Emergency Power: The ABCs of UPS
By James Piper, P.E. - April 2008 - Power & Communication
Uninterruptible power supplies (UPS) have become essential ingredients in providing reliable operation of critical electronic equipment. Managers can specify them to protect sensitive equipment ranging from a single computer to an entire building full of electronic equipment. They also are important components of many organizations’ emergency-preparedness programs.
To be effective, though, engineering and maintenance managers must select the most appropriate system for the type and size of loads they are protecting and establish a comprehensive maintenance program. Selecting the wrong system either will waste money or provide an inadequate level of protection. Failing to maintain the system properly simply will render it unreliable.
Selecting a System
Managers’ UPS options include three basic types of systems: passive standby, line interactive, and double conversion. Each type offers its own features and level of protection. Not all are suitable for all applications, so managers must carefully consider the needs of the application when evaluating their UPS options.
Passive-standby systems are the least expensive option and provide the lowest level of protection. These off-line systems monitor the incoming power and switch to a battery source when an interruption occurs. While the transfer takes place in milliseconds and is suitable for a number of computer-based applications, the loss of power during the transfer can disrupt the operation of some sensitive electronic equipment.
Passive standby systems also do not filter out power-line noise or voltage spikes or sags. These limitations keep their applications primarily to desktops or other similar systems that do not perform critical tasks.
Line-interactive systems insert an inductor or transformer between the connected equipment and the power source. A bank of batteries helps condition the incoming power, as well as filter it.
While line-interactive systems offer more protection than passive-standby systems, they do not fully isolate the equipment being protected from irregularities in the incoming power. Line-interactive systems offer adequate protection for many common facility applications but not enough protection for those that are mission critical.
Double-conversion systems are a true online system. Incoming power is converted from alternating current to direct current, then back to alternating current. A bank of batteries connected to the direct-current portion of the system eliminates the momentary loss of power found in the other two types of UPS systems in the transfer from incoming power to battery-supplied power.
Double-conversion systems fully isolate the protected equipment from the power source, eliminating most power disturbances. Depending on the size of the battery bank and whether a standby generator is part of the system, double-conversion systems provide run times ranging from 15 minutes to as long as the generator’s fuel supplies last.
Double-conversion systems are appropriate for all applications requiring a high level of protection. Their biggest drawback is cost, both the initial cost and the long-term cost to maintain.
Regardless of the system selected for a particular application, managers must be certain it is properly sized for the load it will have to protect. At a minimum, managers should be certain to size all systems to provide 150-200 percent of the connected load. This spare capacity protects the UPS system from additional power loads while equipment is starting, and it provides some room for growth over the life of the system.
It also is important to adequately size the batteries in the system to provide the desired runtime when power is lost. For some applications, it is only necessary to provide power long enough to allow an orderly shutdown of the connected equipment.
But in other applications, the batteries will need enough capacity to provide power for the duration of the most common power interruptions. The nature of the functions being performed by the protected load will determine the required amount of battery capacity.
Keeping it Running
A UPS maintenance program has two major components: battery maintenance and system maintenance. Both are critical to the reliability and long-term performance of UPS systems. The type of maintenance tasks technicians perform and the tasks’ frequency depends to a great extent on the type of batteries installed and their capacity.
For example, smaller passive-standby systems typically use sealed batteries requiring little or no maintenance. Double-conversion systems typically use flooded-cell batteries that require monthly maintenance. But while they require a higher level of maintenance and are more expensive to replace, flooded-cell batteries offer nearly three times the life expectancy of sealed batteries.
Flooded-cell batteries typically are installed in banks and isolated rooms. At least once each month, technicians should inspect and test the room’s ventilation system for proper operation. They should inspect all batteries for proper electrolyte level and signs of leaks.
They should inspect all terminals for corrosion and accumulation of dirt, as well as measure and record the voltage and current of the entire bank. A random number of individual cells should have their voltage recorded and their electrolyte tested. Technicians should record measurements in an ongoing log to track battery performance.
Once every six months, technicians should inspect and retorque all battery connections. Loose connections result in a buildup of heat at the battery terminals, decreasing system capacity, reducing battery life and creating a potential fire hazard.
Once each year, they should load-test the battery bank to determine its capacity. Load testing requires disconnecting the UPS from its power source and allowing the batteries to supply power to a connected load, typically a load bank. The test continues until the system design run time is reached or until the system shuts down due to low battery voltage. During the test, technicians should thermally scan all connections to identify any that are loose or corroded.
The UPS system itself also requires regular maintenance. At least once each year, technicians should take the system offline and inspect all components for corrosion and heat damage. A thermal scanner will help identify loose connections and components that are possibly overheating.
Finally, they should remove dirt and dust from all components, particularly those where the accumulation would interfere with heat transfer. They also should retorque all power connections according to the manufacturer’s specifications. The system manufacturer also might require additional measurement and calibration tasks.
James Piper, P.E., is a national consultant based in Bowie, Md. He has more than 25 years of experience with facilities maintenance and management issues.