First appearing at lower power levels, transformerless uninterruptible power system (UPS) designs have been around for over a decade. With the advances in technology over the years, utilizing transformerless UPS modules for high-power enterprise data centers has become a major interest.

The driving force behind this has been cost and size, alongside demands to improve operating efficiency. Transformerless UPS are generally recognized as more efficient and having a higher power factor than an equivalent transformer-based design, therefore operating costs can be lower.

Over the last decade, the gap between transformerless and transformer-based UPS has reduced as manufacturers have improved component count, design, control and manufacturing techniques to both designs. In terms of online performance, both designs provide the same level of performance and are classified as voltage and frequency independent systems. Their principal differences are their effects on upstream supplies and the operating environment.

Understanding the Two Designs
The transformer-based and transformerless approach both use double conversion process to provide power protection for mission-critical applications. Double conversion is the ultimate in power protection as it removes a wider range of disturbances and provides a seamless transition to battery power. Moreover, both designs can be operated in parallel to achieve higher levels of availability and resilience.

The primary difference between the two technologies is the use of transformers in the design. A transformer-based UPS may use a transformer before the rectifier and requires an isolation transformer after the inverter to derive the voltage being delivered to the critical load.

A transformerless UPS design, meanwhile, utilizes developments in power and control electronics technology to eliminate the need for an isolation transformer on the output of the inverter. Advancements in power semiconductors and control have also allowed the pulse width modulation (PWM) switching frequencies to increase, allowing the use of Insulated Gate Bipolar Transistors (IGBT) within the rectifier stage.

Which Design to Choose?
Choosing between a transformerless or transformer-based UPS requires careful consideration. Consideration needs to be given to:

• Initial purchase cost – Due to the removal of transformers and the reduced need for additional space, transformerless modules will be less expensive. This becomes deceptive however, if an engineer determines that an input/output isolation is required outside of the UPS, this result in higher overall purchase costs.
• Operating costs – Both UPS designs have similar high efficiency and full-load performance factors.
• Physical size – Transformerless UPS topologies have emerged as a solution for the demand for efficient, flexible, smaller, lighter weight UPS systems to accommodate today’s quickly changing IT requirements.
• Installation environment – Transformerless UPS modules are ideal in locations that have limited spaces or when budget does not permit to purchase additional space.
• Availability – Transformer-based online UPS boasts several intrinsic advantages compared to its transformerless counterparts. For high risk, mission-critical systems such as telecoms, healthcare, industrial and military environments, transformer-based UPS provides a safer and more robust solution. Any mission-critical operation requires the utmost reliability --- this requires deployment of proven solutions to minimize the likelihood of potential equipment failure. 

Emerson Network Power explains in a whitepaper how transformerless UPS models should be considered a viable option for lower power, small and medium business applications where achieving the highest availability is not the top concern and space and weight restrictions inhibit the use of traditional transformer-based designs.