A variety of design approaches are used to implement UPS systems, each with distinct performance characteristics. The most common design approaches are as follows:
The Standby UPS is the most common type used for Personal Computers. The transfer switch is set to choose the filtered AC input as the primary power source (solid line path), and switches to the battery / inverter as the backup source should the primary source fail. When that happens, the transfer switch must operate to switch the load over to the battery / inverter backup power source (dashed path). The inverter only starts when the power fails, hence the name "Standby."
The Line Interactive UPS is the most common design used for small business, Web, and departmental servers. In this design, the battery-to-AC power converter (inverter) is always connected to the output of the UPS. Operating the inverter in reverse during times when the input AC power is normal provides battery charging.
When the input power fails, the transfer switch opens and the power flows from the battery to the UPS output. The Line Interactive design usually incorporates a tap-changing transformer. This adds voltage regulation by adjusting transformer taps as the input voltage varies. Voltage regulation is an important feature when low voltage conditions exist, otherwise the UPS would transfer to battery and then eventually down the load. This more frequent battery usage can cause premature battery failure. However, the inverter can also be designed such that its failure will still permit power flow from the AC input to the output, which eliminates the potential of single point failure and effectively provides for two independent power paths. This topology is inherently very efficient which leads to high reliability while at the same time providing superior power protection.
Line Interactive: High efficiency, low cost, high reliability coupled with the ability to correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5kVA power range.
The Standby On-Line Hybrid is the topology used for many of the UPS under 10kVA which are labeled "online." The standby DC to DC converter from the battery is switched on when an AC power failure is detected, just like in a standby UPS. The battery charger is also small, as in the standby UPS. Due to capacitors in the DC combiner, the UPS will exhibit no transfer time during an AC power failure. This design is sometimes fitted with an additional transfer switch for bypass during a malfunction or overload.
This UPS design, is a new technology introduced to eliminate the drawbacks of the Double Conversion On-Line design and is available in the range of 5kVA to 1 MW. Similar to the Double Conversion On-Line design, the Delta Conversion On-Line UPS always has the inverter supplying the load voltage. However, the additional Delta Converter also contributes power to the inverter output. Under conditions of AC failure or disturbances, this design exhibits behavior identical to the Double Conversion On-Line.
In the Delta Conversion On-Line design, the Delta Converter acts with dual purposes. The first is to control the input power characteristics. This active front end draws power in a sinusoidal manner, minimizing harmonics reflected onto the utility. This ensures optimal conditions for utility lines and generator systems and reduces heating and system wear in the power distribution system. The second function of the Delta Converter is to charge the battery of the UPS by drawing power and converting it to the appropriate DC charging voltage.
The Delta Conversion On-Line UPS provides the same output characteristics as the Double Conversion On-Line design. However, the input characteristics are extremely different. With full Power Factor Correction, the delta conversion on-line design provides both input power control and output power control. The most important benefit is a significant reduction in energy losses. The input power control also makes the UPS compatible with all generator sets and reduces the need for wiring and generator over sizing.
