is the lack of a big, bulky, and heat-generating transformer. Transformers are expensive too, so eliminating them reduces initial capital costs. 

Main benefits of Transformeless UPS: 

  • Physical: reduced size and weight (a factor for space-restricted data centres) 
  • Operational: higher energy efficiency (particularly at lower loads), lower noise levels, and less heat 
  • Cost: lower purchase, installation, and running costs (i.e. needs less air conditioning) 


One of the main drawbacks with transformer-free UPS systems is they can’t clear and isolate internal faults as well as a transformer-based unit.  

A solution to this is installing isolation transformers to mirror the strength of a transformer-based system, but this would significantly increase cost and footprint, while also introducing additional points of failure. 

Another issue with transformerless UPS power supplies is their power strength limitations. To achieve larger kW size or redundancy, several transformer-free UPS modules need to be paralleled together – the more modules (and components), the greater the likelihood of failure. 


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