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Energy

The Types Of UPS Systems
update on 2012-04-14

Uninterruptible Power Supply (UPS) systems are an important element where reliable power supply is critical, for example, in data centres. Five of the most common UPS systems will be discussed here. By Sim Liang Kee, head of solutions engineer, Schneider Electric Singapore

There is much confusion in the marketplace about the different types of Uninterruptible Power Supply (UPS) systems and their characteristics. For example, it is widely believed that there are only two types of UPS systems, namely standby UPS and on-line UPS. However, they do not correctly describe many of the UPS systems available. There are five common types of UPS systems available, today, including standby, line interactive, standby-ferro, double conversion on-line and delta conversion on-line. Each of these UPS systems has their respective advantages, disadvantages and practical applications as below.

The Standby UPS
The standby UPS is the most common type used for desktop computers. In the block diagram illustrated in Figure 1, 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.’

Benefits And Limitations:
High efficiency, small size and low cost are the main benefits of this design. With proper filter and surge circuitry, these systems can also provide adequate noise filtration and surge suppression. Limitations include the need for battery usage during brownouts. The design, too, gets impractical beyond a volume of 2kVA and, as such, makes this design best value for personal workstations.

The Line Interactive UPS
The line interactive UPS, illustrated in Figure 2, 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 power flows from the battery to the UPS output. With the inverter always on and connected to the output, this design provides additional filtering and yields reduced switching transients when compared with the standby UPS topology.

Benefits And Limitations:
High efficiency, small size, low cost and high reliability coupled with the ability to correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5 kVA power range.

The Standby-Ferro UPS The standby-ferro UPS was once the dominant form of UPS in the 3-15 kVA range. This design depends on a special saturating transformer that has three windings (power connections). The primary power path is from AC input, through a transfer switch and transformer and then to the output. In the case of a power failure, the transfer switch is opened and the inverter picks up the output load.

Benefits And Limitations:
In the standby-ferro design, the inverter is in the standby mode, and is energised when the input power fails and the transfer switch is opened. The transformer has a special ‘ferroresonant’ capability, which provides limited voltage regulation and output waveform ‘shaping’. The isolation from AC power transients provided by the ferro transformer is as good as, or better than, any filter available. However, the ferro transformer itself creates severe output voltage distortion and transients, which can be worse than a poor AC connection.

Standby-ferro UPS systems are frequently represented as on-line units, even though they have a transfer switch, the inverter operates in the standby mode, and they exhibit a transfer characteristic during an AC power failure. Figure 3 illustrates this standby-ferro topology.

High reliability and excellent line filtering are this design’s strengths. However, the design has very low efficiency combined with instability when used with some generators and newer power-factor corrected computers, causing the popularity of this design to decrease significantly.

The Double Conversion On-Line UPS
This is the most common type of UPS above 10 kVA. The block diagram of the double conversion on-line UPS, illustrated in Figure 4, is the same as the standby, except that the primary power path is the inverter instead of the AC main.
In the double conversion on-line design, failure of the input AC does not cause activation of the transfer switch because the input AC is charging the backup battery source that provides power to the output inverter. Therefore, during an input AC power failure, on-line operation results in no transfer time. Both the battery charger and the inverter convert the entire load power flow in this design.

Benefits And Limitations:
This UPS provides nearly ideal electrical output performance. However, the constant wear on the power components reduces reliability over other designs. In addition, the input power drawn by the large battery charger may be non-linear which can interfere with building power wiring or cause problems with standby generators.

Delta Conversion On-Line This UPS design, illustrated in Figure 5, is a newer, 10 year old technology introduced to eliminate the drawbacks of 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 behaviour identical to the double conversion on-line.

Benefits And Limitations: The most important benefit of this UPS system 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 oversizing. During steady state conditions, the delta converter allows the UPS to deliver power to the load with much greater efficiency than the double conversion design and, as such, makes it highly suitable for large installations.

In Conclusion
As demonstrated above, significant differences in the respective UPS systems offer theoretical and practical advantages and no single UPS type is ideal for all applications. Nevertheless, the basic quality of design implementation and manufactured quality are often dominant in determining the ultimate performance achieved in the customer application.