UPS Uptime: Making it Happen

Testing and maintenance of uninterruptible power systems ensures performance when electrical problems occur

By Mohammad Qayoumi  

As facilities grow increasingly reliant on clean power for essential operations, the pressure rises on maintenance and engineering managers to ensure that electrical distribution systems operate as designed and deliver clean, reliable power.

Today, most computer systems used in facilities demand reliable power service with a minimum uptime of 99.999 percent to 99.99999 percent, depending on whether it functions as a local server or main data center. To achieve this level of reliability, systems can only be down for a total of 30 seconds to 3 seconds in one year, respectively.

Fortunately, as a result of technical advances, uninterruptible power systems (UPS) — which less than a decade ago were a luxury affordable for only a select few facilities — can deliver such reliability economically.

As with every other sophisticated technology, UPS reliability is tied to its maintenance, which has become an important topic for most managers. Since UPS have no moving parts, the overwhelming majority of failures are not due to wear and tear.

As a result, good diagnostic tools and a thorough testing and maintenance program for UPS electronics are essential. For the UPS ancillary equipment, such the batteries or generators, however, wear and tear is a factor managers must consider.

UPS units fall into three categories: online, line-interactive, and offline.

The Online Option

An online UPS provides complete protection against even momentary power glitches, failures and surges. With an online UPS, normal power is not connected directly to the load. Instead, normal power charges a battery bank, which powers the UPS inverter and serves the load.

So electricity goes through two conversions before it gets to the load. First, it is converted from AC to DC to maintain the battery charge, then from DC to AC by the inverter to service the load. In this way, an online UPS provides complete protection and offers a smooth transition with zero switching time from main power to the backup power. Online UPS are the most expensive of the three types, and their highly desirable characteristics make them good candidates for mission-critical loads.

The isolation they offer also provides a number of benefits. First, managers do not have to be concerned with other power quality issues, such as blackouts, brownouts, lightning or other power surges, and line glitches.

Second, almost all online UPS have a static bypass, which allows technicians to perform maintenance without shutting down the system. Also, if a UPS is subjected to overload conditions of possibly up to 150 percent, the unit will transfer to the bypass, and an alarm sounds to inform the operator that the load is unprotected.

The important consideration here is that the load is still serviced — albeit by normal unprotected power — until the operator can address the problem. By contrast, if a line interactive or standby unit is subjected to overload conditions, they commonly will sound an alarm and shut down. Rarely do these units have a bypass path that can still maintain the power flow to the unit.

Line-interactive UPS units are the mid-range units. They are connected to the load on a standby mode. Under normal conditions, a load is served directly by utility service. When an abnormal power condition occurs, normal power is interrupted to the load, and backup power takes over. Although the switchover happens within milliseconds, the delay can be detrimental to some loads.

Finally, an off-line UPS is the most cost-effective type. As with interactive units, under normal conditions the load is served by the utility power, and the UPS is dormant. But during an abnormal condition, the units require 2-10 milliseconds to switch on. This delay might acceptable for many loads, but in some applications, critical data could be lost or corrupted.

Maintenance Matters

Managers generally can choose between two approaches when it comes to maintaining UPS — using in-house staff or contracting out the service. Similarly, internal staff can monitor the operation of UPS, or a third party can do so at a remote location.

Managers who choose to use in-house staff must have qualified and well-trained individuals to perform these tasks. They also will have to identify the vulnerable areas of a UPS. In many cases, the most common failures occur with ancillary devices, such as batteries and generators.

For example, temperature rise in batteries is a major source of concern because it hampers shelf life and voltage control. High ambient temperatures cause increased chemical activity within a battery, shortening its life.

Overcharging and undercharging also can harm batteries and reduce their useful life. And if a battery is working very hard, the probability of failure increases significantly.

Managers and front-line technicians also must remember that some battery types, such as nickel-cadmium, have inherent memory. If a battery is partially charged several times after use, it remembers the lower charge level and in the future will never recharge to its full capacity. Technicians can avoid this situation by fully charging and completely draining batteries several times.

Oscilloscopes: A Closer Look

If a UPS has an emergency generator, it is important to have an effective preventive maintenance program to keep it in a good working condition. Periodically, the generator must be tested based on manufacturer specifications. A functioning emergency generator ensures that the UPS can serve critical loads during long outages.

Some of the tools, equipment and technology that front-line technicians require to test, inspect and troubleshoot UPS include oscilloscopes, digital multimeters, and transient disturbance analyzers. These tools enable technicians to carry out a testing and maintenance program that meets facilities and organizational needs. For more information on multimeters and disturbance analyzers, see the article below, “Tools of the Trade.”

An oscilloscope graphically displays the shape of an electrical signal. In most cases, technicians can determine the shape of the voltage signal, the voltage level, and the frequency of the power from this display. Moreover, the oscilloscope shows if a line voltage has any distortions, noise, transients or harmonics. The operator can observe the voltage wave form and identify and pinpoint problems.

Oscilloscopes can be analog or digital. Analog units continually monitor the signal and display it on the screen in real time, while digital units capture an abnormal event and feed the data to a computer for further analysis.

Oscilloscopes are indispensable for troubleshooting and repairing electronic equipment, including UPS. Because one can follow the signal shape and level in every part of the UPS, technicians can locate the defective part rather quickly. Oscilloscopes are expensive and sometimes not available.

Managers and technicians would be hard-pressed to start too early in implementing a UPS maintenance program. Once a problem arises, facilities will need a UPS to perform as intended.

Tools of the Trade

High-tech tools for UPS maintenance need not be high priced. Technicians often can carry out their tasks using simpler, less expensive tools.

For example, disturbance analyzers have been developed specifically for power-quality measurements. These devices measure and identify power disturbances of very short duration, such as electrical transients and noise. In addition, they can measure long-duration disturbances, such as swells, sags, outages, overvoltages and undervoltages.

They can measure whether power systems experienced any transients or noise. Users can set thresholds and leave the instruments unattended to record disturbances over a long period of time. Moreover, a disturbance analyzer can record the time of an event, along with its magnitude and duration. They also can analyze the shape and frequency of such abnormal occurrences.

Some units record power-quality information on a paper tape, while other units feature attachments that allow users to store data on disk, as well. Today, many units have programming capability, in addition to an RS232C interface, which allows the unit to be connected to a PC for further analysis.

Another tool, a digital multimeter, can measure the voltage and current levels of an electrical system. They also can measure the resistance of the electrical circuits. These capabilities can greatly assist in pinpointing faulty electronic components of a UPS.

The accuracy and ranges of these meters vary greatly. Some units can measure both AC and DC voltage and currents. Measurements are taken in real time and can be displayed on a digital readout.

Moreover, many units have a memory that allows instantaneous values to be locked and recalled later. Some also have an RS232C interface that enables users to download readings to a PC or a printer.

— Mohammad Qayoumi

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  posted on 9/1/2002   Article Use Policy

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