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The Different Types of UPS Systems
Selecting the right backup power system isn’t always easy, there are many different types of uninterruptible power supplies (UPS) and their attributes often cause confusion. There are generally three different UPS technologies and they are often categorised as standby UPS and on-line UPS. Various vendors routinely produce models with similar designs, but with very different performance characteristics.
In this blog post we aim to help you understand how to properly identify and compare systems.
A variety of design approaches are used to implement UPS systems, each with distinct performance characteristics. The most common design approaches are:
Standby / Off-Line
The standby UPS
An Off Line UPS, often referred to as a standby UPS, is the most common type used for desktop computers. The transfer switch is set to choose the filtered AC mains input as the primary power source and switches to the battery / inverter as the backup source should the primary (mains) source fail. When that happens, the transfer switch must operate to switch the load over to the battery / inverter backup power source. The inverter / battery 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 adequate noise filtration and surge suppression.
The line interactive UPS
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 and is used to charge the battery in normal mode when the mains is present.
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 cleaner power than an Off-Line / standby UPS. 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, without this the UPS would transfer to battery when the voltage drops which could create a situation where the battery is already discharged and unable to support the load when a mains failure occurs. 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.
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 double conversion on-line UPS
For the most critical of loads ta double conversion on-line UPS is really the only option. This is the most common type of UPS above 10 kVA. With a double conversion on-line UPS, 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 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.
This UPS provides nearly ideal electrical output performance. But the constant wear on the power components reduces reliability over other designs.
Summary of UPS types
Each type of UPS types is appropriate for different uses, and no single UPS type is ideal for all applications. Some attributes of a UPS, like efficiency, are dictated by the choice of UPS type. Since implementation and manufactured quality more strongly impact characteristics such as reliability, these factors must be evaluated in addition to these design attributes.
The most important question to ask yourself is how important is my load? For the most critical equipment you should always consider an On-Line UPS, if power quality is an issue but your budget is limited, a line interactive UPS is likely to give you what you need and if all you really need is 5-10 minutes of battery backup for a non-critical device then Off-line may be for you.
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