UPS Systems - Steps towards more efficient static UPS systems
The selection of static UPS systems has tended to focus on system reliability
represented by availability and mean time between failure (MTBF). However, as energy costs have skyrocketed, two issues are conspiring to make efficiency central to UPS evaluation; a focus on total cost of ownership (TCO), and green or environmental considerations says Shri Karve, business development director, APC by Schneider Electric
UPS efficiency is invariably quoted at near full load, but when operating with a light load efficiency can drop substantially. Losses fall under three categories: no-load loss, proportional loss, and square-law loss.
No-load losses can be considered as an overhead loss as they are independent of load and result from the need to power components like transformers, capacitors, and communication cards. Since they represent more than 40% of all losses they are the largest opportunity for improving UPS efficiency. Proportional losses increase as load increases and a larger amount of power must be "processed" by components in the power path. As the load increases on the UPS, the electrical current running through its components increases causing losses with the square of the current sometimes called square-law losses.
System Design and Efficiency
When designing systems, it is important to balance levels of availability (i.e., how mission critical is the protected load), with overall capital costs, installation and engineering costs and operational expenses (TCO). However, system design has an impact on overall efficiency e.g., in a resilient, 2N system both sides of the "N" must be capable supporting the full load in the event of a failure. Even with one of the UPS operating at full capacity, the maximum efficiency of the system would be <50%. In reality it is highly unlikely that the UPS would be operating at full capacity and therefore inefficiency levels would be raised. And, of-course, there would be the cost impact of servicing and maintaining two units.
For more details about system design, White Paper #75 "Comparing UPS System Design Configurations" available from www.apc.com/gb provides a useful guide.
Improving UPS Efficiency
To meet today's efficiency demands, UPS manufacturers can utilise three factors to reduce UPS losses; technology, topology, and modularity.
Technology
IGBTs enable the power conversion process to be operated in a "high frequency pulse-width-modulation (PWM)" mode which reduces the size of filter components leading to further efficiency improvements. Replacing analog controls with more advanced DSP controls can improve efficiency through intelligent adaptive switching, especially at lighter loads. In addition, DSP controls are lower power and allow a substantial reduction in no-load losses.
Topology
Two principal topologies used in large UPS systems are double conversion on-line and delta conversion on-line. In delta conversion UPS, efficiency is improved mainly by reducing no-load losses and by a reduction in square-law losses. By using the input transformer in a series arrangement, the UPS input current and output voltage can be fully regulated without having to convert all incoming power to DC and back to AC again.
Modularity
The closer a UPS operates to capacity, the more efficient it will be. Modularity allows users to size the UPS system closely to the load and to scale up as the power requirement grows.
Modularity enables improved serviceability and reduced maintenance requirements through self-diagnostics and user replaceable modules.
Conclusion
As total cost of ownership has become a key decision factor when specifying large UPS systems, the differentiating value is efficiency. With the exception of Rotary solutions, UPS technologies continue to evolve toward greater electrical efficiency. In order to maximise energy use and reduce wastage through losses, it is both financially and environmentally expedient to specify the most efficient UPS and to design and operate systems as close to the protection needs of the load as possible while not ignoring future requirements. UPS products and solutions from APC by Schneider Electric, with published efficiency numbers provide greater evidence for decision making.
For more details, please see White Paper #108, Making Large Static UPS More Efficient, available from www.apc.com/gb
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