A New Solution for HarmonicsGenerated by Variable Speed Drives

In addition to its harmonic mit-igating capabilities, this widespectrum harmonic f ilter helpsprotect a VSD from transientovervoltages caused by capacitorswitching and other fast changingloads. It is suitable for virtuallyany application involving a VSDor similar 3-phase, 6-pulse diodebridge rectif ier load.

Harmonic ProblemsThe front-end diode bridge recti-

fiers of 3-phase, 6-pulse static powerconvertors (ac-dc), such as those foundin variable speed drives, are considerednonlinear because they draw current ina non-sinusoidal manner. The currentharmonics they generate are defined

by the following formula:

h = np ±1

Where:

h = the harmonics generated

n = any integer (1, 2, 3, etc.)

p = the rectifier pulse number

A simple 6-pulse rectifier (p =6) is shown in Figure 1. Withoutany harmonic treatment, the totalharmonic current distortion (THID)of this rectifier would be in the100-140% range with the predomi-nant harmonics being the 5th and7th. The 11th, 13th, and other higherorders are also present but at lowerlevels. In the example shown in

Figure 1, the 5th harmonic currentis about 75% of the fundamental(60Hz) current and the 7th nearly60%. This means that a rectifier ofthis configuration, which draws100A of 60Hz current, will alsodraw 75A of the 5th harmonic cur-rent and 60A of the 7th harmonic.

Power distribution systems carry-ing a heavy nonlinear load compo-nent will often experience problemscaused by excessive harmonic cur-rents. Problems that arise include:

• Power factor correction capaci-tor failures due to overloadingand/or system resonance

he need for harmonic mitigating devices is

growing because of the rapid increase in vari-

able speed drive (VSD) usage in industrial and

commercial applications and the corresponding growth

in harmonic-related problems. A new state-of-the-art

passive universal harmonic filter (UHF) is designed to

enhance the conversion of ac power to dc power within a

VSD or other equipment with a 3-phase, 6-pulse diode

bridge rectifier front-end.

Tony Hoevenaars, P.E., MIRUS International Inc., Toronto, Ontario

T

Editor’s Note:This article focuses on a

specif ic new product fromMIRUS International Inc.Although this article is manu-facturer-oriented, based uponthe information we observedwhen the product was dis-played at our recent PowerQuality ’99 conference, PQMagazine believes this prod-uct deserves to be presentedto our readers so that theymay investigate its claims ontheir own.

DECEMBER 1999

• Overheating cables, transformersand other distribution equipmentreducing their life span

• High voltage distortion (typical-ly in the form of flat-topping),especially when operating onweak sources such as emergencygenerators

• False tripping of circuit breakers

• Premature failure of rotatingequipment (motors, generators,etc.)

• Misoperation or component fail-ure in PLCs, computers or othersensitive loads

Existing Methods ofHarmonic Treatment forVSDs

There are several methodsavailable for treatment of VSDharmonics. Ac input reactors(either 3 or 5% impedance) arethe most commonly used treat-ment. They have a relatively lowcost but are only moderatelyeffective in reducing harmoniccurrent distortion (see Figure 2and Table 1 for typical values).The high impedance of ac inputreactors helps protect the drivefrom transient overvoltagescaused by capacitor switchingand/or fast changing loads butthey can often introduce trouble-some voltage drops at the rectif ierinput. Some VSDs are equippedwith a dc link reactor that isslightly more effective at reducingharmonic currents than the acreactor, and it does not cause anac voltage drop. The dc link reac-tor, however, is somewhat lesseffective than the ac reactor inovervoltage protection.

Conventional tuned LC or trapfilters, as their name implies,require tuning to a specif ic har-monic frequency. Usually, 6-pulserectif ier loads are tuned to themost predominant harmonic – the5th. Their effectiveness is limited,

however, unless multiple tunedelements are incorporated toremove the 7th and other higherorder harmonics. They are proneto problems such as importationof harmonics from upstream non-linear loads and the introductionof a leading power factor.

By treating a wider spectrum ofharmonics, low-pass filters aremore effective than tuned filters,but are also more expensive.Although they address some of theissues associated with tuned fil-ters, they are not problem-free.Specifically, their large seriesinductor necessitates the use of alarge capacitor bank to compen-sate for voltage drops. Thesecapacitors create a leading powerfactor that may cause excitationcontrol and voltage fluctuationproblems with generators.

In multipulsed systems, thedrive manufacturer will phase shiftbetween multiple front-end recti-fiers to cancel harmonics. Some18 and 24-pulsed systems canachieve THID levels of < 8%, butthey require a large footprint andare quite expensive. The applica-tion of phase shifting transformerscan be a very cost-effectivemethod of harmonic treatmentwhere multiple 6-pulse VSDs arein operation. A quasi 12-pulsescheme (i.e., cancellation of 5th

and 7th harmonics) can be createdby phase shifting one VSD againsta second similar VSD. The 18 and24-pulse schemes require threeand four VSDs, respectively.

Active f ilters treat harmonicsby measuring the level of harmon-ic current present in the systemand injecting currents of oppositepolarity to cancel them out.Excellent performance can beachieved but reliability is some-times an issue and their high costhas limited their use. Due to thedynamic characteristics associatedwith detection and treatment, fastchanging conditions may not beadequately addressed.

The Universal HarmonicFilter vs. OtherConventional PassiveFilters

The UHF is a purely passiveseries connected device, whichcan be installed at the input ofany 3-phase, 6-pulse diode bridgerectif ier to dramatically reduce itsinput current harmonics. Its revo-lutionary design achieves cancel-lation of all the major harmoniccurrents generated by the rectif ier,resulting in THID of <8% andoften as low as 5%. This meetsIEEE std 519 harmonic currentlimits for all but the weakest ofsupply sources. The unique fea-ture of the UHF is its 3-phasereactor design consisting of multi-ple windings on a common mag-netic core. This reactor allows forthe use of a much smaller capaci-tor bank without sacrif icing f ilterperformance or introducing unac-ceptable voltage drops. Capacitivereactive power is typically 3-4xlower than that of conventionalf ilters. This is signif icant inreducing cost and space require-ments. Moreover, it preventspower system interaction prob-lems that often result from a lead-ing power factor.

The large capacitor banksfound in both tuned and low-passf ilters present a capacitive reac-tance to the system, especiallyunder light loads. This is a bene-f icial feature where inductiveloads require a compensatingreactance to improve a low dis-placement power factor. But inmost VSD applications, displace-ment power factor is close tounity even though overall powerfactor may be low due to the har-monic content in the current.Compensation for inductive loadsis usually not necessary and, infact, can cause problems, espe-cially when supplied by an emer-gency standby generator. To

address this issue, some f ilter manufacturers offermechanisms for switching out the capacitors underlight loads, which increases cost and complexity.Even under no load conditions, the capacitive reac-tance of the UHF is so low that switching out thecapacitors is unnecessary.

Another concern with tuned f ilters is that unlessthey incorporate a detuning reactor in series with thesupply feeder, they can easily be overloaded byattracting harmonics from upstream sources. Thedetuning reactor will introduce a voltage drop at thedc bus as load is applied to the VSD. The multiplewinding configuration of the UHF, on the otherhand, prevents the attraction of harmonics fromupstream sources without introducing an excessivevoltage drop as VSD load increases.

The f iltering effectiveness of a tuned f ilter isdependent upon the amount of harmonics present atuntuned frequencies as well as the residual at thetuned frequency. To obtain performance better than15% THID, multiple tuned branches are oftenrequired. Low-pass f ilters achieve <12% THID butrequire relatively large capacitor banks. Even largercapacitors are required if further reduction in THIDis desired. The UHF reduces current distortion to<8% over the entire operating range and typicallyachieves near 5% THID at normal operating levels.

Simulation of Harmonic FilterPerformance

As previously mentioned, many of today’s VSDsare equipped with either ac line reactors or dc linkchokes to reduce harmonic current distortion.Computer simulation allows us to compare thesesolutions with the UHF. The circuit diagram shownin Figure 1 was used to simulate a 5kW, 380V 6-pulse diode bridge rectif ier. Five different harmonictreatment schemes were analyzed as follows:Scheme 1 – No harmonic treatment

Scheme 2 – With 3% ac line reactor

Scheme 3 – With 8 mH dc link choke

Scheme 4 – With both ac line reactor and dc link choke

Scheme 5 – With universal harmonic filter

The chart in Figure 2 plots the THID for eachscheme over the full operating range of the rectif ier.With no harmonic treatment, THID ranged fromnearly 180% at extremely light loading to 140% atfull load. The reactors implemented in Schemes 2, 3and 4 reduced the THID by approximately the sameamount. THID ranged from 100% near no load to 30-

Figure 1. Simple 3-phase, 6-pulse diode bridge rectifier withno harmonic treatment.

Figure 2. Total Harmonic Current Distortion through computersimulation of a 5kW, 6-pulse rectifier with various forms of pas-sive harmonic treatment.

Figure 3. dc bus voltage through computer simulation of a 5kW,380V 6-pulse rectifier with various forms of passive harmonictreatment.

35% at full load. The UHF was byfar the most effective in reducingTHID. At full load the THID wasreduced to about 5% and itincreased to 8% under more light-ly loaded conditions.

The computer simulations areconfirmed by f ield tests on a

60HP, 480V PWM variable speeddrive (see Table 1). While thereactors reduced current distor-tion by about 50%, the UHFachieved more than a 10 timesreduction. The current waveformwas nearly sinusoidal with itsspectrum containing only small

traces of harmonic content. Inaddition, removal of the harmoniccurrent improved power factor tovirtual unity.

The computer simulationshowed the variation in dc busvoltage at the output of the recti-f ier with the various forms ofharmonic treatment. With no har-monic treatment, the dc bus volt-age was fairly stable over theentire operating range (between545V to 520V). Additionalimpedance introduced by thereactors had a fairly signif icantimpact on the dc voltage level. Asexpected, the worst case was withScheme 4 when both the ac linereactor and the dc link chokewere in the circuit. At the fulloperating mode, the dc bus volt-age was nearly 10% lower thanfor the rectif ier with no treat-ment. In contrast, when equippedwith the harmonic f ilter, the dcbus voltage remained very stable,always at or above the valueswithout treatment.

Table 1. Performance comparison of various passive harmonic treatments on a 3-phase, 6-pulse, 60 HP Variable Speed Drive.

HARMONIC MITIGATING PRODUCTS

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Essential For:• Banks and Financial

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Providers• Broadcasting Studios• Telecommunications

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- new and old• School Computer Labs• Variable Speed

Drive UsersMirus International Inc.

6805 Invader Cres., Unit #12, Mississauga, ON, Canada L5T 2K6www.mirusinternational.com • mirus@mirusinternational.com

1-888-TO MIRUS or 905-565-6900 • Fax: 905-565-6911

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Reprinted with permission from the December 1999 issue of Power Quality Assurance.®

Copyright 1999, PRIMEDIA Business Magazines & Media Inc. All rights reserved.