Why Worry About Harmonics

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October 2006

Why Worry About Harmonics?Because They May Be a Hidden Source of Inefficiency In Your Plant

By Louis Hapeshis, P.E.

Many modern-day water andwastewater facilities aretaking advantage of theadvances in power

electronics to improve theiroperations. Among thedevices plant operators are

employing to this end arevariable frequency drives,

ozone generators, and UVfiltration systems. These

devices offer benefits suchas improved energyefficiency, reduced

maintenance and operatingcosts, and increased reliability. However, they all share one common operating concern that couldcause many problems if not considered when designing or operating a water facility i.e., thesedevices are all nonlinear or harmonic producing loads on the power system.

Fluid handling within waterand wastewater facilities is

generally the largestconsumer of electricity. Mostfluid handling systems

include pumps that utilizevariable frequency drives tooptimize the pump speedwhile minimizing energy

consumption. Since variablefrequency drives produceharmonics on the power

system, it is important forusers to understand thenegative impact harmonics can have.

Harmonics

Figure 1. Determining characteristic harmonics

Figure 2.Harmonic components of a distorted waveform.


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Current drawn by nonlinearloads is periodic but notsinusoidal. Periodic waveforms are described mathematically as a series of sinusoidal waveformsthat are summed together. Sinusoidal components are integer multiples of the fundamentalfrequency (60 Hz in the United States). Harmonics are multiples of the fundamental frequency, asshown in Figure 1. Total harmonic distortion is the contribution of all the harmonic frequencycurrents to the fundamental. The third harmonic has a frequency of 180 Hz, or three times the

fundamental frequency of 60 Hz. An example of a nonsinusoidal waveform and its harmoniccomponents is shown inFigure 2.

A true-RMS meter is requiredto measure current orvoltage that containsharmonics. Significant errormay be present in readingstaken using a commonaveraging meter.

Linear & Nonlinear Loads

A linear load is one in whichthe current is proportional to

the voltage. In general, the current waveform wil l have the same shape as the voltage waveform.Some examples of linear loads are induction motors, resistance heaters, and incandescen lights.

The current waveform of anonlinear load is shapeddifferently than the voltagewaveform. Some examples of

nonlinear loads are gas-discharge lamps, switchingpower supplies,

uninterruptable powersupplies (UPSs), variable

frequency drives (VFDs), DCmotor drives, and arcingequipment. An example oflinear and nonlinear load current is shown in Figure 3.

Harmonic Current FlowThe current drawn bynonlinear loads passes

through all of the impedancebetween the system sourceand load. This currentproduces harmonic voltages

for each harmonic as it flows

through the systemimpedance. These harmonic

voltages sum and produce adistorted voltage whencombined with thefundamental. The voltagedistortion magnitude isdependent on the sourceimpedance and the harmonic voltages produced. Figure 4 illustrates how the distorted voltage is

created. As illustrated, nonlinear loads are typically modeled as a source of harmonic current. With

Figure 3. Linear & nonlinear load current.

Figure 4. Creation of distorted current.

Figure 5. Typical six-pulse ACdrive components

Figure 6. Nonlinear load and power-supply modeling


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low source impedance, the voltage distortion will be low for a given level of harmonic current. If theharmonic current increases and/or a resonant condition develops, the system impedance willincrease. This increased system impedance can increase voltage distortion significantly. Often, anincrease in system impedance is the result of power factor correction capacitors creating a parallelresonance at a particular harmonic frequency.

How Variable Frequency

Drives Cause HarmonicsAll variable frequency drivescause harmonics because of

the nature of the front-endrectifier design illustrated inFigure 5 for a typical six-pulse configuration. This isthe standard elementarypower circuit for most pulsewidth modulated variablefrequency drives sold today.

Some manufacturers offer

an alternative designincorporating a higher pulseorder front-end, particularlyin large horsepower

configurations. Mostcommon are 12 and 18-pulse designs, which extend the first characteristic harmonic to the 11thand 17th, respectively. The 12-pulse configuration is detailed further on page 9.

Current distortion occurswhen incoming AC voltage isrectified by the three-phase

full-wave diode bridge,which charges the capacitorin the DC link. As the motor

draws current from the DCbus through the inverter, thepotential on the capacitorsfalls below the incoming line

voltage. A forward-biasedpair of diodes beginsconducting and recharges

the DC bus capacitors.Conduction takes place foreach positive and negativehalf cycle of each phase

thereby creating the familiardouble-pulse current shown

previously in Figure 3. If thesource impedance is low, thecapacitor recharges veryquickly resulting in a high

current peak and highcurrent distortion. Withmoderate AC sourceimpedance, the capacitor

recharges over a longerperiod resulting in a more sinusoidal current.

Figure 7. Typical 12-pulse ACdrive components

Figure 8. Typical harmonic trap filter configuration


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The degree and magnitude of the harmonics created by the variable frequency drive is a function ofthe drive design and the interaction of the nonlinear load with the distribution system impedance.The power source line impedance ahead of the controller will determine the magnitude of harmoniccurrents and voltages reflected back into the distribution system. Figure 6 illustrates thisrelationship.

The distorted current reflectedthrough the distribution impedancecauses a voltage drop or harmonic

voltage distortion. Assuming thesource is inductive, this relationshipis proportional to the distributionsystem available fault current.

High fault current (stiffsystem) Distribution system impedanceand distortion is low. System can adsorb high levels of harmonic current.

Low fault current (soft system) Distribution system impedance and distortion is high. System can adsorb only low levels of harmonic current.

StandardsIEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electrical PowerSystems, limits harmonic distortion at the Point ofCommon Coupling (PCC). The PCC is generally

accepted as the point where an individual utility customer shares a connection with othercustomers. Recognizing that voltage distortion results from harmonic currents, the standard alsoplaces limits on harmonic current injection by individual customers. Current limits vary from 5

percent to 20 percent total demand distortion (TDD), depending on the size of the customer relativeto the stiffness of the utility circuit.

Although IEEE 519 discusses harmonic issues that occur inside a facility, the limits only apply at theutility/customer interface. At individual equipment, general guidelines are applied.

Most systems can tolerate moderate amounts of current distortion. However, continuous voltage

distortion greater than 8 percent to 10 percent THD can cause damage (chronic exposure) ordisruption (acute) to equipment.

Effects & Negative ConsequencesThe effects of harmonics on circuits are similar to the effects of stress and high blood pressure onthe human body. High levels of stress or harmonic distortion can lead to problems for the utilitydistribution system, plant distribution system, and any other exposed equipment. Effects can range

from spurious operation of equipment to a failure of important plant equipment, such as machinesor transformers.

For water facilities specifically, harmonics can cause conductor overheating, nuisance tripping ofthermal devices, data faults in sensitive electronic equipment, generator faults, and reduced motorand transformer life. These issues can become expensive for facilities due to increased

maintenance and operating costs, not to mention the possibility of sewage spills resulting in finesand the negative impact on the environment and community.

Some of the negative ways that harmonics may affect plant equipment are listed below:

Power system resonance: A power distribution system has a natural resonant frequency

Figure 9.Typical active filter application


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determined by the inductances and capacitances in the system. Adding power factor correctioncapacitors in the system (or nearby on the utility) can lower the resonant frequency to the range ofpredominant harmonics produced by nonlinear loads. Parallel resonance results in high harmoniccurrents and voltages. Nuisance capacitor fuse operations and eventual failure commonly ensue.

Fuses and circuit breakers: Due to the increase in the RMS value of current and higherfrequency content, harmonics may cause false or spurious operations.

Transformers: Winding loses are increased due to the higher frequency current components. Thiscauses additional heating in the transformer windings. Fully loaded, a standard transformer is only

rated to carry 5 percent harmonic current distortion. K factor-rated units are recommended forhigher levels of harmonics.

Generators: Regulators, excitation systems, and even governors can be affected by nonlinearloads. Generators typically have much higher source impedance than the utility, which limit theamount of harmonic producing load they can serve. Harmonic loads also increase winding and rotortemperatures. The alternator size may need to be increased when serving loads with high harmonic

content.

Motors: Harmonic voltage distortion has a similar impact on motors as voltage phase imbalance.

The higher frequency components cause additional rotor heating and will shorten motor life.

Drives/power supplies: Severe harmonic voltage distortion, particularly that caused by notching,can disrupt voltage sensing circuits such as those for timing, peak measurement, or synchronization.

Telephones: Older hard-wired telephone networks may experience interference in the presence ofpower system harmonics.

Evaluating System HarmonicsIn order to prevent or correct harmonic problems that could occur within an industrial facility, anevaluation of system harmonics should be performed if the facility conditions meet one or more of

the criterion outlined below. The application of capacitor banks in systems where 20 percent or more of the load includes otherharmonic generating equipment or where background distortion exceeds 2 percent.

The facility has a history of harmonic-related problems, including excessive capacitor fuseoperation. Large single nonlinear loads are being added greater than about 10 percent of the transformerrating.

Many small identical nonlinear loads are being added that operate together. In facilities where restrictive power company requirements limit the harmonic injection back intotheir system to very small magnitudes.

When coordinating and planning to add an emergency standby generator as an alternate powersource for nonlinear load.

Often, the vendor or supplier of nonlinear load equipment, such as variable frequency drives, can

evaluate the effects that the equipment may have on the distribution system. This usually involvesdetails related to the design of the distribution system similar to the data required when performing

a short-circuit study. Such a study should consider all harmonic sources, including the utilitybackground distortion.

Reducing Harmonics

There are many ways to reduce harmonics, ranging from variable frequency drive designs to theaddition of auxiliary equipment. Following are some of the more common methods used today forcontrolling power system harmonics.

Power system design: Harmonic problems can be kept in check by limiting the nonlinear load to30 percent of the supply transformer rating. However, with power factor correction capacitors


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installed, resonant conditions could potentially limit the percentage of nonlinear loads. Use thefollowing equation to determine if a resonant condition is likely to occur at an undesirablefrequency:

If hr is close to a characteristic harmonic, such as the fifth or seventh, there is a possibility that aharmful resonant condition could occur.

Multi-pulse converter design: In a 12-pulse configuration, the front-end rectifier circuit uses 12diodes instead of six. When properly designed, this configuration practically eliminates the fifth and

seventh harmonics. The disadvantages are cost and construction due to the requirement for a delta-delta /delta-wye transformer pair or three-winding transformer to accomplish the 30-degree phaseshifting necessary for proper operation. This configuration also affects the overall drive system

efficiency rating because of the voltage drop associated with the transformer requirement. Figure 7illustrates the typical elementary diagram for a 12-pulse converter front end. Higher pulse orders

are also possible, thereby reducing more of the lower harmonics.

Pseudo 12-pulse systems: This configuration uses phase-shifting transformers to cancel fifthand seventh harmonics at the common bus. For cancellation to occur, the nonlinear loads must beoperated simultaneously and have similar characteristics. One cost-effective implementation of this

concept uses delta-wye isolation transformers on a few large variable frequency drives, while usingsmaller and less costly line reactors on smaller VFDs.

Isolation transformers: An isolation transformer provides several advantages. First andforemost, it provides impedance to the drive, which reduces current distortion. It obviously resolvesvoltage mismatch between the supply and the load. If the secondary is grounded, it isolates groundfaults and reduces common mode noise.

Line reactors: A line reactor provides the impedance to reduce harmonic current, similar to anisolation transformer, but with a smaller size and cost. Line reactors (also referred to as inductors)are available in standard impedance ranges from 1.5 percent, 3 percent, 5 percent and 7.5 percent.

Where system voltage is on the lower end of nominal, the greater impedance values should beavoided.

Passive filters: Passive or trap filters employ passive elements (capacitors and inductors) totrap or absorb harmonics. An inherent benefit of all passive filters is power factor correction. Passivefilters are generally configured to remove only one or two specific harmonics. Passive filters are

generally regarded as unsuitable for filtering third harmonics. For this reason, they are best suitedfor applications in which third harmonics are not an issue, power factor correction is required, andspecific harmonics such as fifth or seventh are creating the problem. Passive filters are ideal forsystems that have a high percentage of six-pulse drives and other linear loads. However, the filters

may need to be returned for changes in the power system. Filters can be designed for several

nonlinear loads or for a single load (Figure 8).

Broadband filters: By treating a wider spectrum of harmonics, broadband or high-pass filters canbe more effective than tuned filters, but they can also be more expensive. Shunt-connected high-pass filters are frequently tuned above the seventh harmonic and used in conjunction with single-tuned filters to target the lower frequencies. They usually include resistors, which can be an

operating problem.

A series broadband filter improves upon former broadband harmonic filtering techniques and

broadens the range of suitable applications. This is a conventional low pass filter created with an


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inductor and capacitor. Series broadband filters are applied to individual loads or groups of loads ina system. They can be applied on SCR rectifiers, including phase control and pre-charge front ends,as well as six-pulse rectifiers using AC line reactors or DC chokes. The latest design accomplishesbroadband harmonic filtering with about half the capacitance of previous designs. This minimizesleading current under light load conditions and improves compatibility with standby powergenerators.

Active filters: In contrast to passive filters, active filters monitor the load current and inject aharmonic current of equal magnitude but opposite polarity to dynamically cancel harmonic loadcurrents. The active harmonic filter can be an economical solution for applications where the

harmonic load is either 30 percent of the total transformer capacity or several hundred kVA. Theyprovide a cost-effective alternative to 18-pulse technology when several drives are installed in onelocation. Unlike passive filters, active filters cannot be overloaded if the level of harmonicsincreases. Active filter units can also be paralleled to accommodate increases in nonlinear load. Anillustration of how an active filter is applied is shown in Figure 9.

Avoiding Harmonic Issues in Water FacilitiesThere are many solutions available to eliminate harmonic issues, including line reactors, active andpassive filters, isolation transformers, and multi-pulse drives. Fortunately, there are manycompanies experienced in identifying and solving harmonic issues in existing facilit ies.

The first step to take, if you suspect your facility has harmonic issues, is to have a power systemstudy performed. This study should tell you how your facility compares to the IEEE519 Standard.The next step would be to evaluate the causes of your harmonic issues, and determine the most

cost effective way to eliminate them. Following the installation of the chosen harmonic mitigationequipment, a final study should be performed to ensure compliance with IEEE519.

New facilities should be designed with the IEEE519 Standard in mind. Designing a facility that

meets IEEE519 will ensure that harmonic issues are avoided. This usually involves writingequipment specifications that reference the IEEE519 Standard. There should be a preliminary powersystem study performed using a software package that considers all of the proposed equipment for

the facility. These specifications and preliminary studies should be followed up with an actual powersystem study at the new facility to ensure that it meets IEEE519.

The bottom line is that it is possible to take advantage of the advances in power electronics thatallow water facilities to operate reliably while reducing overall operating costs. The variablefrequency drives, ozone generators, and UV filtration systems that util ize power electronics are vitalcomponents in todays water facilities. When consideration is given to harmonic issues, these

facilities can benefit from the reduced operating costs and increased reliability offered by powerelectronic devices.

Louis Hapeshis. P.E., is a senior engineer for the Square D Power Systems Engineering Groupwithin Schneider Electrics North American Operating Division. He is responsible for power systemstudies, design, and monitoring services for Schneider Electric customers, as well as engineeringsupport for the Schneider Electric Water & Wastewater Competency Center and Square D field

service operations. He is knowledgeable in power distribution, control systems, networkcommunications, automation control products, variable frequency drives, and power quality. Mr.

Hapeshis earned his bachelors degree in electrical engineering with an emphasis on power systemsfrom Clemson University in 1995. He is a member of the Institute of Electrical and ElectronicsEngineers. He is a licensed professional engineer in South Carolina and Georgia. Mr. Hapeshis canbe reached [email protected] or 864 886-1383.

www.squared.com

References

1. Murphy and F.G. Turnbull, Power Electronic Control of AC Motors, Pergamon Press, Elmsford,New York, 1988.


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2. Industrial and Commercial Power Systems Analysis, ANSI/IEEE Std. 399-1990, Chapter 10.3. Electric Power Distribution for Industrial Plants, ANSI/IEEE Std. 141-1986,Chapter 8.4. IEEE Recommended Practices and Requirements for Harmonic Control inElectrical Power Systems, ANSI/IEEE Std. 519-1992.5. John F. Hibbard and Michael Z. Lowenstein, Meeting IEEE 519-1992

Harmonic Limits, TCI (Trans Coil, Inc.), 1993.6. In Tune with Power Harmonics, John Fluke Mfg. Co., Inc., 1991.7. Ed Palko, Living with Power System Harmonics, Plant Engineering, June

18, 1992, pages 48-53.8. Harmonic Filtering - A Guide for the Plant Engineer, CommonwealthSprague Capacitor, Inc., 1991.9. An Overview of Power System Harmonics, W. Mack Grady, Professor, Dept. of Electrical &Computer Engineering, University of Texas at Austin, Austin, Texas.10. Power System Harmonics, Square D Product Data Bulletin No. 8803PD9402, August, 1994.11. Effects of Harmonics On Equipment, IEEE Transactions on Power Delivery, Vol. 8, No. 2, April1993, V.E. Wagner, Chairman.12. Electrical Power System Harmonics Design Guide, R.C. Dugan, M.F. McGranaghan, McGraw-Edison Power Systems, P.O. box 440 Canonsburg, Pennsylvania, Second Edition, Sept., 1988.

13. Harmonic Distortion Accelerates Fuse Aging Failures, Maintenance Tech. Online.

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Why Worry About Harmonics