Keep the power on: Testing uninterruptible power supplies (UPSs)

Server farms and data centers that constitute the world's data infrastructure must run even in the event of a power failure. Uninterruptible power supplies (UPSs) keep the power coming when utility power fails. Not only must a UPS provide the power needed to keep equipment running, but it must also cut in without any interruption whatsoever; even a short dip in power can cause data loss or equipment failure.

MGE (www.mge-ups.com) produces large UPS systems that provide 60-Hz, three-phase, 480-V power at 300 kVA to 500 kVA at its Costa Mesa, CA, facility. (The company produces 50-Hz systems in France.) The supplies take stored DC energy from a battery and invert it into AC. They switch to battery power should utility power fail.

Because MGE produces these large power supplies in small quantities (10 to 20 each week), the company's technicians perform many tests without automation. To perform a test, a technician measures input and output voltage and current on all phase legs. He or she connects a current clamp to each leg, then connects the clamps to a chart recorder. Six other channels on the chart recorder measure voltage in each leg.

A chart recorder monitors input and output voltage and current during a simulated power failure. The recorder proves that the UPS under test switches to battery power.

Using a circuit breaker, a technician cuts utility power for 10 s to 30 s while the chart recorder measures voltage and current. A test verifies that the output voltage and current remain within limits with minimal phase shift. A power analyzer monitors the utility power for dips, spikes, and harmonics.

Technicians also test each UPS for overload conditions. Using loads located outside the building, but controlled from the test area, technicians measure voltage and current at 150% of rated load. The battery charger should shut down in an overload condition.

The UPS has a graphical display that lets users, factory technicians, and service technicians monitor its operation. Diagnostic software lets factory technicians calibrate the supply, and it lets both factory and service technicians troubleshoot the supply.

Factory technicians use DMMs to measure leg-to-leg voltages with greater accuracy and resolution than they can get with the chart recorder. They connect laptop computers to the UPS under test through RS-232 ports. Using diagnostic software and a command-line interface, technicians calibrate the UPS by entering correction values.

A UPS also contains setup software that's accessible to field engineers who customize the supply at installation. Settings include cut-in time (1 s to 20 s), which sets the time that it takes the UPS to switch back to utility power once the power is restored.

Technicians also use other test equipment as needed. For example, an oscilloscope lets them view waveforms to troubleshoot a supply.

“Read the fine print,” said Michael Loberg, power systems production supervisor. “In using test-and-measurement equipment, it is important to read the entire manual. In one case, a 3000-A-rated current clamp burned during power runs at around 2500 A. The continuous rating of these units was only 2000 A, making for a very expensive repair bill.”