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Harmonic Reduction Methods

Application Note

New InformationApril 2007

Overview

There are several basic methods for reducing harmonicvoltage and current distortion from non-linear distributionloads such as adjustable frequency drives (AFDs). Followingis a description of each method, along with each methodsadvantages and disadvantages.

Harmonic Reduction Methods

No Method UsedAssuming an AFD with a 6 diode input bridge, and no linereactor, DC choke, or filter applied, IEEE519 1992 guidelinesmay not be met even if the total connected AFD loads areless than 10%. Since many AFDs require a minimum 1 3%input impedance, the AFD requirements may not be metwithout a line reactor or additional impedance.

Note: The SVX9000 comes standard with a nominal 3% input imped-ance except for the Compact NEMA Type 1 design in the M3b enclo-sure.

Figure 1. No Method Used

Advantages

No added cost

Easy to package

Easy to sell

Easy to apply

Disadvantages

Potentially high levels of harmonic current and voltagedistortion

AFD is more susceptible to damage caused by linetransients

AFD impedance requirements may not be met

DC ChokeThis is simply a series inductance on the DC side of thesemiconductor bridge circuit on the front end of the AFD. Inmany ways, the DC choke is comparable to an equivalentAC-side line reactor, although the %Total HarmonicDistortion (THD) is somewhat less. The DC choke provides a

greater reduction primarily of the 5th and 7th harmonics. Onhigher order harmonics the line reactor is superior, so interms of meeting IEEE guidelines, the DC choke and linereactor are similar. If a DC choke (or line reactor) is appliedon all AFDs, it is possible to meet IEEE guidelines where upto 15% to 40% of system loads are AFDs, depending on thestiffness of the line, the amount of linear loads and the valueof choke inductance. A harmonic analysis is required toguarantee compliance with guidelines.

Figure 2. DC Choke

Advantages

Packaged integrally to the AFD

Can provide moderate reduction in voltage and currentharmonics

Less voltage drop than an equivalent line reactor

Disadvantages

Less protection than other methods for the AFD inputsemiconductors

May not reduce harmonic levels to below IEEE519 1992guidelines

Impedance is typically fixed by design (not selectable)

Not available as an option for most AFDs, including theSVX9000

AFD Motor

AFD

DC Reactor

Motor

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AP04014001E

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Effective: April 2007

Harmonic ReductionMethods

Line ReactorA line reactor is a three-phase series inductance on the lineside of an AFD. If a line reactor is applied on all AFDs, it ispossible to meet IEEE guidelines where up to 15% to 40% ofsystem loads are AFDs, depending on the stiffness of theline and the value of line reactance. Line reactors are

available in various values of percent impedance, mosttypically 1 1.5%, 3% and 5%.

Note: The SVX9000 comes standard with a nominal 3% input imped-ance except for the Compact NEMA Type 1 design in the M3b enclo-sure). A harmonic analysis is required to guarantee compliance withguidelines.

Figure 3. Line Reactor

Advantages

Low cost

Can provide moderate reduction in voltage and currentharmonics

Available in various values of percent impedance

Provides increased input protection for AFD and its semi-conductors from line transients

Disadvantages

May require separate mounting or larger AFD enclosure(for SVX9000 if more than 3% is required)


12-Pulse Converters

A 12-pulse converter incorporates two separate AFD inputsemiconductor bridges, which are fed from 30 phaseshifted power sources with identical impedance. Thesources may be two isolation transformers, where one is adelta/wye design (which provides the phase shift) and thesecond a delta/delta design (which does not phase shift). Aline reactor of equal impedance to the delta/wye transformermay also be used in lieu of the delta/delta transformer.

The 12-pulse arrangement allows the harmonics from thefirst converter to cancel the harmonics of the second. Up toapproximately 85% reduction of harmonic current andvoltage distortion may be achieved (over standard 6-pulseconverter). This permits a facility to use a larger percentageof AFD loads under IEEE519 1992 guidelines than allowableusing line reactors or DC chokes. A harmonic analysis isrequired to guarantee compliance with guidelines.

Figure 4. 12-Pulse Converter

Advantages

Reasonable cost, although significantly more than reac-tors or chokes

Substantial reduction (up to approx. 85%) in voltage andcurrent harmonics

Provides increased input protection for AFD and its semi-

conductors from line transientsDisadvantages

Impedance matching of phase-shifted sources is critical toperformance

Transformers often require separate mounting or largerAFD enclosures

May not reduce distribution harmonic levels to belowIEEE519 1992 guidelines

Cannot retrofit for most AFDs

12-Pulse DistributionThis is similar to a 12-pulse converter, on a macro scale. Iftwo AFDs of equal horsepower and load are phase shiftedby feeding one AFD from a delta/wye transformer, andfeeding the second through a delta/delta transformer or a

line reactor of equivalent impedance, performance similar to12-pulse may be achieved. The cancellation will degrade asthe loads vary from AFD to AFD, although as the load on asingle AFD decreases, the individual distortion contributionpercentage decreases, resulting in less of a need forcancellation.

It is possible for a facility with a large number of AFDs tofeed two halves of the distribution from phase-shiftedtransformers, yielding a large reduction in harmonic levelsfor minimal cost, and allowing a higher percentage of AFDloads under IEEE519 1992 guidelines. A harmonic analysis isrequired to guarantee compliance with guidelines.

Figure 5. 12-Pulse Distribution

Advantages

Cost may either be low or high depending onimplementation

Provides substantial reduction (50 80%) in voltage and

current harmonics


Disadvantages

Cost may be low or high depending on implementation

Impedance and load-matching of phase-shifted sources iscritical to performance

Transformers will require separate mounting


MotorAFD

AFD Motor

AFD

AFD

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Harmonic Trap FiltersHarmonic trap filters are usually used in conjunction with aline reactor, and are usually placed on individual AFD loads.They are usually an L-C filter installed in a shuntarrangement on the line side of the AFD, and are tunedsomewhat below the 5th harmonic, which is the largest

component of harmonic distortion. A significant amount of7th harmonic distortion will also be absorbed. Additionalfilters tuned to higher order harmonics may also be used.

More care is needed with the application of harmonic trapfilters than with other methods, since they will tend to try tofilter the entire distribution system of harmoniccomponents. If additional AFD or non-linear loads are addedwithout filtering, the previously installed filters may becomeoverloaded (they are generally fused for protection). The linereactor is used in conjunction with the filter to minimize thepossibility of this occurring and to enhance filterperformance. A harmonic analysis is required to guaranteecompliance with guidelines.

Figure 6. Harmonic Trap Filters

Advantages

Allow a higher percentage of AFD system loads than linereactors and chokes

Disadvantages

High cost

Separate mounting required


Care is needed in application to ensure that the filter willnot become overloaded

Distribution changes, such as adding AFDs, could lead tooverloading

Broadband FiltersThese filters are similar to trap filters but have some majordesign differences. As trap filters are connected in parallel tothe AFD, broadband filters are connected in series with theAFD and carry the full AFD current. This difference providesadded protection for the input power section of the AFD.

Broadband filters require no tuning, improve power factorfor the system and minimize all harmonic frequencies,including the 3rd harmonic. Additionally, they avoid systemresonance and importation of outside harmonics.

Figure 7. Broadband Filters

Advantages

Allows a higher percentage of AFD system loads than linereactors and chokes


Provides added protection for AFD input power section

Provides system power factor correction

Disadvantages

High cost

Separate mounting required


Could result in leading power factors at during lightlyloaded conditions

Requires modification to match with an AFD using inter-nal line reactors, such as the SVX9000

Clean Power (18-Pulse Converter)This method is similar to 12-pulse converters, althoughinstead of using two phase-shifted power sources andsemiconductor bridges, three are used. Eaton uses aspecially wound autotransformer (Differential Delta) and18-input semiconductors. When this arrangement is used,over 90% of harmonic currents are canceled (typical totalharmonic distortion of 2 3%).

Figure 8. Clean Power (18-Pulse Converter)

AFD Motor

AFD

DC Load Output

ACInput

DifferentialDelta

Transformer 12

1

23

3

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Eaton Corporation

Electrical Group1000 Cherrington Parkway

Moon Township, PA 15108

United States

877-ETN CARE (877-386-2273)

Eaton.com

2007 Eaton CorporationAll Rights ReservedPrinted in USAPublication No. AP04014001E/CPGApril 2007

Advantages

Virtually guarantees compliance with IEEE519 1992


Up to four times the harmonic reduction of 12-pulsemethods

Smaller transformer than isolation transformer used in12-pulse converter

Disadvantages

Can be more expensive than other methods (for up to fourtimes the harmonic reduction of 12-pulse methods)

Larger and heavier magnetics than some other methods

Active FiltersThis method uses sophisticated electronics and powersection IGBTs to inject equal and opposite harmonics ontothe power system to cancel those generated by otherequipment. These filters monitor the non-linear currentsdemanded from non-linear loads (such as AFDs) andelectronically generate currents that match and cancel thedestructive harmonic currents. Active filters are inherently

non-resonating and are easily connected in parallel withsystem loads.

Figure 9. Shunt Active Filter

Figure 10. Series Active Filter

Advantages

Guarantees compliance with IEEE519 1992 if sizedcorrectly

Harmonic cancellation from the 2nd to 51st harmonic

No series connection provides easy installation with nomajor system rework

Provides VAR currents, improving system power factor

Disadvantages

Can be more expensive than other methods due to thehigh performance control and power sections

The filters input semiconductors are exposed to linetransients

For assistance in the application of AFDs or harmonicreduction methods, or for a free harmonic analysis, contactyour local Cutler-Hammer distributor or representative.

AFD

ShuntActiveFilter

Motor

AFD

SeriesActiveFilter

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