The varied types of uninterruptible power supplies (UPS) and their attributes often cause confusion in the data center industry. For example, it is widely believed that there are only two types of UPS systems, namely standby UPS and on-line UPS. These two commonly used terms do not correctly describe many of the UPS systems available. Many misunderstandings about UPS systems are cleared up when the different types of UPS topologies are properly identified. UPS topology indicates the basic nature of the UPS design.
A variety of design approaches are used to implement UPS systems, each with separate performance characteristics. The most common design approaches are as follows:
- Line interactive
- Double conversion on-line
- Delta conversion on-line
The standby UPS
The standby UPS is the most common type used for desktop computers. In the block diagram explained in Figure 1, the transfer switch is set to choose the filtered AC input as the primary power source, and switches to the battery 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 backup power source.
The inverter only starts when the power fails, hence the name “standby.” 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 acceptable noise filtration and surge suppression.
Figure 1 Standby UPS
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 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. 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.
Figure 2 Line interactive UPS
The standby-ferro UPS
This design depends on a special transformer that has three windings. The primary power path is from AC input, through a transfer switch, through the transformer, and to the output. In the case of a power failure, the transfer switch is opened, and the inverter picks up the output load.
In the standby-ferro design, the inverter is in the standby mode, and is energized 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. But the ferro transformer itself creates severe output voltage distortion and transients, which can be worse than a poor AC connection. Even though it is a standby UPS by design, the standby-ferro generates a great deal of heat because the ferro-resonant transformer is inherently inefficient.
Figure 3 Standby-ferro UPS
The double conversion on-line UPS
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.
Figure 4 Double conversion
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 which 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.
The delta conversion on-line UPS
This UPS design, was introduced to eliminate the drawbacks of the double conversion on-line design and is available in sizes ranging from 5 kVA to 1.6 MW. Similar to the double conversion on-line design, the delta conversion on-line UPS always has the inverter supplying the load voltage. But, the additional delta converter also contributes power to the inverter output. Under conditions of AC failure or disturbances, this design shows behavior identical to the double conversion on-line.
Figure 5 Delta conversion on-line UPS
The following table shows some of the characteristics of the various UPS types. Some qualities of a
UPS, like efficiency, are dictated by the choice of UPS type.
Various UPS types are appropriate for different uses, and no single UPS type is ideal for all applications. The intent of this article is to contrast the advantages and disadvantages of the various UPS topologies on the market today.
Important differences in UPS designs offer theoretical and practical advantages for different purposes.
The basic quality of design implementation and manufactured quality are often dominant in determining the ultimate performance achieved in the customer application.