Part 3 Power Quality: Definition and Discussion

Published by

  • John D. Kueck and Brendan J. Kirby, Oak Ridge National Laboratory
  • Philip N. Overholt, U.S. Department of Energy
  • Lawrence C. Markel, Sentech, Inc.

Published in Measurement Practices for Reliability and Power Quality: A Toolkit of Reliability Measurement Practices, 2004

Prepared by Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-6285 managed by UT-BATTELLE, LLC for the U.S. Department of Energy under contract DE-AC05-00OR22725

The IEEE Standard Dictionary of Electrical and Electronics Terms defines power quality as “the concept of powering and grounding sensitive electronic equipment in a manner that is suitable to the operation of that equipment.” Power quality may also be defined as “the measure, analysis, and improvement of bus voltage, usually a load bus voltage, to maintain that voltage to be a sinusoid at rated voltage and frequency.”2

Today’s electronic loads are susceptible to transients, sags, swells, harmonics, momentary interruptions, and other disturbances that historically were not cause for concern. For sensitive loads, the quality of electric service has become as important as its reliability. Power quality is a new phenomenon. Events such as voltage sags, impulses, harmonics, and phase imbalance are now power quality concerns. Power quality problems have a huge economic impact. As a result, any discussion of power system reliability must also include power quality.

The body of literature on reliability indices and calculation techniques represents a fairly mature discipline. In contrast, power quality references are works in progress, often revised and frequently outdated. There are several reasons for this:

  • Reliability and availability describe clearly defined events—loss of power. Power quality incidents are often momentary—a fraction of a cycle—and hard to observe or diagnose. Power quality measurement devices had to be developed so that the phenomena could be observed before power quality could be analyzed.
  • The growing digital load, and the increased sensitivity of some of these loads, means that the definition of a power quality incident keeps changing. Ten years ago, a voltage sag might be classified as a drop by 40% or more for 60 cycles, but now it may be a drop by 15% for 5 cycles.
  • The constituencies concerned with power quality are very diverse—utilities, regulators, facilities managers, equipment manufacturers, electrical engineers, electrical inspectors, building designers, electricians. All these groups have different definitions, objectives, responsibilities, criteria, and levels of sophistication in measurement and modeling capabilities.
  • Power quality often involves safety issues (e.g., grounding and elevated neutral voltages) that were not ever a part of reliability assessment.
  • Power quality involves design issues, such as the stiffness of the user’s distribution system, that did not have such an impact on operational reliability before.
  • Power quality problems can easily cause losses in the billions of dollars, and an entire new industry has recently grown up to diagnose and correct these problems.
  • Often, power quality problems can best be addressed with local corrective actions, and these local devices are undergoing a revolution themselves, with changes occurring rapidly.

There are many measures and indices of power quality. Some of the more common indices are the following:

  • Total harmonic distortion (THD): The ratio of the RMS value of the sum of the individual harmonic amplitudes to the RMS value of the fundamental frequency
  • K factor: The sum of the squares of the products of the individual harmonic currents and their harmonic orders divided by the sum of the squares of the individual harmonic currents
  • Crest factor: The ratio of a waveform’s peak or crest to its RMS voltage or current
    Flicker: A perceptible change in electric light source intensity due to a fluctuation of input voltage. It is defined as the change in voltage divided by the average voltage expressed as a percent. This ratio is plotted vs the number of changes per minute to develop a “flicker curve.”

There are many more indices and definitions of power quality. A list of power quality standards is provided in Appendix B. The definitions are rapidly changing, and often quite specialized in their application. For example, there is a harmonic voltage factor for motor application that is similar to, but not the same as, THD. The motor harmonic voltage factor is defined in NEMA Standard MG-1, a standard for motors and generators.

Typically, electrical engineers who work in various fields of electrical engineering—power systems, communications, computers—are familiar with the indices and definitions that pertain to their particular disciplines. There is also a new group of consultants who deal exclusively with power quality problems. Some industries are also developing their own standards for power quality; these are discussed further in Appendix F.

Some power quality problems are supplied to customers’ load through the utility distribution system, and some are caused by the customers themselves. Many problems originate with one customer and travel through the distribution system, and even the transmission system, to impact other customers. Some manufacturers are now equipping their products with filters and short-term storage devices so that they will be immune to many power quality problems. Local solutions to power quality problems tend to be the most cost-effective.

Knowing what power quality to expect from the power supplier is critical to designing power quality tolerance in end-use equipment, thus benefiting the customer, the utility, and the equipment manufacturer.


2. Gerald Heydt, Electric Power Quality, Stars in a Circle Publications, December 1991.

Appendix B Power Quality Standards, Guidelines, and Measurement

The following is a list and brief synopsis of many of the standards for power quality. Again, this list is not intended to capture every single standard, but rather the significant ones that are often invoked or referenced currently.

IEEE Std. 141-1995: Red Book IEEE Recommended Practices for Electric Power Distribution for Industrial Plants
Organization: IEEE
Targeted industry segment: Industrial and commercial facilities
Limitations: This standard is directed more towards good electrical power engineering practice and is not focused on power quality.
Strengths: A thorough analysis of basic electrical systems is presented. Guidance is provided in design, construction, and continuity of the overall system.
Other: Recommendations are made regarding system planning, voltage considerations, surge voltage protection, flicker, protective devices, grounding, and other issues.

IEEE Std. 519-1992: IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems
Organization: IEEE
Targeted industry segment: Industrial and commercial power systems with non-linear loads
Limitations: This standard addresses steady-state operation only, and not transients.
Strengths: The practices are used for guidance in the design of power systems with non-linear loads, such as adjustable speed drives and uninterruptible power supplies.
Other: The standard also discusses power system response characteristics, the effects of harmonics, methods for harmonic control, and recommended limits.

IEEE Std. 1159-1995: IEEE Recommended Practice for Monitoring Electric Power Quality
Organization: IEEE
Targeted industry segment: Utilities and industrial and commercial power systems
Limitations: This standard deals with electromagnetic disturbances such as voltage dips, notching, oscillatory transients, and frequency variations.
Strengths: The standard provides guidance in the monitoring, classification, and correction of a wide range of steady state and transient phenomena. It defines disturbances in 24 categories of typical characteristics of power system electromagnetic phenomena.
Other: The standard provides guidance on troubleshooting, interpreting data, analysis tips, and verifying the solution. Some IEEE groups developing sections of this standard are the following:

  • UIEEE P1159.1 Task Force on Recorder Qualification and Data Acquisition Requirements for Characterization of PQ Events
    This task force is developing the Guide for Recorder and Data Acquisition Requirements for Characterization of Power Quality Events. This guide will establish the data acquisition attributes necessary to characterize the electromagnetic phenomena listed in Table 2 of IEEE Std. 1159-1995. The guide includes definitions (in conjunction with P1433), instrumentation categories, and technical requirements that are related to the type of disturbance to be recorded. The objective of this guide is to describe the technical measurement requirements for each type of disturbance in Std 1159-1995. Measurement requirements of these types of disturbances are not currently covered by other standards.
  • IEEE P1159.2 Task Force on Characterization of a Power Quality Event
    This task force is developing a recommended practice for converting a suitably sampled voltage and current data set into specific power quality categories. Appropriate definitions, categories, and sampling rates are being developed by other task forces. The emphasis is on compatibility between power delivered by power suppliers and power needed by equipment manufacturers. The translation from sets of digital data to statistically comparable events could be used for comparing power suppliers, comparing susceptibility qualities of equipment, and evaluating performance vs specification or contract.
  • IEEE P1159.3 Task Force on the Transfer of Power Quality Data
    This task force is developing a recommended practice for a file format suitable for exchanging power-quality-related measurement and simulation data in a vendor-independent manner. (Definitions and event categories are being developed by other task forces.) Many simulations and measurement and analysis tools for power quality engineers are available from numerous vendors. Generally, the data created, measured, and analyzed by these tools are incompatible among vendors. The proposed file format will provide a common ground to which all vendors could export and from which they could import to allow the end user maximum flexibility in choice of tool and vendor.

IEEE Std. 1100-1999: Recommended Practice for Powering and Grounding Electronic Equipment (Emerald Book)
Organization: IEEE
Targeted industry segment: Industrial and commercial electric power distribution systems.
Limitations: This standard is directed toward the designers and users of industrial power systems. It is not intended for utility distribution or transmission systems.
Strengths: The standard provides information on the electrical environment, conducting power surveys, monitoring and metering equipment, power conditioning equipment, and wiring and grounding for power quality. Historically, most power quality problems in an industrial environment resulted from improper wiring or grounding. The standard has been revised to include additional information on the sensitivity of industrial environments.

IEEE 1250-1995 IEEE: Guide for Service to Equipment Sensitive to Momentary Voltage Disturbances
Organization: IEEE
Targeted industry segment: Utilities and industrial and commercial power systems.
Limitations: The purpose of this guide is to assist in identifying potential problems and to suggest effective ways to satisfy sensitive equipment voltage problems.
Strengths: The guide describes many common problems, such as capacitor switching, motor starting, and tap changing.
Other: The guide provides data on various types of sensitive loads―including computers, process control, and adjustable speed drives―and suggests solutions and measures, such as grounding, circuit design, and surge protection.

IEEE Std. 1346-1998: IEEE Recommended Practice for Evaluating Electric Power System Compatibility with Electronic Process Equipment
Organization: IEEE
Targeted industry segment: Utilities and industrial and commercial power systems.
Limitations: This standard deals with planning and designing a power supply system so that compatibility issues with electronic process equipment are resolved.
Strengths: The standard provides guidance in methods for analysis of power systems in evaluating the compatibility of service quality with the equipment that uses the electricity. This standard addressesbthe issue of how service quality affects the end user. The standard provides worksheets to provide an estimate of the number of disruptions, financial loss, and analysis of alternatives.
Other: This first edition of the standard provides a methodology for voltage sags; later editions will deal with issues such as harmonics and transients. The purpose of this document is to recommend a standard methodology for the technical and economic analysis of compatibility of process equipment with the electric power system. The emphasis is on the new digital loads of microprocessors and power electronics equipment. This document does not intend to set performance limits for utility systems, power distribution systems, or electronic process equipment. Rather, it shows how the performance data for each of these entities can be analyzed to evaluate their compatibility in economic terms. The recommended methodology will also provide standardization of methods, data, and performance of power systems and equipment in evaluating compatibility so that compatibility can be discussed from a common frame of reference. The methodology is intended to be applied at the planning or design stage of a system; consequently, it does not discuss troubleshooting or correcting existing power quality problems. [See H index.html, IEEE Electric Power System Compatibility with Electronic Process Equipment (P1346)H.]

IEEE Std. P1564 IEEE: Recommended Practice for the Establishment of Voltage Sag Indices
Organization: IEEE
Targeted industry segment: Utilities and industrial and commercial power systems.
Limitations: This is a draft standard in preparation. It will provide sag indices to indicate the different performance levels at the transmission, substation, and distribution circuit levels.
Strengths: The standard will provide guidance in characterizing sags in terms of indices.
Other: The standard should help utilities and manufacturers compute the advantages and disadvantages of various connections to the electrical system.

CBEMA Curve and IEEE Standard 446-1995: IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications (Orange Book)
Organization: Computer Business Equipment Manufacturers Association (CBEMA) and IEEE
Targeted industry segment: Computer manufacturers, building electrical system designers.
Limitations: Criteria for tolerance of computer equipment to voltage variations. The CBEMA curve has been applied to many types of electronic equipment and needs to be updated to reflect current state-of-the-art of electronic equipment.
Strengths: This is a widely accepted and recognized standard.
Other: The CBEMA Curve is a part of IEEE 446. Cognizant industry groups are involved in updating the curve and specifying the situations where it is applicable. The CBEMA curve is discussed further in Chapter 3 of this report under “A Basic Generally Accepted Level of Reliability.

Appendix F Industry Initiatives to Define Power Quality: Discussion of the SEMI, CBEMA and ITIC Curve

Semiconductors Manufacturers’ Institute

One industry that has established its own specific level of power quality is the semiconductor manufacturing industry. The Semiconductors Manufacturers’ Institute (SEMI) has developed a power quality need curve that simply shows the minimum voltage vs time that their equipment is expected to ride through an outage (Figure F.1). With this curve, semiconductor manufacturers can specify tools, adjustable speed drives, controllers, etc. that are designed to function during power quality events. The manufacturers can also specify the needed DER to ensure that they can ride through events worse than those covered by the need curve.

Figure F.1. Semiconductors Manufacturers’ Institute provisional specification for voltage sag ride-through capability.

SEMI #2844 is the ride-through limit curve for semiconductor tools

  • The curve was developed from an analysis of 30 monitor years of disturbance data collected at major semiconductor sites.
  • The proposed curve should result in less than one event per site per year.
  • However, the curve requires 80% voltage at the longer durations, 1 second to 10 seconds.
  • The curve is based on minimal use of energy storage devices; instead, it suggests the careful selection of devices such as tools, relays, and power supplies.
  • The curve assumes direct connection to transmission; connection through a distribution feeder may result in sags and durations that would fail to meet the curve.
  • DER will enable the curve to be met in locations where direct connection to transmission is not possible.

The CBEMA curve is the defined power quality acceptability level defined by a group of computer manufacturers

  • The CBEMA curve requires a return to 90% voltage after one minute.
  • The CBEMA curve is more restrictive for the first 12 cycles.
  • DER with energy storage and a power electronics interface could easily enable any manufacturing facility to meet either curve.
  • The SEMI curve was created because so much SEMI equipment could not meet the CBEMA curve.

Figure F.2 is a histogram of sag and interruption rate magnitude. The solid line indicates the CBEMA acceptance limit, and the light blue dashed line indicates the SEMI 2844 acceptance limit. (This figure is from the EPRI Distribution Power Quality Study.) The numbers in the cells indicate the probability of a sag of that voltage and duration. These are based on 1-minute aggregations from 6/1/93 to 6/1/95. The SEMI curve is not as restrictive as the CBEMA limit.

Figure F.2. A histogram of sag and interruption rate magnitude. The solid line indicates the CBEMA acceptance limit and the dashed line, the SEMI 2844 acceptance limit. The numbers in the cells indicate the probability of a sag of that voltage and duration, based on 1-minute aggregations from 6/1/93 to 6/1/95. Source: EPRI Distribution Power Quality Study.

Information Technology Industry Council Curve

ITIC, the successor organization to CBEMA, has developed the ITIC Curve, a recommended capability curve for single-phase data processing equipment operating at 120 V. The ITIC Curve provides an easier graphical format to reproduce and requires improved ride-through capability for minor voltage sags (Figure F.3). However the curve is still general in nature and does not reflect typical performance for any particular type of equipment.

Figure F.3. ITIC curve defining voltage sag ride-through design goals for manufacturers of information technology equipment (applies to single-phase 120/240-V equipment).

Published by PQTBlog

Electrical Engineer

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