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Comparison between AC and DC UPS

  • Comparison between AC and DC UPS

    What are the main types of UPS?

    UPSs come in three major varieties, which are also known as topologies:

    Single-conversion systems

    In normal operation, these feed incoming utility AC power to IT equipment. If the AC input supply falls out of predefined limits, the UPS utilizes its inverter to draw current from the battery, and also disconnects the AC input supply to prevent backfeed from the inverter to the utility. The UPS stays on battery power until the AC input returns to normal tolerances or the battery runs out of power, whichever happens first. Two of the most popular single-conversion designs are standby and line-interactive:

    • Standby UPSs allow IT equipment to run off utility power until the UPS detects a problem, at which point it switches to battery power. Some standby UPS designs incorporate transformers or other devices to provide limited power conditioning as well.

    Line-interactive UPSs regulate input utility voltage up or down as necessary before allowing it to pass through to protected equipment. However, like standby UPSs, they use their battery to guard against frequency abnormalities.


    Double-conversion systems


    As the name suggests, these devices convert power twice. First, an input rectifier converts AC power into DC and feeds it to an output inverter. The output inverter then processes the power back to AC before sending it on to IT equipment. This double-conversion process isolates critical loads from raw utility power completely, ensuring that IT equipment receives only clean, reliable electricity.

    In normal operation, a double-conversion UPS continually processes power twice. If the AC input supply falls out of predefined limits, however, the input rectifier shuts off and the output inverter begins drawing power from the battery instead. The UPS continues to utilize battery power until the AC input returns to normal tolerances or the battery runs out of power, whichever occurs sooner. In case of a severe overload of the inverter, or a failure of the rectifier or inverter, the static switch bypass path is turned on quickly, to support the output loads.



    Multi-mode systems

    • These combine features of both single- and double-conversion technologies while providing substantial improvements in both efficiency and reliability:


    • Under normal conditions, the system operates in line-interactive mode, saving energy and money while also keeping voltage within safe tolerances and resolving common anomalies found in utility power.


    • If AC input power falls outside of preset tolerances for line-interactive mode, the system automatically switches to double-conversion mode, completely isolating IT equipment from the incoming AC source.


    • If AC input power falls outside the tolerances of the double-conversion rectifier, or goes out altogether, the UPS uses the battery to keep supported loads up and running. When the generator comes online, the UPS switches to double-conversion mode until input power stabilizes. Then it transitions back to high-efficiency line-interactive mode.

    How do I choose the right UPS for the job?

    Question number one is should you get a single-conversion, double-conversion or multi-mode UPS? The answer depends largely on how important energy efficiency is for your organization relative to protection. Single-conversion UPSs are more efficient than double-conversion devices, but offer less protection. That makes them a good fit for loads with a higher tolerance for failure. More specifically, standby UPSs (the most basic type of single conversion UPS) are generally the best option for smaller applications, like desktop and point-of-sale solutions, while line-interactive UPSs are typically preferable for smaller server, storage and network applications located in facilities with access to relatively trouble-free AC utility power. Double-conversion UPSs, which provide the highest levels of protection, are less efficient but are usually the standard choice for protecting mission-critical systems. Multi-mode UPSs, although they may be more expensive than either single or double conversion systems, are the best choice for companies looking to achieve an optimal blend of both efficiency and protection.


    Energy efficiency comparison

    The total efficiency of a direct current system can be made greater than in present ac systems owing to elimination of the extra conversion step of the inverters. The centralization of rectifiers and PFC circuits can be made more efficient than when each single device includes rectifying the ac to dc with (more or less effective) power factor correction. Moreover, all equipment connected to the mains network would be embraced by the PFC technique in contrast to now, when a lot of small, and indeed also large, equipment is not included. Use of dc will provide higher energy efficiency and reduced losses when DC/DC converters are used in electric equipment instead of AC/DC power supplies.

    The efficiency of electric power distribution and equipment can become 5-20 % higher as compared to the present ac solutions. DC/DC converters can reach an efficiency of 85-90 % as compared to AC/DC power supplies which provide an efficiency of 65-75 %, typical values of PC power supplies . 

    Even if you compare best-in-class AC/DC to DC/DC you find a 2-5% advantage to the DC/DC, at a lower cost.

    A number of factors contribute to the lower power losses. Such factors are fewer conversion steps and that continuously running inverters, used in AC UPS equipment, no longer are needed. The reduced losses in the power supplies also means less cooling requirements for the premises.

    The buildings can be designed for an effective connection of alternative energy sources, for instance photovoltaic- or fuel cells. Connection of such cells to a direct current system is highly efficient as no losses for transformation to alternating current will occur.

    Altogether, the efficiency of the use of electrical power is estimated to increase by 5-20 % for applications that could be supplied with direct current. This estimation also takes into account losses of a potentially needed transformer to isolate the dc plant from the incoming ac grid. If the better efficiency of the DC UPS means that otherwise necessary cooling equipment can be eliminated or reduced, a figure up to 30 % may be reached

    The most common class of AC UPS has 85 % efficiency , but many UPS systems operate on lower efficiency. If connected directly to a high voltage line a best in class DC UPS can operate at 98 % efficiency.



    2. My Ton, Ecos Consulting, Brian Fortenbury. EPRI, High Performance Buildings: Data Centers Uninterruptible Power Supplies (UPS) December, 2005


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