A New Solution for Harmonics Generated by VSDs

1 A New Solution for HarmonicsGenerated by Variable Speed DrivesIn addition to its harmonic mit-igating capabilities, this widespectrum harmonic filter 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 rectifier 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 definedby the following formula:h= np 1 Where:h= the Harmonics Generated n= any integer (1, 2, 3, etc.)

2 P= the rectifier pulse numberA 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 5thand7th. The 11th, 13th, and other higherorders are also present but at lowerlevels. In the example shown inFigure 1, the 5thharmonic currentis about 75% of the fundamental(60Hz) current and the 7thnearly60%. This means that a rectifier ofthis configuration, which draws100A of 60Hz current, will alsodraw 75A of the 5thharmonic cur-rent and 60A of the distribution systems carry-ing a heavy nonlinear load compo-nent will often experience problemscaused by excessive harmonic cur-rents.

3 Problems that arise include: Power factor correction capaci-tor failures due to overloadingand/or system resonance he need for harmonic mitigating devices isgrowing because of the rapid increase in vari-able speed drive (VSD) usage in industrial andcommercial applications and the corresponding growthin harmonic-related problems. A new state-of-the-artpassive universal harmonic filter (UHF) is designed toenhance the conversion of ac power to dc power within aVSD or other equipment with a 3-phase, 6-pulse diodebridge rectifier front-end. Tony Hoevenaars, , MIRUS International Inc., Toronto, OntarioTEditor s Note:This article focuses on aspecific new product fromMIRUS International this article is manu-facturer-oriented, based uponthe information we observedwhen the product was dis-played at our recent PowerQuality 99conference, PQMagazine believes this prod-uct deserves to be presentedto our readers so that theymay investigate its claims ontheir own.

4 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 forVSDsThere 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 1for typical values).

5 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 rectifierinput. 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 specific har-monic frequency. Usually, 6-pulserectifier loads are tuned to themost predominant harmonic the5th.

6 Their effectiveness is limited,however, unless multiple tunedelements are incorporated toremove the 7thand 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 they address some of theissues associated with tuned fil-ters, they are not , 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 multipulsed systems, thedrive manufacturer will phase shiftbetween multiple front-end recti-fiers to cancel Harmonics .

7 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 ( , cancellation of 5thand 7thharmonics) can be createdby phase shifting one VSD againsta second similar VSD. The 18 and24-pulse schemes require threeand four VSDs, filters treat harmonicsby measuring the level of harmon-ic current present in the systemand injecting currents of oppositepolarity to cancel them performance can beachieved but reliability is some-times an issue and their high costhas limited their use.

8 Due to thedynamic characteristics associatedwith detection and treatment, fastchanging conditions may not beadequately addressed. The Universal HarmonicFilter vs. OtherConventional PassiveFiltersThe UHF is a purely passiveseries connected device, whichcan be installed at the input ofany 3-phase, 6-pulse diode bridgerectifier to dramatically reduce itsinput current Harmonics . Its revo-lutionary design achieves cancel-lation of all the major harmoniccurrents Generated by the rectifier,resulting in THID of <8% andoften as low as 5%. This meetsIEEE std 519 harmonic currentlimits for all but the weakest ofsupply sources.

9 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 sacrificing filterperformance or introducing unac-ceptable voltage drops. Capacitivereactive power is typically 3-4xlower than that of conventionalfilters. This is significant 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-passfilters present a capacitive reac-tance to the system, especiallyunder light loads.

10 This is a bene-ficial 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 for inductive loadsis usually not necessary and, infact, can cause problems, espe-cially when supplied by an emer-gency standby generator. Toaddress this issue, some filter manufacturers offermechanisms for switching out the capacitors underlight loads, which increases cost and under no load conditions, the capacitive reac-tance of the UHF is so low that switching out thecapacitors is concern with tuned filters is that unlessthey incorporate a detuning reactor in series with thesupply feeder, they can easily be overloaded byattracting Harmonics from upstream sources.