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PowerViews
Transformer or Transformer-free -- that is (not necessarily) the question
Bill Campbell, Senior Product Manager, AC power Systems, Emerson Network Power
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Initial Costs (CAPEX)
Due to the absence of transformers, most transformer-free UPS modules will be less expensive than the transformer-based models. They will also have a smaller footprint, reducing the need for additional space in the data center. The cost/benefit of these systems can be deceptive if an engineer determines that an input and/or output isolation transformer(s) is required. The addition of transformer requirements outside the UPS results in higher overall purchase costs, increased total footprint, and increased labor to install and wire the additional equipment. Plus, the external transformer may not be as well-coordinated as it would be with a transformer-based UPS design.
Operating Costs (OPEX)
Both the transformer-free and transformer-based UPS modules can achieve similar efficiency and full-load performance factors, with transformer-free having the slight edge. Both designs typically feature efficiency optimization options, including enabling the critical load to normally operate in the UPS bypass mode (“eco-mode”) and transferring back to the UPS inverter within a few milliseconds when the input source has an out-of-limits disturbance.
Technological evolution is constantly impacting the relative efficiency of transformer-based and transformer-free solutions. After years of optimizing performance, transformer-based UPS systems have achieved a relatively flat efficiency curve from 30 percent to 80 percent loading where typical Tier 3 and Tier 4 data centers operate. The latest transformer-free designs also have very flat curves down to as low as 20 percent of capacity, and have efficiencies in the 96 percent range in double conversion mode. A study of the whole system design is necessary to determine the relative efficiencies as the addition of transformers, and the efficiency of those transformers, will have an impact.
At the end of the day, transformers, whether internal or external to the UPS, are necessary to establish circuit isolation and local neutral and grounding points, as well as to provide voltage transformation points. This facilitates, for example, the implementation of very high power density installations based on 600 Volt distribution sources, subsequently stepped down to 208/120 Volts for IT load applications. When utilized in conjunction with the UPS internal dc link, dc-to-ac output and ac-to-dc input isolation can be provided, reducing or eliminating the risk of dc faults propagating upstream or downstream of the UPS. Because of data center designer requests, most leading UPS suppliers continue to offer both topologies as part of transformer requirement solutions.
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About the Author
As Senior Product Manager, AC Power Systems, Emerson Network Power, Bill Campbell is responsible for the integration of outside-sourced ancillary power equipment, such as switchgear, energy storage and other alternative ac and dc energy systems, with Liebert UPS modules for high-availability, high-efficiency UPS systems.
We welcome the opportunity to publish your opinions. Please email us at editorial@darnell.com.
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