Learn Why UPS (Uninterruptible Power Supply) are Critical to Control Systems

Learn about the functions and types of uninterruptible power supplies (UPS), as well as why they are critical in control systems.
The first continuous power supply designs were developed in the early twentieth century, with the first UPS system patent issued in 1934.
Several decades later, during the PC era, when hard drives were less resistant than they are today, the modern UPS emerged.
Physical damage and corrupted directories in the hard disk were more likely to result from a sudden loss of power in a server rack.
What exactly is a UPS?

A UPS (uninterruptible power supply) is a system that serves two purposes:

It's an emergency power device that acts as a backup source of energy in the event of a utility power outage.
Depending on the length of the outage, a UPS can hold a system running until services or generators are restored, or it can give enough time to completely shut down the system and prevent data loss. It guards against power spikes and sags that can damage sensitive electronic equipment.
Solid-state drives (SSDs) are now standard in most computers, since they are virtually free of moving parts and can handle power outages better.

Other devices and IT infrastructure have improved in terms of reliability, and boot times have decreased significantly.

Despite these advances, UPSs remain an important solution that provides efficiency and improves a company's continuity.
Despite the fact that the United States' power grid is rated at 99.9% reliable, this figure nevertheless equates to about 9 hours of power loss each year for businesses and the average household. Extreme weather incidents, such as the early February Texas winter storm, highlighted the importance of getting easy access to backup power sources.
UPS Varieties

The three major forms of UPS, also known as topologies, are as follows.

  • Single conversion (standby and line-interactive)
  • Double conversion
  • Multi-mode
Single Conversion

UPS systems continuously track utility AC input and turn to battery power when the AC input falls outside of predetermined limits.
When the AC input returns to normal or the charge levels are exhausted, battery power is used.
Single conversion UPSes are divided into two types: standby and line-interactive.

The most simple and commonly available UPS style is the standby UPS.
These are designed for a variety of commercial consumer electronics, including PCs, monitoring and security devices, banking, and point-of-sale services.

In the event of typical input power issues (undervoltage, power surges), a standby UPS switches to battery backup power.
Inverters are used to convert the DC power from the battery to AC power for output.
Figure 1. The BR700G from Schneider Electric, a UPS unit commonly used with PCs. Image used courtesy of Dell
Transformers are used in line-interactive UPSs to control irregular input AC power conditions without having to turn to battery power.
Line-interactive UPSs, which are commonly used in consumer electronics, can also be a secure solution for mid-range servers.
The ability to use battery power to defend against frequency anomalies is a major benefit over standby UPSs.

On average, line-interactive UPSs are more costly than standby UPSs.
Double Conversion
Power is converted twice in double conversion systems, also known as "online" UPSs.
The input AC power is converted to DC and fed to an inverter in the first stage.
The DC power is then converted to AC and sent to the output, where it is used to power attached equipment.
Figure 2. The Eaton 9PXM UPS system for data centers, retail, and other mission-critical IT applications. Image used courtesy of Eaton
When the input power falls beyond the defined tolerance window, the UPS switches to battery power and feeds the output inverter, much like single conversion systems.

This form of UPS provides a high degree of connected component isolation from utility power and is commonly used in essential IT infrastructure including server rooms.
Multi-mode UPSs, as the name implies, combine features from single and double conversion systems to provide increased security and performance.

On standard input power conditions, a multi-mode UPS acts as a line-interactive UPS.

The UPS switches to double conversion mode in the event of a power surge or sag to help isolate the attached equipment.
The UPS serves as a standby and uses the backup battery power if there is a total loss of utility power.
UPS in Control Systems
Depending on the type of subsystem, uninterruptible power supplies in control systems can have different dimensions.
Server Rooms and Data Centers
Integrating UPS units is most popular and well-studied in data centers and server rooms, which play a critical role in many modern automated system infrastructures.

The aim is to keep vital servers going during a power outage, so these areas use either multi-mode or double conversion.

The backup power might not be enough to keep the machine running depending on the length of the power outage and the size of the UPS.
In this scenario, the UPS allows for a soft system shutdown, which helps prevent data loss and prepares the system for a smooth restart.
Figure 3. A DiamondPlus 1100 hot-swappable UPS from Mitsubishi Electric. Image used courtesy of Mitsubishi Electric
Switching time is an important feature of UPSs in these applications.

To avoid disrupting the operation, switching time must be small or imperceptible since certain data transfers occur in milliseconds.
Many UPS manufacturers are concentrating on improving this functionality.
Physical Servers and PLCs
Integrating a UPS, on the other hand, for PLC-controlled systems and certain physical servers may be more difficult, and the benefits are not well known.

The problem arises when vast amounts of energy are used in movements and processes.
In this scenario, designing a UPS that can provide backup power for the duration of a power outage might not be feasible or cost-effective.
The benefit of using backup power to interrupt and shut down devices more gradually remains.
A UPS, for example, will sense a lack of input power and prepare the control system to stop following a smooth ramp down transition, as in conveyance systems.

This has the potential to prevent collapses and crashes, and hence has safety implications.
Control systems also use decentralized physical servers (Windows-based in many cases).

These servers, like PCs, must be shielded from power surges while also allowing for a proper shutdown in the case of a power failure.

Most industrial systems, whether they use PLCs, decentralized physical servers, data centers, or server rooms, require a double conversion or multi-mode UPS.

These UPS models help prepare for a smooth system restart by preventing data loss.

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