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August 20, 2012

Transformer or Transformer-free -- that is (not necessarily) the question

Bill Campbell
Senior Product Manager, AC power Systems, Emerson Network Power

Power Channels: Energy Efficiency, Power Components, Switch-Mode Power

One of the debates currently occurring in the data center market is over which is the best UPS design to use in the data center, transformer or transformer-free. As in most debates, the sides have been drawn and the arguments focus on which technology is better. While an understanding of the technology is important, the focus should instead be on the particular application in question.

There is growing interest in using transformer-free UPS modules in higher power, three-phase mission critical power backup applications (e.g.; 200 kW to 5 MW). However, many organizations are still unclear about which architecture—transformer-based or transformer-free— is best suited for a particular application.

In general, transformer-based UPS systems are highly robust and excel at providing the highest capacities and availability while simplifying external and internal voltage management and fault current control. The latest transformer-free designs offer better efficiency, smaller footprint and improved flexibility while providing high levels of availability. Driven by data center designer demand, most leading UPS suppliers offer both topologies.

Presently, large transformer-free systems are constructed of modular building blocks that deliver high power in a lightweight, compact package. This modular design offers advantages when the timing of future load requirements is uncertain by allowing capacity to be more easily added as needed, either physically or via control settings. On the other hand, a modular design means higher component counts, which may result in lower unit Mean Time Between Failure (MTBFu) and higher unit service rates.

For high-power enterprise data centers and other critical applications, a state-of-the-art transformer-based UPS still provides an edge in availability. Transformers within the UPS provide integrated fault management and galvanic isolation as well as greater compatibility with critical power distribution system requirements that should be considered when designing a high availability UPS system. Technology developments and configuration options allow the latest transformer-based designs to operate at higher efficiencies compared to previous designs, making them more comparable to the transformer-free models in terms of efficiency.

However, if operational efficiency, expansion flexibility or limiting UPS footprint are of paramount importance, and other appropriate measures are instituted to provide an acceptable level of availability, transformer-free technology may be the optimal choice. In general, 200 kW is a threshold below which the space, weight and cost advantages of transformer-free UPS systems outweigh the robustness and higher capacity capabilities of transformer-based systems. These under-200 kW applications can benefit from the high efficiency and excellent input power conditioning through active components offered by transformer-free designs. In addition, the scalability of a modular transformer-free UPS can help avoid over-provisioning while maintaining operational efficiency.

When considering the total cost of ownership for these two architectures, it is important to include both the initial upfront or capital expenses (CAPEX) as well as the ongoing or operating expenses to power, maintain and service various options (OPEX ).

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