Impact of Variable Speed Drives on Electrical Systems

Transcription

1 Impac of Variable Speed Drives on Elecrical Sysems Marin Bürker, Developmen Manager Elecronics a ebm-paps Mulfingen GmbH & Co. KG, Germany Armin Hauer, Advanced Technology Manager a ebm-paps Inc. Farmingon, CT, SA INTRODCTION One undispued requiremen for a high performance sysem is he abiliy o closely mach is oupu o he acual demand. Comfor, healh, process variance, sound emissions, and energy performance benefi from enabling moors for variable speed operaion. Wha are he poenial side effecs of his prescripion? Power line harmonics, addiional capaciance, or neural conducor curren? DEFINITIONS AND STANDARDS Draf sandard IEC x (1s ediion) inroduced he erm power drive sysem PDS. I encompasses an elecric moor wih an associaed elecronic variable speed drive wihou resricions o specific echnologies. This sandard will address he energy efficiency of hese producs. AHRI Performance Raing of Variable Frequency Drives is limied o sandard inducion moors in combinaion wih jus VFDs, as is ile implies precisely. One raing requiremen of he laer sandard aside from efficiency is Power Line Harmonic Currens. I sandardizes he es mehod for harmonics caused by VFDs. Limis for equipmen emissions are specified in EN and in EN , grouped by applicaion and by nominal curren consumpion. IEEE sandard 519 Recommended Pracices and Requiremens for Harmonic Conrol in Elecrical Power Sysems is concerned abou he same phenomena bu from he perspecive of an overall plan, an enire faciliy, or he elecric uiliy supplier. I does include recommended limis. PDS such as VFDs and elecronically commuaed (EC) moors require unique power elecronic unis, eiher physically separae from he moor or wihin he moor casing. Such uni convers AC from he power line source o DC power ha is emporarily sored in he so-called DC link circui. The DC link volage is hen invered o a frequency- and ampliude-modulaed 3-phase AC oupu volage ha hen finally excies he moor windings. The inverer oupu frequency and volage ampliude deermine he resuling moor shaf speed. Figure 1 illusraes he flow of power in a PDS moor elecronics uni. POWER LINE CRRENT HARMONICS The converer wih a passive recifier bridge and is highly capaciive DC link circui draws non-sinusoidal, non-linear power line inpu curren. The values are summarized in able 1. Descripion Quaniy ni No miigaion AC line filer choke Inegraed DC link filer choke AC line inpu passive harmonics filer Acive converer Real Power P kw Effecive Curren I eff A rms Apparen Power S kva Power Facor λ [ ] Toal Harmonic Curren Disorion THD i % <2 Table 1: Three-phase PDS a nominal load Three quaniies describe power line inerference commonly:! 1

2 Displacemen power facor cos φ quanifies he phase shif beween volage and curren in an AC circui wih purely sinusoidal waves. Oherwise i is no useful. I is a non-dimensional quaniy. A sinusoidal volage applied o a pure resisor causes a sinusoidal curren wihou phase displacemen and wihou drawing reacive power (Sidebar formulae). The cos φ in his case is 1.0. If a capacior or an inducor were added o his circui hen a reacive power componen would occur and cos φ would be <1.0. True power facor λ is he quoien of effecive power in was over apparen power in vol-amps. Indirecly, i quanifies he reacive power of a load caused by boh he phase displacemen and by disorion from he sine wave of he fundamenal frequency. Only if he curren waves and he volage waves are sinusoidal hen he values of λ and cos φ are idenical. Toal harmonic curren disorion THDi is he raio of he geomeric sum of all curren harmonics up o he forieh order over he fundamenal curren expressed in percen. Non-linear loads quie commonly produce THDi > 100%. Then he harmonics conen of his curren is greaer han is fundamenal wave. Displaced sinusoidal curren waves and disored currens are generally unfavorable for power supply sysems. The associaed reacive power adds o he load of generaors and of uninerrupible power supplies (PS). Harmonics cause addiional iron losses in power disribuion ransformers. Correspondingly, reacive power couns owards he sizing crieria of power sources including cables, swiches, fuses and breakers. Elecric uiliies and suppliers of sandalone power disribuion sysems herefore benefi from a power facor close o uniy and from low oal harmonic curren disorion. Above menioned sandards help o assess hese quaniies and o esablish various limis. Line reacors and DC link filers provide mehods for PDS o comply wih hese sandards. These filers emporarily sore some of he absorbed elecrical energy and miigae disored curren a he inpu of PDS. LINE REACTORS A line reacor raing of 2% means ha he volage drop across is filer choke a nominal curren load amouns o 2%. Oher common line reacors are 3% and 4%. However, heir volage drop on occasion reduces he maximum speed of a moor and he maximum possible oupu in cases of oher load ypes. DC LINK FILTER CHOKE The filer is inegraed direcly in he elecronic power uni which avoids addiional wiring in he field and minimizes he insallaion space requiremens. The PDS layou accouns already for he filer volage drop and he manufacurer guaranees he suiabiliy of his filer based on he overall specificaion of he PDS. Inegraed DC link filer chokes or line reacors usually yield sufficien miigaion for PDS o mee he requiremens of IEC However, sensiive applicaions such as off-grid power sysems may demand more severe power line curren disorion limis. Wha ofen sems from aemps o save cos on generaors or PS is increased power source impedance which is hen manifesed in low relaive shor-circui power Rsce. If his siuaion leads o very low THDi specificaions for elecrical loads hen comprehensive measures mus be aken o miigae harmonic curren emissions. AC LINK INPT PASSIVE HARMONICS FILTER These filers are voluminous and expensive bu hey miigae harmonic curren emissions well. ACTIVE CONVERTER Acive converers have become available recenly. As inegral pars of he elecronic power unis hese can saisfy he mos sringen specificaions, reaching THDi of 2% or less.! 2

3 PART LOAD OPERATION Design-phase discussions of harmonics and of power facor mosly cener on full load operaion. Once a echnician runs an acual bench es o evaluae equipmen, he power harmonics analyzer may display a λ and a THDi ha appears worse han adverised. When ha occurs, he PDS likely operaes a par load. I does consume reduced power as prediced and he curren draw of he fundamenal frequency does drop accordingly. The reducions of he higher order harmonic currens are less pronounced. Remember ha boh λ and he THDi are raios. The absolue values of boh he numeraors and he denominaors grow smaller disproporionally. Therefore, λ and THDi values rarely maer for par load operaion. STANDBY OPERATION A so-called DC link capacior miigaes DC volage ripple a he converer oupu. If he moor draws no curren from he DC link, he capacior will remain charged. The DC link capacior for his operaing sae canno displace he curren wave, does no consume idle power, and generaes no power line harmonics. Simply, no charging curren flows in a charged capacior. Sandards for elecromagneic compaibiliy EMC limis demand ha a PDS be equipped wih an EMC filer. sually, filer capaciors are conneced beween he power source phases or beween a phase and he neural conducor. The necessary capaciance depends on he emissions specrum and he level of miigaion. Typical overall values vary from 100 nf o 10 µf. This resuls in capaciive idle currens. Power rickles no jus during sandby bu for as long as he PDS remains conneced o he AC power line. A filer sized for a 5 A nominal phase curren draws effecive power of jus abou 4 W while he reacive power consumpion is approximaely 100 VAr. This capaciive amoun provides a sligh compensaion when a PDS operaes in parallel o inducive loads such as sandard moors. The DC link capacior indeed buffers his power. Bu compared o he idle curren of he EMC is is very low charge curren of no consequence. Harmonics miigaion does no provide a reason for separaing a PDS from line power when he moor speed is se o zero. Addiional conrol and swich gear would be necessary. Frequen power on and power off cycling reduces he life expecancy of PDS so ha his mehod is accepable rarely. SINGLE-PHASE POWERED PDS Triplen harmonic currens ypically are negligibly low when a converer is powered from a hree-phase (3ph) supply. However no such naural, inernal cancelaion exiss wih single-phase (1ph) supplied PDS. Here he hird harmonic if lef unmiigaed easily approaches he ampliude of he fundamen curren. is maybe 0.5 which hen indicaes ha his uni draws reacive power 3 imes he value of is effecive power. Happily, a 1ph powered PDS is generally jus a small fracion of he load for a branch circui, much less for an overall power disribuion sysem. All repuable manufacurers have me applicable produc sandards for sandalone PDS. Surprises only loom when an insallaion has many small PDS operaing in parallel. Serious examples are packaged erminal air condiioners in hoels, hydronic circulaion pumps, furnaces and air condiioners in ownhouses, evaporaor fans in supermarke display cases, and fan powered filer unis in indusrial cleanroom ceilings o name a few. A 3ph disribuion ransformer can supply an array of single-phase loads. The ransformer is uilized bes if he overall load is similar for all hree legs: A 3ph ransformer wih wye conneced secondary windings and he cener of he hree phases wired o he oupu as a neural provides a so-called four-wire 3ph supply. The hree phases are 120 apar elecrically. The 1ph loads are wired beween his neural and any of he hree phases. The resuling phase-o-neural volage ampliudes are hen 1 3 imes he ransformer s phase-o-phase oupu volage.! 3

4 Wih purely linear loads he hree sinusoidal phase curren waves will cancel each oher ou in he neural conducor enirely. Wih non-linear loads, however, he hree hird harmonic phase currens are exacly in phase so ha he 180 Hz curren harmonic in he neural conducor is he aggregae of he hree individual hird harmonics. (Figure 15). This mechanism generaes an effecive neural conducor curren value ha amouns o a muliple of he phase curren and he risk of overheaing. If he ransformer s secondaries are dela conneced hen here is no neural and he 1ph loads are supplied from wo phases of his hree-wire 3ph sysem. The hird harmonic appears rapped in he dela loop bu his curren sill conribues o dangerously unsuspeced phase conducor load.! Quaniy ni Acive PFC Passive PFC No PFC Effecive phase curren I rms A Effecive neural curren I rms A Table 2: Comparison of 1ph PDS power facor conroller wih a 4-wire 3ph source The soluion for his phenomenon is an acive power facor conroller PFC for single-phase powered PDS. I provides elecrical performance close o ideal sinusoidal condiions (Table 2, single phase comparison) and consiss of an added circui beween he converer and he DC link (Figure 16). An acive PFC does ake up addiional space in he PDS, reduces maximum oupu power slighly, and increases he PDS firs cos. Is savings ouweigh more cosly and complex miigaion in he above described cases on he side of he power source. CONCLSION An elecrical sysems design process should always include a judgmen abou THDi. Non-linear curren draw causes non-linear volage drop across he power source impedance. The engineer eiher specifies a suiable power supply and low impedance disribuion sysem or encouners added cos for miigaion on all non-linear loads. A harmonic curren emission limi sricer han EN and EN prevens he use of proven, sandard PDS, may jeopardizes he cos savings poenial from variable speed sysems.! 4

5 Figure 1: Topology of a 3ph PDS EMC filer Converer DC link Inverer L1 L2 L3 Moor 3ph AC power source DC link Moor inpu

6 Figure 2: Power line curren unmiigaed

7 Figure 3: 3ph PDS wih 2% line reacor Line reacor (exernally) EMC filer Converer DC link Inverer L1 L2 L3 Moor 3ph AC power source DC link Moor inpu

8 Figure 4: Power line curren wih line reacor

9 Figure 5: 3ph PDS wih DC link filer choke EMC filer Converer DC link Inverer L1 L2 L3 Moor 3ph AC power source DC link Moor inpu

10 Figure 6: Power line curren wih DC link filer choke

11 Figure 7: 3ph PDS wih passive harmonics filer L1 L2 L3 Passive harmonics filer (exernally) EMC filer Converer DC link Inverer Moor 3ph AC power source DC link Moor inpu

12 Figure 8: Power line curren wih passive harmonics filer

13 Figure 9: 3ph PDS wih acive converer L1 L2 L3 L1 L2 L3 EMC filer Acive converer DC link Inverer Moor 3ph AC power source DC link Moor inpu

14 Figure 10: Power line curren wih acive converer

15 Figure 11: Harmonics of hree-phase PDS for par load operaion See original XLS file

16 Figure 12: 3ph PDS curren harmonics a nominal load

17 Figure 13: Oscillogram of 3ph PDS sandby operaion

18 Figure 14: 3ph PDS curren harmonics for sandby operaion

19 Figure 15: Third harmonic neural problem wih 1ph PDS 4)wire$3ph$ source$ 60$Hz$ L1$ Neural$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$180Hz$ 1ph$PDS$ 1ph$PDS$ 1ph$PDS$ L3$ L2$

20 Figure 16: 1ph PDS wih acive PFC Converer Acive PFC Inverer DR D T 1 T 2 T 3 ~ T 7 C Moor T 4 T 5 T 6 R 2 R 1 = = 1ph AC power source DC link Moor inpu