Intelligent, industrial UPS solutions
Smarter uninterruptible power supplies improve uptime
By Ken Allwine
Loss of power constantly threatens to disrupt control systems in every industry. A loss of power, however brief, can cause multiple issues from process disruption to component damage and loss of revenue. The latest generation of uninterruptible power supplies (UPS) takes the guesswork out of implementing an industrial level battery back-up solution to prevent process downtime.
For years, most panel builders and users would simply place an inexpensive “consumer electronics” level UPS at the bottom of the cabinet in order to withstand just a few minutes of a power interruption. The issues with using these UPS systems are numerous and can cause headaches when dealing with maintenance. The biggest issue with this style of UPS package is the battery. Most of these UPS systems do not have the ability to provide any insight into the performance or health of the battery. This prompts the common practice of replacing the battery on a scheduled maintenance cycle—whether the battery needs to be replaced or not.
Advanced UPS systems now available for control cabinets solve these battery maintenance issues. Instead of guessing when it is time to replace the battery or using a service schedule, high-end UPS platforms indicate the lifespan of the batteries. Not only do they provide a “state-of-charge” indication, they also provide a “state-of-health” and “state-of-function” indication. “State of health” provides real-time feedback as to the immediate availability of the battery. It provides a 0-100 percent scale indicating how well the battery is working. It also shows when it is time to change the battery out for a new one, using a year and month countdown function. The advanced features of these UPS products will provide a diagnostic output, usually either through a dry contact or software indication when the battery life reaches a pre-determined level. Advanced UPSs allow this to be configured by the user. These functions allow the customer to change the battery on a “just-in-time” schedule rather than replacing the battery before its time has come.
Other issues with using classic, inexpensive UPS units include: scalability, output voltage, monitoring, and configurability. Because the classic UPS batteries are internal, there is no possible way to change the battery capacity. In many situations, this causes users to buy a much larger UPS than necessary just so they can have a longer run-time. The advanced UPS products available for the industrial control cabinet (UL508 applications) are of a multi-piece design. The typical topology is a three-piece product grouping. This includes an AC/DC power supply, a UPS control unit, and an external battery module. Using a multi-product solution has quite a few advantages. Primarily, it allows users to fully scale their solution to the size required.
With a single housing product, you are locked into a predetermined wattage size and back-up time. With a modular solution, you can scale the UPS output, usually from 60 watts up to 960 watts. The battery can also be scaled to the back-up time that is required. Many solutions provide batteries from 1.3Ah to 38Ah capacity. Having the ability to select the UPS size and the battery size individually allows users a wide range of applications—and they only need to pay for the sizing that the solution requires. This modularity also creates a solution that can provide battery back-up power lasting for only a few minutes or over several days.
When it comes to battery maintenance, a modular solution is ideal for customers. When the UPS does indicate that it is time to replace the battery, maintenance personnel do not have to shut down the process. They easily put the UPS into “service-mode” and disconnect and replace the battery cells. New battery housing designs even eliminate the need to remove anything from the DIN rail. A “trap door” opens to easily remove the battery cells from the assembly. With the UPS having a service-mode setting, the UPS only bypasses the battery circuit, continuing to provide output power from the power supply to the load. This function allows the process to continue running without loss of time or revenue.
All mainstream UPS solutions used in control panels (classic style or DIN rail) utilize some type of lead acid-based battery technology. Most common are valve regulated lead acid (VRLA) batteries. They are of a time-tested technology and have become the commonly used battery because of their charge density and cost point. However, the overriding constraint in their use is the limited temperature range needed for optimal operation. On average, it is between 0°-40°C (32°-104°F). In an outdoor application or on a factory floor, this range can be easily surpassed. The higher or lower the operational temperature, the more limited the battery’s lifespan. To resolve this issue, the highest level DIN-rail UPS products are now capable of using not only VLRA batteries, but also wide temperature range (WTR) VLRA batteries, lithium-ion batteries, and dual-layer capacitor storage devices. These UPS systems automatically detect what energy storage device is connected and configures the charging parameters to fit the requirements of that device. Lithium-ion and dual-layer capacitor technologies provide an extended temperature range, as well as a much longer operational lifetime (see Table 1).
Another issue in dealing with most of the classic UPS solutions—and even many of today’s DIN-rail mount UPS solutions—is that they typically are only available in 120 V AC output configurations. With more control systems now running completely on 24 V DC, they become less of a viable solution because an intermediary 120 V AC to 24 V DC power supply is required—adding cost and size to the system.
With 24 V DC control circuits so prevalent in control systems, it becomes illogical to use an AC UPS on a primarily 24 V DC panel. Many of the 24 V DC-based UPS solutions are designed as “offline” products. This simply means that the UPS and battery circuit sit in a standby mode until the battery is needed. The UPS allows the DC current and voltage to flow through the UPS undisturbed. When the UPS senses a loss of power from the input source (power supply), the UPS will switch to the battery as the primary power source. Advanced systems are also considered “bump-less.” The UPS has a specific amount of capacitance built in that can provide the full amount of rated output power while the UPS transfers from the main input power to battery storage device. This prevents the UPS output voltage from dropping below the voltage range of the loads connected to it, such as a PLC or an industrial PC.
Once in battery mode, more advanced UPS products are capable of enacting a safe shutdown sequence of either the PLC or PC. When connected to a PC, the UPS is able to provide commands that allow a secondary piece of archival software to be launched, saving critical system data before the shutdown is completed. After the PC has been fully shut down, the UPS is also able to automatically restart a DC-powered PC once the main input power has returned, eliminating the need to have someone physically start the PC.
A major advancement with the latest UPS technology is with software and diagnostic tools. Most of the systems that have been available only had basic forms of feedback either through LED indicators, relay outputs, or a rudimentary software interface, typically through an RS232 connection. Advanced UPS systems now have expanded software capabilities that not only provide real-time status to the UPS and connected batteries, but also provide the ability to configure the operational behavior of the UPS.
New software packages allow users to view performance status of the battery packs that includes voltage, current, temperature, and battery size. If multiple batteries are connected in parallel to expand runtime, the user will be able to view each battery cell individually. Users are also able to modify how the onboard diagnostic relay contacts work. Relay contacts can be reassigned to provide an output of battery lifetime, performance level, and other options. This flexibility indicates to users when a specific problem has occurred with the power system. Advanced software features also allow users to send an email when specific conditions are met. These emails can contain pre-defined messages that provide maintenance personnel with particular work instructions. The software will also data-log every event that happens with the input power, battery status, and alarm conditions.
The newest versions of these UPS systems also become addressable locations on a network. Interfaces, such as Modbus-RTU and EtherNet/IP, provide access to the monitoring information without using a software interface on a PC. This also allows a single HMI location to poll multiple UPS locations for battery and power status of each.
With advanced software, expanded battery options, and easy maintenance and modularity, recent UPS innovations provide more flexibility than ever before. These UPS products take much of the guesswork out of the implementation of a battery back-up system. From auto battery recognition to email capabilities, these solutions are the most intelligent solutions available.
ABOUT THE AUTHOR
Ken Allwine (email@example.com) is a lead product specialist for the Power Solutions group at Phoenix Contact. He has been employed there for 13 years and responsible for power supplies and UPS systems for five years.