UPS DESIGN Which type of uninterruptible power supply gives the best data protection?
There are broadly three types of static, or solid-state, uninterruptible power supplies available: Off-line, line-interactive, and on-line. Here, we look at these topologies and their relative merits.
Practically all organisations today depend on sets of sensitive electronic equipment to handle their everyday IT, communications, and automation functions. As such equipment is susceptible to electrical power glitches or failures, uninterruptible power supplies (UPSs) are normally built into power distribution networks to provide protection from such events.
If a power failure does occur, the UPS battery takes over and continues to support the UPS’s critical load until either
- mains power is restored
- or a local generator can start up and take on the critical load
- or the critical load can be shut down in a safe and orderly fashion
However, while UPSs are therefore highly desirable if not mandatory, their precise design and level of protection depends on each organisation and their individual circumstances. For a data centre or high street bank, for example, the consequences of any service interruption could be so severe that the best available UPS protection must be installed; cost is not the key consideration. Other organisations, though, may not have such critical or sensitive equipment, and will seek to balance their UPSs’ performance against capital and operating cost considerations.
UPS suppliers have responded to this diversity of requirements by offering three UPS categories: Off-line, line-interactive, and on-line. All share conceptually common components, but vary in terms of their topologies. If we start by looking at the generic UPS components, and then at how they can be combined for each category, we can understand how users can match the choices available to their particular priorities.
Generic UPS components
Most UPS systems today are based on solid state, or static technology, as opposed to earlier rotary (motor-generator) based systems. All such systems have four fundamental components:
- a rectifier, which converts AC mains voltage into a DC supply which can be used for charging the UPS battery
- the battery itself. Currently, data centre UPSs most commonly use lead-acid batteries, but lithium-ion types are increasingly popular as well. Battery capacity is typically sufficient to fully support the critical load for five to 15 minutes, although extra capacity can usually be added.
- an inverter, which converts the DC supply back into AC, used for feeding the critical load
- a static switch, which switches the critical load between the raw mains supply and the UPS inverter output.
Figure 1 below shows these components assembled into a typical basic UPS design.
Off-line UPS systems
An off-line system’s most distinctive aspect is that, during normal operation, the utility mains supply is fed directly to the static switch and then the load. Power is also fed through the rectifier to keep the battery charged. If the supply fails, or transgresses preset voltage or frequency limits, the static switch changes over, and starts to draw power from the inverter – which in turn is supplied by the battery.
This means that, during normal operation, the load is subject to any mains disturbance that is not sufficient to trigger a changeover, although most UPSs now include a degree of spike suppression and radio frequency filtering in their bypass circuit.
When the static switch changes over from a failing mains supply to the inverter output, there is an inevitable although brief interruption of typically two to 10ms in the load supply. While most loads can ride through this interruption safely, this factor should always be considered when choosing a UPS topology.
On-line UPS systems
In an on-line system, electricity flows from the utility mains supply through the UPS rectifier/charger and inverter components, and on to the critical load, under normal conditions. The load is therefore protected from any mains-borne anomalies, as it is receiving a clean voltage waveform generated by the UPS inverter.
If the mains supply does transgress preset voltage or frequency limits, or fails entirely, the inverter continues to feed the load without interruption. It simply starts to draw power from the battery – which is connected to the DC bus – rather than the rectifier. This changeover is entirely transparent to the load, which experiences no interruption to supply.
Line-interactive systems are hybrid devices that attempt to offer a higher level of performance than conventional off-line designs by adding voltage regulation to the bypass line. The two most popular types employ either a buck/boost transformer or a ferroresonant transformer.
Which UPS topology should I choose?
On-line UPSs clearly offer the best protection, as only they provide true ‘no break’ power during a mains failure. Additionally, they provide a protected, conditioned voltage waveform to the critical load throughout normal operation. However, they are more expensive than off-line types in terms of both capital and operating costs. The rectifier and inverter are permanently on load and have small but inevitable operating inefficiencies; these increase energy and cooling costs. Capital costs are greater because higher-rated components are used.
Nevertheless, any operator with responsibility for supporting a critical load with unacceptable consequences of failure will always choose an on-line option. Any extra expense associated with the UPS becomes insignificant when compared with the financial losses and possibly brand damage that could arise from an unmanaged power failure.
Yet off-line and line interactive systems also have a place. Some users may decide that a temporary loss of electronic processing function is sustainable, or they simply have equipment robust enough to ride through mains anomalies or failures. Accordingly, they can afford to take advantage of the off-line system savings.
Information in this article is based on The UPS Handbook, published by Kohler Uninterruptible Power. You can obtain your own copy of this Handbook here. It contains detailed information on modern UPS design, as well as how to specify, install and maintain a UPS solution appropriate to your site.