Off-grid UPSs, including those with line regeneration, are often preferred to online UPS systems in small offices and data centers. Because they have lower acquisition and operating costs despite relatively poorer protection. This approach does not take into account existing differences between online UPS systems. Such as the ability of some online systems to operate in eco mode, which can provide better power protection at a lower cost.
Simple off-line UPS systems connect the load directly to the incoming AC grid. Line-interactive systems are also able to correct the UPS output when the input AC voltage deviates from preset limits by using an automatic voltage regulator based on an autotransformer. In the event of significant line voltage fluctuations or failures, these systems switch the UPS to battery operation.
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Because of their direct connection to the incoming AC mains, off-line UPSs, including line-interactive systems, have higher efficiency than online UPSs. However, an unstable grid environment, such as in the summer due to air conditioning or during stormy winter periods with frequent power surges or outages. Can cause these systems to switch to battery power frequently. This exposes the critical load to potential failures due to inconsistent or failed transmissions or battery failures due to multiple battery discharges, which drastically shortens battery life.
An online UPS system, often referred to as a double conversion system, first converts the input AC voltage to a stabilized DC voltage, which is then converted back to AC voltage to continuously supply the critical load with a pure, stabilized sinusoidal output, either from the input line via an AC/DC converter or from batteries in the event of a power failure.
The online double-conversion topology described above, while providing the best protection, reduces the efficiency of a single-phase UPS to about 90% compared to the 97% achieved by line-interactive systems. The additional operating costs associated with lower efficiency can increase the initial cost of the UPS by about 10% per year if cooling costs are not considered.
Recently, some manufacturers have started to offer online UPS systems with Eco mode of operation for single-phase, low-power online systems. In Eco mode, the online UPS can operated directly from the mains and automatically switches to online mode. If the AC mains fails or the set limits are exceed.
However, there is a big difference between a line-interactive UPS and an online system operating in Eco mode. A line-interactive UPS system MUST operate continuously from the AC mains, using the battery backup mode. For a short period of time, while an online system can operate continuously in both modes.
A line-interactive UPS will operate from the mains even in unstable mains conditions with frequent switching to batteries, as explained earlier. An online UPS behaves similarly under similar conditions when switched to eco mode. However, most of the hazards described here can avoided by selecting a double-conversion mode during unstable grid periods. Some UPS systems automatically switch to online mode. Even when Eco mode selected, when an unstable utility environment is detect.
The benefits associated with Eco mode are very enticing. It offers the best protection and high efficiency at the same time. Making it something like the best solution of all worlds. UPS systems with Eco mode offer an average energy saving of about 5%. When considering only the efficiency of the UPS. Taking into account the cooling needs, it is also possible to save about 10% of unnecessary energy consumption. Which reduces the initial investment in the UPS by almost 20% per year compared to a traditional online UPS.
It should noted that the Eco function generally not priced. Its implementation largely software-based, and manufacturers prefer not to increase the price of UPSs. Therefore, online UPS systems with Eco mode should preferred unless other considerations outweigh them.