Firestone, click here...

Minding Power Quality

Understanding what creates harmonics, transients and other electrical anomalies can reduce downtime and damage to digital equipment

By Lindsay Audin October 2004 - Power & Communication

As facilities and businesses become increasingly dependent on digital equipment, the quality of electricity that powers that equipment becomes more important. Computer processors, servers and certain pieces of telecommunications equipment are extremely sensitive to the quality of electricity fed through plug outlets.

Momentary glitches in a power feed can cause a system to malfunction, shut down or sustain damage that is expensive to repair, if it is repairable at all.

But what exactly is meant by power quality problems? Service interruptions caused by blackouts? Variations caused by lightning? Or does poor power quality refer to the level of service supplied by utility customer service departments?

Blackouts are considered an issue of reliability, not quality, while lightning — always potentially very damaging — is an issue unto itself. And poor utility customer service? While it may be a problem, it’s not related to power quality.

Understanding Power Quality

On an oscilloscope, standard alternating current (AC) power looks like a wave with peaks and valleys occurring 60 times a second. Any variation in this sine wave — be it in amplitude, frequency or consistency — can a reduce power quality. Three-phase power, which is provided to all facilities where large motors are in use, is like three electric services. Each feed is equally out of phase with the others so that 180 pulses of power are seen every second instead of only 60. Any variation in the voltage or timing among such phases may also be considered a reduction in power quality.

Most, perhaps as much as 80 percent, of the power quality problems experienced in buildings originate on the customer side of the meter. The problems that might occur in a building include:

  • Harmonics: minor variations in voltage superimposed on the 60-cycle waveform, typically appearing as miniature versions of that wave form.
  • Spikes and sags: significant variations in the peak voltage supplied to a building or a load within the building, typically lasting from a few thousandths of a second to a few seconds.
  • Transients: over- and under-voltage conditions lasting for minutes or hours, or momentary power interruptions.
  • Noise: static transmitted over power lines that interferes with power line carrier communications.
  • Excessive voltage on the neutral line: Power is transmitted through the “hot” line of a circuit which is completed by a neutral line, which should not be carrying significant voltage.
  • Poor grounding: For safety, circuits are grounded; failure to pass stray voltage to the ground may create a safety problem or damage equipment.
  • Low power factor: Without getting too technical, power factor is a numerical way to describe synchronization of the power and voltage wave forms; some loads grab voltage and current at different rates, causing these two waves to get out of sync. A perfect power factor is 1.0. Many buildings average between .7 and .9.

All of the above can create problems for commercial, institutional, industrial and even residential buildings. With sensitive digital equipment, including that used in telecommunications centers, Web hosting facilities, data centers and Internet hotels — some of the fastest-growing sources of new power loads — power quality will only become a more critical issue. While estimates vary, experts who have studied the financial impact of power quality problems place the cost in the billions of dollars a year, primarily because of lost production and disrupted operations.

Measuring Power Quality

Variations from the ideal sine wave are described in several ways. Harmonics, for example, are defined by the intensity and frequency of deviations as a percent of peak voltage and the shape of those miniature multiples of the 60-cycle wave previously mentioned.

The Institute of Electrical and Electronic Engineers (IEEE) maintains its Standard No. 519, most recently updated in 2000, for describing acceptable utility power and deviations from it. To define a window of spikes, sags or other variations that can be tolerated by solid-state equipment, the Information Technology Industry Council (ITIC), formerly the Computer and Business Equipment Manufacturers Association, or CBEMA, developed a chart showing the intensity of such variations and their duration.

The oft-mentioned “six 9s” (i.e., 99.9999 percent), when related to power services, are more a description of reliability than power quality. They indicate how much time, annually, power can be expected to be lost. Six nines would represent a cumulative loss of power for only about 30 seconds a year, which is 0.0001 percent. Most utility power is more than 99 percent reliable.

Symptoms of power quality problems can appear when certain building systems, including some types of security devices, simply stop operating. Older book detection systems in libraries and book stores began to fail when electronic ballasts, which can create harmonics, were installed in nearby fluorescent lighting fixtures. Some types of clocks, such as those used in schools, may lose the ability to be reset when sending corrective signals through power lines.

Harmonics and low power factor can be created by some types of equipment, such as early model variable-speed drives, the placement of many desktop computers on a single circuit and devices with solid-state transformers. Even some types of lighting dimmers can corrupt the 60-cycle wave form while controlling voltage.

In general, low levels of harmonics and power factor approaching .8 will not cause serious problems. Even so, it sometimes pays to improve power quality because many utilities now charge for low power factor through a kilovolt-amp charge that appears on monthly bills.

If power factor on a circuit is significantly reduced, current levels, as detected by circuit breakers, could rise, causing breakers to open or fuses to blow. Excessive voltage on the neutral sometimes causes wiring to fail or burn out or reduces the life of some components, such as backup batteries on emergency lighting.

Elusive Problems

Sags and spikes can play havoc with sensitive manufacturing processes, laboratory equipment and some medical devices that demand consistent power. Large printing facilities and other continuous-feed systems are especially vulnerable when a temporary drop in motor speed causes a production line to suddenly back up, resulting in a loss of product and significant downtime while the process is cleaned out and restarted.

In another case, failure of a building’s step-down transformer can send unusually high voltage into a 120-volt panel, burning out the internal transformers in a variety of printers, monitors and other office equipment before the problems are diagnosed and corrected.

Differences in voltage or phasing in three-phase power feeding large motors can cause vibration, noise, overheating and other problems that may burn out compressors, pumps and other equipment dependent on such motors.

It is often harder to find and define a power quality problem and its source than it is to cure it. Many problems are like ghosts, appearing and disappearing without leaving a clear idea of what was just seen. Some only show up when several factors occur simultaneously. Others appear once and are never seen again.

Utility providers can be a good source of information but may be limited in their capacity to correct problems on the customer side of the meter. Liability issues sometimes restrict a utility’s ability to offer advice. An electrician experienced in diagnosing power quality issues could be needed, along with sophisticated metering and recording devices, to ensure that the right solution is applied.

Where low power factor exists, capacitors added to affected circuits may be sufficient to address the problem. Distributing solid-state devices, such as personal computers, among several circuits may improve power factor occurring on a problematic circuit.

Harmonics can be reduced by choosing better quality equipment — including ballasts — installing filters and correcting feeder lengths between loads and variable-speed drives. In some cases, it is more cost effective to simply replace vulnerable equipment with newer models immune to the problem.

Correcting chronic over- and under-voltage conditions could require working with the local utility to improve distribution delivery service, such as the distribution transformer feeding the building, or addressing capacity issues related to customer-owned transformers. In some cases, the situation can be improved by merely tightening loose panel connections found to be hot because of excess resistance. Also, reducing excessive loads on circuits or distributing high startup or intermittent loads, such as high-efficiency motors and electric heater coils, will reduce short-duration voltage sags.

To smooth out such variations and provide short-term power that allows orderly shut down of equipment or to transfer equipment loads to on-site generators, some facilities have installed various forms of power storage, such as uninterruptible power supply systems.

Never assume that a problem is caused by a recent change at a facility. In some cases, mistakes in original wiring did not create problems until some circuits became fully loaded, or other changes to the building shifted loads to panels near sources of vibration that loosened connections. Neutral-to-ground voltage created by wiring errors sometimes remain hidden until water penetration or damaged insulation exposes the problem.

Utility as a Source of Problems

Although the majority of power quality problems are unrelated to a utility distribution system, some major cases of stray voltage — such as the electrocution of a New York woman walking her dog — have been documented. Transformer failures, meanwhile, have created high-profile lawsuits.

A recent failure of a distribution transformer in upstate New York resulted in excessive voltage being sent to residential and small commercial electric services, resulting in burnouts of expensive appliances and business equipment. While admitting the failure, the utility disclaimed responsibility and is now threatened with lawsuits and an investigation by the public utility commission.

Unless negligence can be proven, lightning is beyond utility control, even though the enormous power surges lightning creates can be transmitted through power lines before burning the lines out.

To protect against such problems, whole-building surge suppression systems are available and can be installed at the building electric service entrance point as part of the main switchgear. Essentially circuit breakers sensitive to incoming voltage, such breakers open when sensing major over-voltages, thereby disconnecting the building from the utility until reset manually or through a building management system.

If it can be clearly shown that a utility is at fault, working with its engineers may resolve the issue. If not, be ready to file a formal complaint with the public utility commission. Often merely filing the complaint is sufficient to move the issue toward resolution. Otherwise, pursuit of legal action may be necessary.

Lindsay Audin is president of EnergyWiz, an energy consulting firm based in Croton, N.Y. He is a contributing editor of Building Operating Management.


Find us on Google+

ASCO. Click here...