Putting Power Quality in Perspective
Anyone who's experienced a brownout or a mysterious series of control system trips should appreciate how power quality, or PQ as it is commonly known, impacts facility operations. As power travels through the wires and energizes downstream equipment, the quality of the power can be altered, making it less suitable for the next device. These changes in power quality, which can include increases and decreases in voltage and other troublesome manifestations, are especially common in systems-intensive industrial and commercial facilities.
It has been estimated that large industrial customers in the U.S. lose up to $114 billion every year due to under- voltage events and sags, and another $39 billion from power interruptions. The fact that the U.S. electric power system, according to the Galvin Electricity Initiative, is designed to operate at a reliability level of three nines-at least 99.9 percent-still equates to supply interruptions in the electricity supply that cost American consumers more than $150 billion every year.
It may come as a surprise to some, but a significant percentage of the cost and effort of maintaining a company's power supply involves identifying and defeating the problems caused by PQ phenomena interacting (Figure 1) with the building's electrical infrastructure and loads. According to industry sources, half of all computer problems and one third of all data losses can be traced back to the power line. Furthermore, some 30-40 percent of all business downtime is power-quality related. A few of the ways that power quality problems impact businesses include:
• Lost productivity, idle people and equipment
• Scrap
• Lost orders, good will, customers and profits
• Lost transactions and orders not being processed
• Revenue and accounting problems such as invoices not prepared, payments held up, early payment discounts missed
• Customer and/or management dissatisfaction
• Overtime required to make up for lost work time.
Traditionally considered “job one” by every electric utility, simply keeping the lights on is no longer enough for today’s automated “high-tech” industrial facility. The fact that most utilities only log outages that last longer than 1-5 minutes tends to gloss over the many momentary interruptions that every facility experiences, and which annually result in millions of dollars in lost productivity for American businesses. Given the breadth and depth of these conditions, it is easy to see how understanding what power quality problems are, how to find them and how to solve or mitigate them will continue to gain importance for facility operators, electrical contractors and utility personnel. In general, power quality phenomena fall within the following categories:
• Steady-state events
• Long-duration events
• Short-duration events
• Transient events
• Frequency events
My Friend Flicker and Other Typical PQ Disturbances
Over the years, power monitoring studies have clearly demonstrated that most industrial plants around the country experience up to two dozen power quality disturbances every year that significantly impact plant operations. About 92-98% are voltage sags due to lighting strikes, accidents, animals or equipment failure on the transmission and distribution grid feeding the plant. Also, most are short-duration events of 1-6 cycles corresponding to the clearing time of upstream breakers, fuses and other utility protective equipment.
The most obvious impact of power quality disturbances is reduced uptime of plant equipment and processes that may run into many hours and many thousands of dollars in scrap, lost production and other costly ramifications. The very equipment at the heart of industrial automation—PLCs, industrial drives, motors, robots, servos, CNC equipment and more—are highly susceptible to power quality variations (Figure 2). There is considerable evidence that industrial plants experience at least 10 to 40 power disturbances every year, mainly from voltage sags. Based on voltage disturbance data from industrial plants, voltage sags occur much more frequently than swells, and it is perhaps surprising that current swells accompanying voltage sag recovery are the root cause of most of the equipment damage.
Power quality anomalies are usually characterized in terms of the effect upon the supply voltage and can be broken down into the following major categories:
• RMS voltage variations, short or long duration, include sags, swells and interruptions. Sags, the most common type of PQ disturbance, usually last from 4-10 cycles and are generated within the facility, not by the utility. Swells, formerly called “surges,” occur when nominal rms voltage increases to 110 percent or more. Interruptions occur when the supply voltage decreases to 10 percent or less of nominal.
• Voltage transients, also known as impulses, are rapid, short-term voltage increases that are categorized as either impulsive (large, short-term waveform deviation) or oscillatory (ringing signal following initial transient).
• Waveform distortion – Harmonics, interharmonics, and sub-harmonics are mainly caused by phase angle controlled rectifiers and inverters and other static power conversion equipment found in variable frequency drives, PCs, PLCs and other devices employing switching power supplies. Harmonics are defined as integer multiples of the fundamental frequency, for example, 300Hz is the 5th harmonic in a 60Hz system. Non-integer multiples produce interharmonics, for example, 190Hz in a 60Hz system. Sub-harmonics provide frequency values less than the fundamental frequency and are typically evidenced by flickering lights. Electrical noise, caused by unwanted broadband signals that distort the power frequency sine waves, is often generated by switching power supplies and can be aggravated by improper grounding methods.
• Voltage imbalance – In three-phase systems, voltage imbalance occurs when the amplitude and/or phase angles of the three voltage or current waveforms are unequal. According to the DOE, imbalance is probably the leading power quality problem resulting in motor overheating and premature failure. If imbalanced voltages are detected, a complete investigation should immediately be made to find out why. • Voltage fluctuation – Sub-harmonics in the range of 1-30Hz result in what is generally called light flicker, an amplitude modulation of the power frequency sine wave. Causes are widespread and include arc furnaces, arc welders, resistance welding machines, lamp dimmers, large electric motors with variable loads, HVAC systems, medical imaging systems and many more. Due to its nature, flicker is difficult to characterize and requires PQ analyzers with considerable processing power to characterize its effects measured as Perceptibility short-term and long-term values, or PST and PLT, respectively, as set forth in IEEE 453.
• Power frequency variation – When powered by a back-up generator, UPS, or other alternative power source, maintaining voltage and frequency stability during load changes is of concern, along with making sure the transfer mechanism synchronizes the frequency and phase angle before the switch from back-up to the grid is made.
Wiring and Grounding
Wiring and grounding play a key role in the proper operation of facility equipment and systems. There is much agreement that the majority of PQ-related problems originate within the facility and that the majority of those problems are wiring and grounding related. Grounding systems and equipment are used to limit the voltage imposed by lightning, line swells or unintentional contact with higher voltages. Grounding systems stabilize the voltage to earth under normal operation and establish an effective path for fault current that is capable of safely carrying the maximum fault current with sufficiently low impedance to facilitate the operation of overcurrrent devices under fault conditions. Grounding systems help protect people and equipment from shock and/or damage.
Some of the things to look for in the facility’s wiring and grounding are bad or loose connections, missing grounding (safety) conductors, multiple bonds of grounding-to-grounded conductor (neutral-to-ground connections), ungrounded equipment, additional ground rods, ground loops, and insufficient size of the grounded (neutral) conductor. The key components of grounding systems are covered in Article 250 of the National Electrical Code (NFPA 70).
In summary, Table 1 lists six most of the most commonly encountered power quality phenomena, along with their probable causes and typical mitigation solutions.
Domestic and International PQ Standards
One of the most important PQ developments in recent years has been the increasing coordination of standards developed by the IEEE in the U.S. and the International Electrotechnical Committee (IEC). For example, IEEE 1159 Recommended Practice for Monitoring Electric Power Quality complements IEC 61000-4-30 Electromagnetic Compatibility (EMC), which is in force in Europe and most of the rest of the world.
Another important industry standard is IEEE 519 (Recommended Practices and Requirements for Harmonic Control in Electric Power Systems). Early iterations of IEEE 519 established levels of voltage distortion acceptable to typical distribution systems; however, as adjustable speed drives, rectifiers, and other non-linear loads became more common, it became obvious that IEEE 519 needed to be revised and updated to reflect changing industry conditions, especially with regard to the relationship of harmonic voltages to the harmonic currents flowing within industrial plants. The updated standard, IEEE 519-1992, established limits for harmonic voltages on the utility transmission and distribution system as well as for harmonic currents within industrial distribution systems. Other convergences of key elements of IEEE / IEC standards include:
• Voltage Sags and Reliability—IEEE 564 / IEC 61000-2-8
• Flicker—IEEE 1453 / IEC 61000-4-15
Power Quality and Energy Audits
It is not unusual for large industrial and commercial power consumers to see electric bills with demand charges as high as 50% of the facility’s actual consumption costs. As an offset, load shedding, peak shaving, installing more efficient lighting and other energy management strategies go far toward helping facility operators lower their demand penalties. However, before any of these strategies can be implemented, it is necessary to first gain an exact picture of how, when and where their energy is being used. This is the necessary first step to managing it. To that end, handheld power analysis instruments are ideal for facility energy studies and carbon footprint calculations, and for taking forward/reverse energy measurements for grid-tied alternative energy systems.
Energy audits come in many forms and can range from simple applications that monitor a single device or machine, to complex monitoring of an entire campus—and anything between. Regardless of a facility’s energy load, most energy audits have much in common. The most important parameters to measure when analyzing electrical energy are typically voltage (V), current (I), watts (W), volt-amperes (VA), volt-amperes reactive (VAR) and power factor (PF). Recorded over time, these basic parameters can provide the necessary information for a complete energy profile.
Voltage and current measurements are used as the basis to compute the other parameters. The parameters can be viewed instantaneously by a variety of instruments, but the key benefit of using an energy analyzer is its ability to record and trend parameters over time. Energy analyzers also compute the demand and energy that utilities use for billing.
What an energy-measuring instrument measures and computes is important, but how it measures can be critical. For example, some inexpensive low-resolution instruments may measure the basic parameters mentioned, but they can miss data and thereby produce false and misleading measurements.
Effective energy-analyzing instruments (Figure 3) should provide a sampling rate that is appropriate for the application while also providing the ability to take continuous readings. Power analyzers typically define sampling rate as the number of measurements taken per AC (60/50Hz) cycle. Because the instrument creates a digital representation of the analog voltage and current being measured, it is generally desirable to use an instrument that provides a higher number of samples per cycle, thus resulting in more accurate measurements of the data being collected.
Users are also encouraged to select an energy analyzer that can measure more than just the basic power parameters, since more advanced parameters may be required to also help understand the quality of the electrical supply, including: voltage and current total harmonic distortion (THD), transformer derating factor (TDF) and crest factor (CF). Additionally, with the advent of alternative-energy applications, parameters such as forward and reverse energy that record the flow of power to and from the grid are often required.
Details of the survey can vary greatly according to the application. The goal of an energy audit is usually to determine the energy profile of the system being monitored. Regardless of application, it helps to know some of the information about what is being monitored, such as the type of load, process or facility. These details are essential for determining the duration of the energy survey.
To obtain a complete picture of the energy profile, it is recommended to monitor several business cycles of the load being audited. For example, an industrial process that cycles (start to finish) every 15 minutes may only need monitoring for approximately an hour to capture multiple cycles and to find out what is usual or typical for that load. An office building cycling on a 24-hour basis may require a much longer survey, such as a week or more, to determine a typical energy profile. A survey replicating a utility bill may require monitoring for multiple utility billing cycles over several months.
PQ Solutions and Strategies
The importance of choosing the right tool to analyze and report the data cannot be overemphasized. But after the data has been analyzed, the next step is to apply the proper equipment or strategy to solve the problems identified by the survey. Solutions for dealing with PQ phenomena can be found under the following general classifications:
• Alternative power sources
• Back-up or standby generators
• Harmonic filters
• K-factor transformers
• Line reactors
• Overvoltage restorers/ stabilizers
• Power factor capacitors
• Power isolation transformers
• Surge-protective devices (SPD)
• UPS systems
• Wiring and grounding
In addition to the above, designing for critical operations is gaining traction in today’s digital economy, as well as other high-reliability applications that typically employ several mitigation strategies to maximize uptime to better than “six nines”—or 99.9999 percent—of availability. Although extremely close, this does not guarantee 100 percent uptime, as even highly redundant systems are susceptible to failures or unanticipated problems. In this scenario, permanent PQ monitoring equipment installed in strategic locations will help significantly to determine what happened and what is needed to prevent a recurrence.
For more information on mitigation strategies for mission-critical applications, the National Electrical Code’s Article 585 “Critical Operation Power Systems” specifically addresses the additional requirements needed to support vital operations requiring 99.9999 percent availability, including data and communications centers, financial and medical facilities and more.
Benefits of Permanently Installed PQ Monitoring Equipment
As opposed to handheld devices, permanently installed power quality monitoring systems (Figure 4) are generally Internet based and allow password-protected access from anywhere in the world. A key advantage of this type of system is the fact that multiple users can simultaneously access the same data, thus allowing the application of valuable input from multiple sources to a given problem. Permanently installed systems can range from a single device up to many hundreds of distributed units, depending on the complexity of the power quality monitoring task. Typical uses include:
• Data acquisition of problematic conditions
• Indication of trends
• Prediction of facility power quality problems
Communications flexibility is an especially useful benefit of the permanently installed system. Not only local and wide area networks (LANs and WANs) are supported, but also land and wireless modems including GSM and GPRS mobile phones. Due to bandwidth issues and other restrictions, however, it is not recommended to employ the latter method when downloading large amounts of data. Most PQ analyzers employ PC-based software programs that provide in-depth analysis of the collected data along with:
• Comparisons of multiple monitoring locations
• Comparisons of current and historical data
• Detailed reportage
• Looking for site trends
Conclusion
Some companies are reluctant to invest in PQ mitigation equipment, either because they lack knowledge about available solutions, or they do not want to allocate funds without a clearly defined return on investment. To help justify the expense, the first step is to determine what the cost of downtime is and how often it occurs. The next step is to determine what is causing the downtime and from that, what solutions are available to solve the problem. The good news for facility operators is that powerful, cost-effective handheld and permanently installed electrical energy monitors and power quality analyzers are now on the market. With the growing emphasis on “green” facilities, instruments from Dranetz and others offer facilities engineers and electrical contractors the ability to monitor, analyze and report on the full spectrum of facility power quality issues impacting the bottom line, as a first step in applying an appropriate, cost-effective PQ mitigation solution.
Ross Ignall is director of product management at Edison, NJ-based Dranetz, the leading provider of intelligent handheld and permanently installed monitoring solutions for electrical demand, energy and power quality analysis. He may be contacted at: (800) 372-6832 or rignall@dranetz.com
Sources
1. “Equipment Failures Caused by Power Quality Disturbances,” Ashish Bendre, Deepak Divan, William Kranz and William Brumsickle. SoftSwitching Technologies (Middletown, WI) and DRS Power and Control Technologies (Milwaukee, WI).
2. “Power Quality Analysis,” published by the National Joint Apprenticeship and Training Committee for the Electrical Industry (NJATC) in partnership with Dranetz (www.dranetz.com). 2010.
