Inspect and replace aging UPS batteries: continuous power depends on preventive maintenance - Testing and diagnostics: testing UPS - uninterruptible power supply

Communications News, Dec, 2001 by Alan Katz, Sr.

Blackouts and power crises aside, installing a UPS is not a guarantee of continuous power without compromise. Performing comprehensive maintenance and testing is the best way to guarantee the continued performance of a UPS.

The power range of a UPS module will dictate what type of testing and maintenance is appropriate. Test procedures for smaller, single-phase UPS modules (under 12 kVA) are often limited to assessing the integrity of the battery bank, and performing a functional test of the device. For larger, three-phase UPSs (above 10 kVA), testing and diagnostics can be more extensive, with internal maintenance best carried out by dedicated factory service engineers.

While UPS quality and design integrity can differ radically from one manufacturer to another, the number one cause of UPS failure during power outages is the battery. In the case of small, single-phase UPS modules, battery life will range from three to six years, with a recommended replacement of batteries every three years.

Battery life is dictated by operating temperature (batteries will lose roughly half their life for every five degrees above 25 degrees Celsius), the number of times the battery has been discharged and recharged, and the quality of the battery charger. Since UPSs last longer than the batteries, selecting a model that has user-replaceable batteries is important.

Performing a load test is the best method to check the integrity of the batteries, accomplished by removing utility power when the UPS is carrying a load similar in power to the critical load. The UPS should effectively power the load close to the manufacturer's rated backup time.

While battery capacity will decrease with the age and condition of the batteries, the batteries should be replaced if the UPS cannot deliver full power for 75% of its rated backup time. Natural aging is just one of the problems encountered during battery tests. Since UPS batteries are configured in series, a poor cell can have an open circuit condition, and immediately compromise the critical load.

For larger, three-phase UPSs, many manufacturers incorporate battery self-test features, but a professionally administered periodic load test, similar to that performed on single-phase UPS, is recommended. Since servicing UPS systems can expose a person to lethal voltages, any internal service of a UPS system (specifically higher power systems) should be performed by a certified UPS service professional. Battery bus voltages on many three-phase UPSs are extremely high (typically between 192 VDC to 960 VDC), limiting service to UPS technicians only.

The condition of individual battery cells can also be assessed using advanced impedance measurement instruments to spot a potentially bad cell in a string of batteries, (UPS batteries are linked in series to obtain a DC voltage close to that of the AC output voltage.) Impedance measurements taken when individual batteries are under load show deviations in their original ohmic resistance--making spotting a bad cell in the chain easier.

Another often overlooked but critical test that should be performed on all UPS systems is a generator test, to determine compatibility with the backup generator.

Administering this test requires that the generator power the UPS at various load levels close to where the UPS will be operating. Problems are most common when the UPS load is very light and the UPS potentially becomes a leading power factor load, making the reactive current difficult to manage for the generator. Step loading (increasing the load instantly from 0 to 50% or 50% to 100%) the UPS and monitoring the UPS's ability to stay online (not switching to battery) when the generator frequency and voltage shifts (providing the shift is below 10% of nominal voltage and frequency) may also be necessary. During generator-frequency changes, the output frequency of the UPS should remain stable at 60 Hz, under all conditions; otherwise, the test can be considered a failure.

Calibration is not an issue, as most modern UPS systems are 100% solid state and self-calibrating. For older UPSs, calibration can be complex, and again, restricted to factory technicians.

Preventive maintenance is the key to keeping a UPS reliable. From a test-and-replace standpoint, the only critical parts of concern (aside from batteries) are filter capacitors and fans. Abide by the manufacturer's replacement schedule (five to 10 years is typical) for these components.

Larger, three-phase UPSs should have periodic (annual or biannual) preventive maintenance inspections. During this time, a factory engineer respects the UPS module internally for any visual sign of deterioration of components like capacitors, torquing of any loose connections, removal of dust or particulates from surfaces, and, lastly, a simulation of all UPS operating conditions.

When investing in a larger centralized UPS system (i.e., 100 kVA or above power rating), purchasers may want to visit the manufacturer and perform all their initial tests at the factory. While most major manufacturers will conduct extensive factory testing on all large UPS modules and provide the test data, witnessing the testing provides a level of reassurance and gives users a better understanding on the operation of their UPS systems.