The Important Differences between Transformer-Free and Transformer-Based UPS

Why do uninterruptible power supplies have transformers? The answer is in one of the most common applications of a transformer: to step up output voltage.

The new power converters introduced somewhere at the start of this century brought not only substantial customer benefits but also removed the requirement to have a transformer. Thus, the age of transformer-free UPS began. Succeeding evolution was quick: Thyristor-based designs – with six-pulse, then later, 12-pulse, thyristor bridges - rapidly paved the way to today’s topology, which is constructed using insulated-gate bipolar transistors (IGBTs). Currently, almost all double conversion UPSs have IGBT power converters and are transformer-free.

Transformer-Based UPS

In a transformer-based UPS, when the mains supply is present, the power flows via the rectifier, transformer and inverter to the output to deliver the critical load. This working mode is referred to as double conversion mode. In double conversion mode, the battery is continuously kept fully charged. During power outages, the battery feeds the inverter, which then delivers continuous power to the critical load through the transformer.

The static bypass serves as an emergency path that is switched in when there is an issue on the double conversion path such as over temperature, overload, or output short circuit. As its name indicates, the maintenance bypass switch links input to output and allows the UPS to be bypassed. Other switches then allow the unit to be separated and serviced.

Transformer-Free UPS

The operational principle of a transformer-free UPS is the same as that of a transformer-based UPS except that - because the IGBTs can deal with high voltages there is no requirement for a step-up transformer after the inverter. This increases energy efficiency - usually from 90 to 96% - which is the industry-standard. Moreover, transformer-free UPSs have a smaller footprint and are lighter, thus reducing investment and running costs.

Due to the higher voltage involved in transformer-free UPSs, an extra converter is added between the battery and the DC bus. This converter enables a continual and precise control of the batteries and offers a clean DC voltage without any ripple, which increases the battery lifetime.

However, those are not the only benefits. Total harmonic distortion is radically reduced and the input power factor (PF) is resistive due to active control of the input currents. This means that the devices upstream of the UPS (eg: generators) do not have to be oversized by a factor of 1.5 (or even more) as is usual with transformer-based UPS.

A few other differences can be identified. When a transformer-free UPS is used, the output impedance and the dynamic reaction to unbalanced loads are better because of the direct control of the output sine wave and the fact that each phase is independently. controlled. The inverter’s output short-circuit capability is also better than a transformer-based UPS.

transformer-based UPS

Figure 1. Single-line diagram of a transformer-based UPS.

transformer-free UPS

Figure 2. Single-line diagram of a transformer-free UPS.


Major Technical Specification Differences (Typical)

Table 1. summary of the major technical differences between the two topologies.

#   Transformer-based Transformer-free
1 Efficiency on double conversion mode 90-92 percent 95-96 percent
2 Efficiency on eco-mode 99 percent 99 percent
3 Current total harmonic distortion (THDi) 30 percent (6-pulse thyristor-based rectifier)
12 percent (12- pulse thyristor-based rectifier)
3-4 percent (IGBT rectifier)
3-4 percent
4 Input PF Low on partial load 0.99 – 0.97 at full and partial load
5 AC ripple on battery Without battery charger more than 5 percent
with battery charger 0.2 percent
0.2 percent
6 Allowed number of battery blocks in series (12 V) Fix (typically 40) Variable from 30 to 50
7 Output impedance High (worse) Low (better)
8 Output fault clearing capability on inverter Typically up to 5 x In Typically up to 3 x In
Up to 4-5 x In or even higher may be option with more cost
9 Output fault clearing capability on bypass Up to 10 x In Up to 10 x In
10 Dynamic response Poor, unbalanced loads affect output voltages Ideal, direct control of the output sine wave, each phase is controlled inde-pendently. Thus, unbalanced loads do not affect the output voltages
11 Weight (one 500 kVA unit) 2.2 – 2.6 tons 1 ton


Major Differences for the Customers (Effect on the Customer Benefits)

Table 2. summary of the major differences between the two topologies for the customers.

#   Transformer-based Transformer-free
1 Investment cost $$$ due to higher UPS cost, higher installation cost (oversizing of the system upstream of the UPS) and larger footprint. $$
2 Operational cost $$$ due to lower efficiency means higher energy costs for both UPS and cooling. $ Higher efficiency reduced power losses and less cooling. Over many years, the saving is significant.
3 Environmental impact for production and transport to final location Higher than transformer-free due to more components (transformer) and bigger mechanical size for the unit Lower than transformer-based thanks to the same arguments
4 Environmental impact for operating the product Lower efficiency means higher power losses thus more energy needed for cooling. Higher efficiency means less power loss and less energy needed for cooling
5 Battery life (lead-acid) May be reduced due to AC ripple Up to 12 years


This information has been sourced, reviewed and adapted from materials provided by Innovative ABB Power Conditioning - Discrete Automation and Motion Division.

For more information on this source, please visit ABB Power Conditioning - Discrete Automation and Motion Division.


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