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Traditional data center UPS systems have for years used some form of double conversion design, taking alternating current (AC), change it to direct current (DC), which charges the batteries and then re-convert it back to AC. These UPS systems have used rather large "modules" to create high capacity systems or to obtain "N+1" redundancy. Three kVA UPSs, for example, could be intended to deliver a maximum of1, kVA, so if any one unit fails or is shut down for service, the full design capacity is still available.
In recent years, the trend has been toward the use of much smaller modules ( kVA to kVA) to make up larger UPS systems. As with anything in engineering there are advantages and disadvantages. The principle advantages touted for the modular approach are the ability to grow capacity as needed (assuming an initial right-sizing) and reduced maintenance cost. The modules are hot swappable and can be returned to the factory by the user for exchange or repair. Modular systems are also generally designed to accept one more module than is required for their rated capacity, making them inherently "N+1" capable at much lower cost than would be possible with the very large system.
The last potential advantage to modular UPS systems is efficiency. A UPS system runs at highest efficiency when it is near its maximum rated capacity. As load level drops, so does efficiency. The losses may not seem great on the surface, but it adds up and as we become increasingly concerned about energy waste and cost, this starts to become a consideration.
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Modular UPS systems can be configured, and readily re-configured, so they are running close to capacity. Large, traditional UPS systems are usually purchased with all the capacity anticipated for the future, so they often run well below capacity for a number of years, if not forever. Redundancy, however, always means running below capacity which also means reduced efficiency. This can be minimized in an "N+1" modular system through careful power management.
However, with any "2N" redundant configuration, regardless of type, it is always necessary to manage power so that no system is loaded beyond% of its capacity, otherwise it will overload if the duplicate, load-sharing system fails. As a result, every UPS running in a "2N" mode operates at less than maximum efficiency. Again, with very careful management, a modular UPS may be configured more closely than a larger, fixed-capacity system, and this might result in some long-term power savings. There are many "ifs", "coulds", and "mays" in this scenario.
Traditional data center UPS systems have for years used some form of double conversion design, taking alternating current (AC), change it to direct current (DC), which charges the batteries and then re-convert it back to AC. These UPS systems have used rather large "modules" to create high capacity systems or to obtain "N+1" redundancy. Three kVA UPSs, for example, could be intended to deliver a maximum of1, kVA, so if any one unit fails or is shut down for service, the full design capacity is still available.
