ROADMAP FOR POWER QUALITY STANDARDS DEVELOPMENT

Copyright Material IEEEPaper No. PCIC-2005-30

David B. Vannoy, P.E.MemberVannoy ConsultingWilmington, DE

Mark F. McGranaghanSenior MemberEPRI PEACKnoxville, TN

S. Mark HalpinSenior MemberAubum UniversityAubum, AL

William A. Moncnef, P.E.Senior MemberHood-Patterson & DewarDecatur, GA

D. Daniel SabinSenior MemberElectrotek ConceptsBoston, MA

Abstract - Power quality standards provide the basis forachieving compatibility between the characterstics of the electricsupply system and end use equipment. They provide themethods for evaluating performance, define equipmentrequirements, and outline relative responsibilities. This paperdescribes the status of important power quality standards aroundthe world and presents a roadmap for ongoing standardsdevelopment.

Index Terms - Power quality, standards, harmonics, flicker,voltage unbalance, voltage sags, transients.

I. INTRODUCTION

The requirements of electrcity users have changedtremendously over the years. Equipment has become muchmore sensitive to power quality variations and some types ofequipment can be the cause of power quality problems.Standards are needed to achieve coordination between thecharacteristics of the power supply system and the requirementsof the end use equipment. This is the role of power qualitystandards.Durng the past 15 years much progress has been made in

defining power quality phenomena and their effects on electricaland electronic equipment. In addition, methods have beenestablished for measuring these phenomena and in some casesdefining limits for satisfactory performance of both the powersystem and connected equipment. In the intemationalcommunity, both IEEE and IEC have created a group ofstandards that addresses these issues from a variety ofperspectives. However, there is a continuous need to definecoordination requirements, methods of assessing performance,and relative responsibilities.The IEEE Standards Coordinating Committee on Power

Quality (SCC22) tracks the development of power qualitystandards and has created a master plan to direct standardsdevelopment efforts in needed areas. In addition SCC22 has afocus on continuing efforts to coordinate intemational powerquality standards to provide consistent requirements andevaluation methods around the world. This paper examinesexisting IEEE and IEC standards and descrbes the need forongoing development.

II. THE ROLE OF POWER QUALITY STANDARDS

Power quality problems ultimately impact the end user.However, there are many other parties involved in creating,propagating, and solving power quality problems. Power qualitystandards must provide guidelines, recommendations, and limitsto help assure compatibility between end use equipment and the

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system where it is applied. The following are basic needs forpower quality standards:

* Definitions, Indices. Standards are needed to definethe indices that are used to characterize performanceand provide definitions for important power qualitycharacteristics.

* Measurement and monitoring procedures.Standardized methods of characterizing performanceand evaluating equipment characteristics are needed.

* Benchmarking. Understanding expected power qualitycharacteristics for different types of systems providesthe basis for establishing guidelines and limits.

* Power Quality Guidelines and Limits. These standardsprovide the "Compatibility Levels" that define theexpected power quality levels. They need to be definedin three categories:

- PQ requirements for the supply system- PQ immunity for equipment- PQ disturbance generation limits for

equipment and end user systems* Application guidelines. Finally, the standards need to

provide guidance in controlling power quality andsolving problems, including methods to understand theeconomics of solving power quality issues at differentlevels.

Ill. POWER QUALITY STANDARDS DEVELOPMENTORGANIZATIONS

The International Electrotechnical Commission (IEC) is themain organization responsible for power quality standardsdevelopment in the intemational community. JEC standards areoften adopted by individual countries as actual performancerequirements. IEEE also has a number of important standardsdevelopment activities in the power quality area and is activelycoordinating with the IEC Working Groups that are prmarilyresponsible for power quality standards.The IEC has defined a category of standards called

Electromagnetic Compatibility (EMC) Standards that deal withpower quality issues. They fall into the following six categores:

1. General. These provide definitions, terminology, etc.(IEC 61 000-1 -x).

2. Environrnent. Characterstics of the environmentwhere equipment will be applied (61000-2-x).

3. Limits. Emission limits define the allowable levels ofdisturbances that can be caused by equipmentconnected to the power system. These standards

05CH37704

were formerly the IEC 555 series but now arenumbered 61000-3-x. For instance, IEC 555-2 hasnow become IEC 61000-3-2.

4. Testing and Measurement Techniques. Theseprovide detailed guidelines for measurementequipment and test procedures to assurecompliance with the other parts of the standards(61000-4-x).

5. Installation and Mitgation Guidelines. These aredesigned to provide guidance in application ofequipment, such as filters, power conditioningequipment, surge suppressors, etc., to solve powerquality problems (61000-5-x).

6. Generic and Product Standards. These will defineimmunity levels required for equipment in generalcategories or for specific types of equipment (61000-6-x).

The following working groups of IEC SC77A are activelydeveloping these standards:

* Working Group 1 - Harmonics and other Low frequencyDisturbances. Focus on limits and methods ofmeasurement for harmonics and interharmonics.

* Working Group 2 - Voltage Fluctuations (flicker) andother Low Frequency Disturbances. Develops limits forvoltage fluctuations caused by end user equipment andmethods of measurement as appropriate. This workinggroup will be working on an update to the document onrreference impedances that can be used for evaluating theimpact of equipment on the system.

* Working Group 6 - Low Frequency Immunity Tests.Develops testing procedures for evaluating equipmentimmunity from power quality variations.

* Working Group 8 - Electromagnetic Interference Relatedto the Network Frequency. This group is addressing thefull range of power quality phenomena on the networkand the interaction issues with consumers.

* Working Group 9 - Power Quality MeasurementMethods. Currently developing IEC 61000-4-30, anoverall guide defining the requirements for power qualitymonitoring equipment.

In the United States, standards are developed by the IEEE,ANSI, and equipment manufacturer organizations, such asNEMA. There are also safety-related standards, like the NationalElectrical Code. IEEE standards generally do not specifyrequirements for equipment. These standards tend to be moreapplication oriented, like IEEE Standard 519-1992, whichprovides recommendations to limi harmonic distortion levels onthe overall power system.The Power Quality Standards Coordinating Committee,

SCC22 was created in 1991 as a coordinating body for powerquality standards in IEEE . Historically this committee met atboth Power Engineering Society meetings and the IndustryApplication Society Annual Meeting to help coordinate thestandards activities under way in each of these societies. Inaddition, SCC22 sponsored standards efforts when no SocietyCommittee sponsor was available. Recently, a Power QualitySubcommittee was created under the Transmission andDistribution Commitee of the Power Engineering Society tosponsor individual working groups and task forces that are

developing standards. Ownership for several of the SCC22sponsored standards Working Groups was transferred to thisnew Subcommfttee. This new subcommfttee coordinates closelywith SCC 22. SCC22 membership is composed of personsactively involved in Power Quality standards development andrepresents a variety of industry segments.A listing of some of the important power quality standards

activities in IEEE is provided in Appendix A. Appendix B gives across reference with important IEC PQ standards.

IV. STEADY STATE POWER QUALITYCHARACTERISTICS

Power quality characteristics and requirements are divided intotwo broad categories - steady state, or continuous,characteristics and disturbances. Steady state characteristicsdefine the requirements for the normal voltage supplied from thepower system and the relative responsibilities of the supplysystem and end users and equipment in maintaining the requiredquality of the voltage. Disturbances, on the other hand, occurrandomly and different methods of describing performance andcoordination requirements are needed.

For steady state power quality characteristics (voltageregulation, unbalance, harmonics, flicker), the levels on thesupply system are coordinated with the characteristics ofequipment to define compatibility levels. Steady statecharacteristics are characterized with trends and statisticaldistributions of the quantity being evaluated. Understanding thatthese characteristics are not defined with a single value butrepresent a range of values with a probability distribution is veryimportant. The concept is illustrated in Figure 1.

p1amingbveb1

Disturbance level

Fig. 1. Compatibility between supply system and end useequipment.

The concept of compatibility levels in Fig. 1 can be expandedto introduce related levels for evaluation of performance. Someimportant power quality levels that are described in thestandards include the following:

1. Compatibility levels. These define the basicexpectation for performance of the supply system.Therefore, they can provide the basis formanufacturers to design equipment for immunity tosupply system power quality variations. Requirementsfor the regulation of the steady state voltage have beenin place for power systems around the world for manyyears. New standards, such as the EuroNorm EN

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50160, "Physical characteristics of electrcity suppliedby public distrbution systems" define the requirementsin other power quality categories (harmonics, voltagefluctuations, unbalance, interruptions, voltage dips).The requirements for system performance in thesedifferent categories are known as "voltagecharacteristics".

2. Planning limits. Planning limits are established byutilities for comparison with actual power quality levels.Measured power quality levels that exceed the planninglevels are an indication of a possible problem on thesystem that should be addressed. There should besome margin between planning levels and requiredvoltage characteristics.

3. Equipment Immunity Characteristics. The equipmentimmunity levels should be coordinated with the voltagecharacterstics to make sure that the equipment canoperate under the full range of possible power qualitylevels. There should be some margin between theequipment immunity levels and the voltagecharacterstics.

Fig. 2 shows these levels with a trend of measured data for anactual power quality characteristic. Note that this could be any ofthe steady state power quality quantities - voltage deviations,unbalance, harmonics, and flicker. It is worthwhile to considerthe status of standards and needs for standards developmentbrefly in each of these categores.

Disturbance magnitudeA1

Equipment immunity test levels

Compatibility level

Utility planning levels

timeFig. 2. Illustration of a measured trend for a power quality

characterstic compared to different levels defined for assessingperformance.

A. Voltage Regulation

There is no such thing as steady state on the power system.Loads are constantly changing and the power system isconstantly adjusting to these changes. All of these changes andadjustments result in voltage varations that are referred to aslong duration voltage varations. These can be undervoltages orovervoltages, depending on the specific circuit conditions.Characteristics of the steady state voltage are best expressedwith long duration profiles and statistics. Importantcharacterstics include the voltage magnitude and unbalance.According to the latest draft of IEEE Standard P1159, IEEERecommended Practice for Monitoring Power Quality, longduration variations are considered to be present when the limitsare exceeded for greater than 1 minute.Most end use equipment is not very sensitive to these voltage

varations, as long as they are within reasonable limits. ANSIC84.1-1995 [11] specifies steady state voltage tolerancesexpected on a power system. It recommends that equipment bedesigned to operate with acceptable performance under extremesteady state conditions of +6% and -13% of nominal 120/240 voltsystem voltage. Protective devices may operate to remove theequipment from service outside of this range.

European limits are specified in EN 50160 [4]. Limits for supplyvoltage magnitude variations are specified for low voltage (LV)systems. The supply voltage rms magnitude, whether line-to-neutral or line-to-line, should be within +/- 10% for 95% of aweek. Voltage magnitudes are characterized by a measurementperod of 10 minutes. The evaluation procedure is that 95% ofthe 1 0-minute values for one week should be within the specifiedlimits. These limits are based on the compatibility levelsspecified in IEC 61000-2-2 [1]. In general, all ten minutes meanrms values of supply voltage are expected to be within +10%/-15%, excluding dips, interruptions and overvoltages.

B. Voltage Unbalance

The most recent version of ANSI C84.1 [11] includesrecommended limits for voltage unbalance on the power system.In the ANSI Standard, unbalance is a steady state quantitydefined as the maximum deviation from the average of the threephase voltages or currents, divided by the average of the threephase voltages or currents, expressed in percent. In theintemational standards, unbalance is more commonly defined asthe ratio of the negative sequence component to the positivesequence component.The primary source of voltage unbalances of less than two

percent is unbalanced single phase loads on a three-phasecircuit. Voltage unbalance can also be the result of capacitorbank anomalies, such as a blown fuse on one phase of a three-phase bank. Voltage unbalance can also be introduced from thetransmission system due to transmission lines that are nottransposed. Severe voltage unbalance (greater than 5%) canresult from single-phasing conditions.

Voltage unbalance is most important for three phase motorloads. ANSI C84.1 recommends that the maximum voltageunbalance measured at the meter under no load conditionsshould be 3%. Motors operating between 1 and 5% voltageunbalance should be derated to prevent overheating. Motorsshould not be operated with over 5% voltage unbalance.The EN 50160 limit for unbalance is 2% for normal systems,

based on the compatibility levels specified in IEC 61000-2-2. Alimit of 3% applies on systems with single-phase loads. Forevaluation, unbalance levels are characterized in 10-minuteperiods. For compliance, 95% of these 10-minute values shouldbe within the limits in a one-week measurement period.

C. Harnonics

Harmonic voltage distortion results from the interaction ofharmonic currents (created by nonlinear loads and othernonlinear devices on the power system) with the systemimpedance. The harmonic standard, IEEE Standard 519-1992,IEEE Recommended Practices and Requirements for HarmonicControl in Electrcal Power Systems [71, has proposed two wayresponsibility for controlling harmonic levels on the powersystem. End users must limit the harmonic currents injectedonto the power system. The power supplier will control theharmonic voltage distortion by making sure system resonant

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conditions do not cause excessive magnification of the harmoniclevels.Harmonic distortion levels can be characterized by the

complete hanmonic spectrum with magnitudes and phase anglesof each individual harmonic component. It is also common touse a single quantity, the Total Harmonic Distortion or THD, as ameasure of the magnitude of harmonic distortion. For currents,the distortion values must be referred to a constant base (e.g.the rated load current or demand current) rather than thefundamental component. This provides a constant referencewhile the fundamental can vary over a wide range.

Harmonic evaluations often involve a combination ofmeasurements and analysis (possibly simulations). It isimportant to understand that harmonics are continuousphenomena, rather than a disturbance (like a transient).Because harmonics are continuous, they are best characterizedby measurements over time so that the time varations and thestatistical characteristics can be determined. Thesecharacterstics describing the harmonic varations over timeshould be determined along with snapshots of the actualwaveforms and harmonic spectrums at particular operatingpoints.Harmonic evaluations on the utility system involve procedures

to make sure that the quality of the voltage supplied to allcustomers is acceptable. IEEE Standard 519-1992 providesguidelines for acceptable levels of voltage distortion on the utilitysystem (Table 1). Note that recommended limits are providedfor the maximum individual harmonic component and for theTotal Harmonic Distortion (THD).

TABLE 1HARMONIC VOLTAGE DISTORTION LIMITS FROM IEEE STANDARD

519-1992

Bus Voltage Maximum Individual MaximumHarmonic Component (%) THD(%)

69 kV and below 3.0% 5.0%115 kV to 161 kV 1.5% 2.5%Above 161 kV 1.0% 1.5%

These voltage distortion limits apply at the point of commoncoupling (PCC), which will be on the medium voltage system formost industral and commercial customers. The concept of thePCC and many other questions related to the application ofharmonic limits are addressed in an application guide forapplying harmonic limits that is currently being finalized - IEEEStandard 519.1 [18]. For instance, discussion of notchingconcems, such as additional zeros crossings and exciting highfrequency resonance, are now addressed in the applicationguide. Concems for important effects of hannonics, liketelephone interference, are also addressed in the applicationguide.

Note that higher voltage distortion levels may be appropratewithin the end user facility and this is being addressed in therevision effort for IEEE Standard 519. Most end use equipmentis not affected by voltage distortion levels below 8%. In fact, thecompatibility level for voltage distortion on LV and MV systemsspecified in IEC 61000-2-2 is 8% (this is the voltage distortionlevel that should be exceeded less than 5% of the time). Therevision of IEEE 519 may also consider revised voltage distortionlimits at other voltage levels to be more compatible withintemational standards.

Most harmonic problems occur at the end user level, ratherthan on the utility supply system. Most nonlinear devices arelocated within end user facilities and the highest voltagedistortion levels occur close to the sources of harnonics. Themost significant problems occur when an end user has nonlinearloads and also has power factor correction capacitors that resultin resonance conditions.

In order to maintain acceptable levels of voltage distortion,harmonic current limits at the PCC are described in IEEEStandard 519 as well. These are summarzed in Table 2.

TABLE 2HARMONIC CURRENT LIMITS FOR INDMDUAL END USERS FROMIEEE STANDARD 519-1992 (EXPRESSED IN % OF THE RATED LOAD

CURRENT, IL)

vv69kV

__ /I, h<ll_ 11h<17 17ch<23 23<h<35 35<h TDD<20 4.0 2.0 1.5 0.6 0.3 5.020-50 7.0 3.5 2.5 1.0 0.5 8.050-100 10.0 4.5 4.0 1.5 0.7 12.0100-1000 12.0 5.5 5.0 2.0 1.0 15.0>1000 15.0 7.0 6.0 2.5 1.4 20.0

69kV < v ' 161kV<20* 2.0 1.0 0.75 _0.3_ 0.15 2.520-50 3.5 1.75 1.25 0.5 0.25 4.050-100 5.0 2.25 2.0 1.25 0.35 6.0100-1000 6.0 2.75 2.5 1.0 0.5 7.5>1000 7.5 3.5 3.0 1.25 0.7 10.0

v > 161kV< 50 2.0 1.0 0.75 0.3 0.15 2.52 50 3.5 1.75 1.25 0.5 0.25 4.0

There are a number of important concepts introduced in thesecurrent limits. For instance, the harmonic limits are dependenton the strength of the system where the customer is connected(ratio of IL to the short circuit current, Isc). Also, a new quantitycalled the Total Demand Distortion (TDD) is introduced asfollows:

V I2TDD= n=2 XlOO%

IL

where:

In= magnitude of individual harmonic components (rmsamps)n = harmonic orderIL = maximum demand load current (rms amps)

Intemational compatibility levels for harmonics are specified inIEC 61000-2-2. These are used to develop utility limits in EN50160. EN 50160 specifies limits for individual harmoniccomponents up to the 25th and for the Total Harmonic Distortion(THD). The limits are not as strct as the recommended limits inIEEE Standard 519 and some efforts to coordinate these limitsare under way in the next revision to IEEE Standard 519. Forinstance, the limit for THD is 8%. These limits are evaluatedusing a measurement procedure defined in IEC 61000-4-7 [19].This involves calculating harmonic values in 3 second perodsand then combining these 3 second values to obtain 10 minutevalues. The limits should be met by 95% of the 10 minutevalues dunng an assessment perod of one week. One of the

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most important standards coordination efforts needed in theharmonics area is to achieve more of a consensus on methodsand indices for measuring and characterzing harmonic levelsusing statistical procedures.

D. Flicker

Voltage fluctuations are systematic vanations of the voltage ora series of random voltage changes, the magnitude of whichdoes not normally exceed the voltage ranges specified by ANSIC84.1. These fluctuations are often referred to as flicker. Theyare characterzed by the magnitude of the voltage changes andthe frequency with which they occur. A plot of the rms voltagemagnitude vs. time can be used to illustrate the variations.The most important impact of these fluctuations is that they

cause varations in the light output of various lighting sources.Sensitivity curves have been developed for incandescent lightingthat show how the voltage fluctuations can cause unacceptablevariations in the light output. These sensitivity curves were usedto specify a measurement device that can characterze thepotential for voltage variations to cause unacceptable light flicker.This measurement device (the flickermeter) has beenstandardized in IEC 61000-4-15 [19] and is now the intemationalstandard for measuring voltage fluctuations and flicker.The orginal flickermeter specification was based on the

effects of voltage fluctuations on a 60 Watt incandescent light on230 volt systems. A 60 Watt incandescent light bulb designedfor 120 volts is not as sensitive to the same voltage fluctuationsbecause the filament is larger (longer time constant) to handlethe higher current levels associated with the same Watt rating.As a result, an additional weighting curve was developed for 120volt applications which are more common in North America. The120 volt and 230 volt weighting curves are compared in Fig. 3.In North America, the flicker measuring procedure should usethe method standardized in IEC 61000-4-15 with the 120 voltweighting curve employed. This has now been formalized in anIEEE standard - IEEE Standard 1453 [10].

W. I'

01 1 10 100 1000 10000changes/minute

Fig. 3. Comparison of 120 volt and 230 volt flickermeterweighting curves (curves are Pst=1 curves for rectangular

variations).Output from the flicker meter consists of two basic quantities:

1. The short term flicker severity, Pst. A Pst value isobtained every 10 minutes. There are 144 Pst sampleseach day. Pa is a per-unitized quantity where 1. per unitrepresents a flicker severty that should correspond

approximately to perceptible flicker in 40 Wattincandescent lights.

2. The long tern flicker severity, PRt. Each Pftvalue iscalculated from 12 successive Pst values using thefollowing formula:

1 12 3

Each of these two basic quantities can be characterzed interms of their statistics. The following statistical quantities arerecommended in a recent report prepared by Cigre C4.07 TaskForce [23]. They should be calculated after measunrng over aperod of time, recommended to be at least one week.

Psjg95% is the Pt level that is exceeded 5% of the time. Thisvalue is compared with planning levels for the system beingevaluated.

Ps,gg% is the Pst level that is exceeded 1% of the time. Thiswould be compared with planning levels with some margin(e.g. planning levels times 1. 0-1. 5)

Plo5% is the Pt level that is exceeded 5% of the time. This isthe value that is compared to voltage characteristics(limits)..

IEC developed standard 61000-3-7, "Assessment of emissionlimits for fluctuating loads in MV and HV power systems" [9] toprovide a procedure for assessing flicker levels and applyinglimits at individual end users connected to the high voltagesystem. This standard was developed in close cooperation withboth the United States and Canada and includes the 120 voltweighting curve described above for the North America systems.

V. STANDARDS FOR POWER QUALITY DISTURBANCESAND RELIABILITY

Disturbances are events that do not occur on a regular basisbut can impact the performance of equipment. They includetransients, voltage varations (sags swells), and interruptions.Interruptions that last more than one minute (sometimes fiveminutes) are usually referred to as outages and are included inreliability statistics. Short interruptions are classified with powerquality variations.

A. Voltage Sags and Interruptions

Voltage sags fall in the category of short duration voltagevariations. According to IEEE Standard 1159 and IECdefinitions, these include variations in the fundamental frequencyvoltage that last less than one minute. These variations are bestcharacterized by plots of the rms voltage vs. time (Fig. 4) but it isoften sufficient to describe them by a voltage magnitude and aduration that the voltage is outside of specified thresholds. It isusually not necessary to have detailed waveform plots since therms voltage magnitude is of primary interest.The voltage variations can be a momentary low voltage

(voltage sag), high voltage (voltage swell), or loss of voltage(interruption). IEEE Standard 1159 specifies durations forinstantaneous, momentary, and temporary disturbances.

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Voltage sags are typically caused by a fault somewhere on thepower system. The voltage sag occurs over a significant areawhile the fault is actually on the system. As soon as a fault iscleared by a protective device, voltage retums to normal on mostparts of the system, except the specific line or section that isactually faulted. The typical duration for a transmission systemfault is about six cycles. Distribution system faults can havesignificantly longer durations, depending on the protectionphilosophy. The voltage magnitude during the fault will dependon the distance from the fault, the type of fault, and the systemcharacteristics.

Fig. 4. Plot of rms voltage vs. tme for a voltage sag (and anactual waveform plot)

End users can evaluate the economics of power conditioningequipment if they have infommation describing the expectedsystem voltage sag performance. A complete methodology forthis evaluation is provided in IEEE Standard 1346 [21]. Theexpected voltage sag performance from the supply system isused in combination with equipment sensitivity characteristics toestimate the number of times per year that a process will bedisrupted. Fig. 5 illustrates the contour plot method ofcharacterizing system performance for these evaluations.

Interruption and Sag Rate Probabitles as a Functionof Event Voltage Magnitude and Duration, z ' I T I = _ _ _ _ _ ~~~~~~~~~~~~-60 s

_ ____ --9e -nt -z -L - - - 4

=-_S=_ ==-- ,0

A 2 4 6 10 20 so 40 so ,o 300o.S000D0.0.n I(Cy...)

Fig. 5. Contour plot method of characterizing system voltagesag performance (IEEE Standard 1346).

There is considerable standards work under way to defineindices for characterizing voltage sag performance. In IEEE, thiswork is being coordinated by IEEE P1564 [16]. The mostcommon index use is SARFI, or the System Average RMS(Variation) Frequency Index. This index represents the averagenumber of voltage sags experienced by an end user each yearwith a specified characteristic. For SARFIx, the index would

indude all of the volage dips where the minimum voltage wasless than x. For example, SARFI7o represents the expectednumber of voltage sags where the minimum voltage is less than70%. The SARFI index and other altematives for descuibingvoltage sag performance are being formalized in the IEEEStandard 1564 Working Group. Fig. 6 is an example of SARFIlevels calculated from a survey of performance for distributionsystems in the United States.

Avg. SAlI Sttistlics for US Distdbu>n Sysftms

:DII

SARFI.70 SARFI40 SARF1410

Fig. 6. Example of voltage sag performance levels (SARFI) fordistribution systems in the United States from the EPRI

Distribution Power Quality (PDQ) project [22]

B. Transients

The term transients is normally used to refer to fast changes inthe system voltage or current. Transients are also in thecategory of disturbances, rather than steady state variations.Transients can be measured by triggering on the abnormalityinvolved. For transients, this could be the peak magnitude, therate of rise, or just the change in the waveform from one cycle tothe next. Transients can be divided into two subcategories,impulsive transients and oscillatory transients, depending on thecharacteristics.

Transients are normally characterized by the actual waveform,although summary descriptors can also be developed (peakmagnitude, primary frequency, rate-of-rise, etc.). Fig. 7 gives acapacitor switching transient waveform. This is one of the mostimportant transients that is initiated on the utility supply systemand can affect the operation of end user equipment. Otherimportant causes of transient voltages include lightning surgesand switching operations within a facility.

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Example Voltage Sag Waveform from FieldMeasurement

120 Durationi110 01.17 Sec910 Min 74.70so - Ave 94. 1170I Max98.58

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 RefCycleTime (Seconds) 48462

100,50I0

-1500 25 50 75 100 125 150 175

Time (mSeconds)

Transient problems are solved by controlling the transient atthe source, changing the characteristics of the system affectingthe transient or by protecting equipment so that it is notimpacted. For instance, capacitor switching transients can becontrolled at the source by closing the breaker contacts close toa voltage zero crossing. Magnification of the transient can beavoided by not using low voltage capacitors within the end userfacilities. The actual equipment can be protected with filters orsurge arresters.

The most well-known standard in the field of transientovervoltage protection is ANSIVIEEE C62.41-1991, IEEE Guidefor Surge Voltages in Low Voltage AC Power Circuits [12]. Thisstandard defines the transient environment that equipment maysee and provides specific test waveforms that can be used forequipment withstand testing. The transient environment is afunction of the equipment or surge suppressor location within afacility as well as the expected transients from the supplysystem.

VI. FUTURE DIRECTION FOR POWER QUAUTYSTANDARDS

Benchmarking efforts from around the world have provided theinitial basis for defining expected power quality perfofmance ofsupply systems. These performance standards should includeat least:

* Interruptions (including momentary)* Voltage sags* Steady state voltage regulation* Voltage unbalance (negative sequence)* Harmonic distortion in the voltage* Transient voltages

There is a need for significant additional research to establishthe relationship between power quality/reliability levels and thevarious characteristics of the supply system. Also, the powerquality/reliability characteristics need to be defined in a morestatistical manner to allow more effective risk assessments byend users using statistical techniques.

In tum, equipment manufacturers must be able to provideinformation describing the sensitivity of their equipment to thesevariations. Wth information on typical system performancebased on historical and calculated data along with information onequipment sensitivity, end users will be able to performeconomic evaluations of power conditioning altematives.Standard procedures for the economic analysis will incorporatestatistical risk assessment methods in the future.Ongoing monftoring efforts and case studies wili provide the

information to characterize system performance and tounderstand the susceptibility of different types of end usersystems. Monitoring of power quality should become a morestandard part of the overall system monitoring (both at the utilitylevel and the customer level). These monitoring efforts shouldbe coordinated between the utility and the customer withemphasis on remote monitoring and data collection systems withmore automated data analysis capabilities. IEC 61000-4-30 [17]provides a good start for standardizing power qualitymeasurements but there is a need for additional standardsdevelopment for monitoring to characterize power quality foradvanced applications.

Analytical tools will also benefit from the increased level ofmonioring and characterization. Models should be improvedand the tools themselves should become easier to use. There isconsiderable opportunity to facilitate analysis of power qualityissues with standard models and modeling techniques.The overall focus needs to be on economics using a systems

approach. We need to develop tools that can help find theoptimum system design including power conditioning forsensitive equipment. The altematives should include improvedimmunity at the equipment level, power conditioning at theequipment level, power conditioning at more centralizedlocations within the end user system, and measures to improveperformance on the utility system.

Fig. 8 illustrates the overall flow of standards developmentactivities in the area of power quality and reliability.Understanding of system characteristics and end use equipmentcharacteristics leads to tools and methods to assessperformance and improve the overall performance in anoptimum manner.

SyF ler dep toactivitiesTPhere ormances n s Eqvlpment

Exrzectatnds n IEuntny

the syste and Asmentpr

Equipme P w forImanca eeoUmis f Impropte.

h smauthe thnsantiI I fid mplemontdoCUserrand

equlpmunt sutluonsto ltmnt impactanI

Fig. 8. Flow of power quality standards development acties.

VIl. CONCLUSIONS

There has been significant progress in the development ofpower quality standards. Recent efforts have been focused onharmonizing standards between IEEE and IEC and this is anongoing pro2ess. Continued effmrtsto understand system powerquality as a function of system characteristics and to coordinatethe system characteristics with the performance of end useequipment are under way. Both system perfommance and enduse equipment characteristics are being described pithmorestandardized methods. This infosmationesll lead to improvedeconomics of power quality management in the future.

Vlil. ACKNOWLEDGEMENTS

The authors acknowledge the contribution and participation ofthe members of the Power Quality Standards CoordinatingCommittee, SCC-22, in developing the information for this paper.

D(. REFERENCES

[1] IEC 61000 2-2, Ed 2, Electromagnetic Compatibility (EMC)- Part 2-2: Environment - Compatibility levels for lOUFfrequency conducted disturbances and signaling in publiclow-voftage power supply systems.

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[2] IEC 61000-2-4, Ed 2, Electromagnetic Compatibility (EMC)- Part 24: Environment - Compatibility levels in industrialplants for low-frequency conducted disturbances.

[3] IEC 61000-2-12, Ed 1, Electromagnetic Compatibility (EMC)- Part 2-12: Environment - Compatibility levels for low-frequency conducted disturbances and signaling in publicmedium-voltage power supply systems.

[4] EN 50160:11 1999, Voltage characteristics of electicitysupplied by public distribution systems.

[5] NRS 048-2:2002, Electicity Supply - Quality of Supply, Part2 - Voltage Characteristics, compatibility levels, limits andassessment methods, 2nd Edition, Draft 4, 05 November,2002.

[6] IEC 61000-3-6, Electromagnetic Compatibility (EMC) - Part3: Limits, Section 6: Assessment of emission limits forharmonics in MV and HV power systems.

[7] IEEE Standard 519-1992, IEEE Recommended Practicesand Requirements for Harmonic Control in Electrcal PowerSystems.

[8] G5/4 - Harmnonics[9] IEC 61000-3-7 - Electromagnetic Compatibility (EMC) -

Part 3: Limits, Section 7: Assessment of emission limits forfluctuating loads in MV and HV power systems.

[10] IEEE Standard P1453, IEEE Guide for Measurement andLimits of Voltage Flicker on AC Power Systems, Draft 4,July 2002.

[11] ANSI C84.1-1995 (R2001), Electrcal Power Systems andEquipment - Voltage Ratings (60 Hz).

[12] IEEE/ANSI C62.41-1991, IEEE Recommended Practicesfor Surge Voltages in Low-Voltage AC Power Circuits.

[13] IEC 61000-2-8 - Electromagnetic Compatibiity (EMC) -Part 2-8: Environment - Voltage dips and short interruptionson public electric power supply systems with statisticalmeasurement results.

[14] IEC 61000-4-11, Electromagnetic Compatibility (EMC) Part4: Testing and Measurng Techniques - Section 1 1: VoltageDips, Short Interruptions, and Voltage Varations ImmunityTests.

[15] SEMI F47-0200, Specification for SemiconductorProcessing Equipment Voltage Sag Immunity.

[16] IEEE Standard 1564 (Draft) - Voltage Sag Indices, Draft 4,2003.

[171 IEC 61000-4-30, Electromagnetic Compatibility (EMC) Part4-30: Testing and Measuring Techniques - Power qualitymeasurement methods, IEEE, January 1997.

[18] IEEE Standard 519.1 - Application Guide for ApplyingHarmonic Limits on Power Systems, (Final Draft forBalloting), 2004.

[19] IEC 61000-4-7, Electromagnetic Compatibility (EMC)Part 4-7. Testing and Measurement Techniques - Generalguide on harmonics and interharmonics measurements andinstrumentation, for power supply systems andequipment connected thereto.

[20] IEC 61000-4-15, Electromagnetic compatibility (EMC) - Part4: Testing and Measurement Techniques - Section 15:Flickermeter - Functional and design specifications

[21] IEEE Standard 1346, IEEE Recommended Practice forEvaluating Electric Power System Compatibility WithElectronic Process Equipment.

[22] EPRI RP3098-1 An Assessment of Distrbution PowerQuality.

David B. Vannoy received his BEE and MEE degrees form theUniversity of Delaware in 1966 and 1967 respectively. He is aregistered Professional Engineer in the State of Delaware. Heworked for over 31 years with Delmarva Power in theEngineering and Operating Departments. He is currently anindependent consultant. Previously, he managed DelmarvaPowees Power Quality Group, which he developed beginning in1987. He is Chairman of the IEEE Power Quality StandardsCoordinating Committee. He is active on numerous IEEE PowerQuality Standards committees and was the founding President ofthe Delaware Valley Power Quality Group, a non-profiteducational forum on power quality.

Mark F. McGranaghan is a Vice President with EPRI Solutions,Inc. in Knoxville, TN. He works with electric utilities worldwide inthe areas of reliability and power quality assessments, systemmonitorng, transient and harmonic studies, and economicevaluations. He is a co-author of the book Electnc PowerSystems Quality and has written numerous IEEE papers. BeforeEPRI Solutions, Mr. McGranaghan worked for ElectrotekConcepts and Cooper Power. He has BSEE and MSEEdegrees from the University of Toledo and MBA from Universityof Pittsburgh. He is active in many IEEE and IEC Standardsactivities.

S. Mark Halpin (M 93, SM 02, F 05) received his BEE, MS,and PhD degrees from Auburn University in 1988, 1989, and1993, respectively. He is currently a professor in theDepartment of Electrical and Computer Engineering atAubum University. His teaching interests include powersystems, control systems, and network analysis. Hisresearch interests are in the areas of modeling and simulationtechniques for large-scale power systems, power systemtransients, and computer algorithms. He is active in the IEEEPower Engineering Society, where he serves as Chair of theTask Force to revise IEEE 519-1992, and Industry ApplicationSociety, where he serves as Chairman of the IAS WorkingGroup on Harmonics.

W. A. Moncrief, PE performs short circuit studies and powerquality investigations at Hood-Patterson & Dewar in Decatur,Georgia. He is the secretary of IEEE Standards CoordinatingCommittee on Power Quality, Chairman of the IEEElPESHarmonics Group, and an editor for IEEE Transactions. He isactive on CIGRE and IEEE standards committees, includingIEEE Std 519 and IEEE Std 1159. He serves on the USNational Committee of ANSI.

D. Daniel Sabin (M 1992, SM 2001) is the Manager ofPQView and Enterprise Applications with Electrotek Conceptsin Beverly, Massachusetts, USA. Dan's primaryresponsibilities involve developing power quality databasesoftware, conducting power quality research, and providingconsultation to electric utilities on power quality monitoringefforts. He was the principal investigator for the EPRIDistribution System Power Quality Monitoring. He has aBSEE from Worcester Polytechnic Institute and a Master ofEngineering degree in Electric Power Engineering fromRensselaer Polytechnic Institute. Dan is a registeredProfessional Engineer in the State of Tennessee. He is thePES Vice Chair of the IEEE SCC22, the secretary of the IEEEPES Power Quality Subcommittee, and the chair of the IEEEP1564 Voltage Sag Indices Task Force.

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APPENDIX AIEEE Power Quality Standards

Organization Std. Title/ScopeANSI/EEE 141 Industrial Electric Power Systems

142 Industrial & Commercial Power System Grounding241 Commercial Electric Power Systems242 Industrial & Commercial Power System Protection399 Industrial & Commercial Power System Analysis446 Industrial & Commercial Power System Emergency Power493 Industrial & Commercial Power System Reliability518 Control of Noise in Electronic Controls519 Harmonics in Power Systems602 Indusrial & Commercial Power Systems in Health Faclities739 Energy Conservation in Industrial Power Systems929 Interconnection Practices for Photovoltaic Systems1001 Interfacing Dispersed Storage and Generation1035 Test Procedures for Interconnecfing Static Power Converters1050 Grounding of Power Stabon Instrumentaion & ControlC62 Guides & Standards bn Surge ProtectionC84.1 Voltage Ratngs for Power Systems & EquipmentC37 Guides and Standards for Relaying & Overcurrent ProtectionC57.1 10 Transformer Derating for Supplying Nonlinear Loads

IEEE P487 Wire Line Communication Protection in Power Stations1100 Powering and Grounding Sensitive Equipment1159 Monitoring and Definition of Electric Power Quality1250 Guide on Equipment Sensitive to Momentary Voltage DisturbancesP1346 Guide on Compatibility for ASDs and Process Controllers1366 System Design Working Group - Reliability IndicesP1409 Custom Power1453 Flicker Measurement and Application Guide1459 Power Definitions for Non-Sinusoidal and Unbalanced ConditionsP1531 Harmonic Filters1547 Distributed Generation Interconnection GuidelinesP1564 Voltage Sag Indices

NEMA PE 1 Uninterruptible Power Supply Specification and Performance VerificationMG 1 Motors and Generators

NFPA 70 National Electric Code75 Protection of Electronic Computer Data Processing Equipment78 Ughtning Protection Code for Buildings

NIST 94 Electric Power for ADP InstallationsSP678 Overview of Power Quality and Sensitive Electrical Equipment

UL 1449 Standards for Safety of Transient Voltage Surge Suppressors

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APPENDIX BCross Reference of Important IEC and IEEE Power Quality Standards

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Category IEEE Standards IEC StandardsGeneral PQ Concepts, Definitions,Indices IEEE 1159 IEC 61000-2-2

IEC 61000-4-30, 61000-4-7Power Quality Measurements j IEEE 1159.1, IEEE 1159.3 (Harmonics), 61000-4-15 (Flicker)

I System PQ Performance IEEE 1250 IEC 61000-2-8

Steady State Regulation, - _ _

Unbalance ANSI C84.1 - _ IEC 61000-2-2

IEC 61000-3-29 61000-3-12, 61000-Harmonics IEEE 519, IEEE 519.1 3-6, 61000-4-7 (Measurements)

IIEC 61000-3-3, 61000-3-7, 61000-4-Flicker IEEE 1453 15 (Measurements)Voltage SagslMomentaryInterruptions/Reliability IEEE 493, IEEE 1346, IEEE 1366 IEC 61000-2-8

N EIEC 61000-4-49 61000-4-5 (botht Transients ANSI/IEEE C62 Series testing standards)