The Impact of Variable Speed Drives on Electrical Systems

Transcription

1 July 2015 REACHING OEM DESIGN ENGINEERS ACROSS CONSMER AND COMMERCIAL MARKETS WORLDWIDE The Impac of Variable Speed Drives on Elecrical Sysems P. 18 INSIDE 15 Conrols & Sensors 23 Plasics 26 HVAC 29 Coils Cover.indd 1 6/17/15 8:05 AM

2 MOTORS The Impac of Variable Speed Drives on Elecrical Sysems Wha You Need o Know Abou Harmonics Bu Were Afraid o Ask O ne 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? Definiions and sandards by marin bürker and armin hauer Marin Bürker, developmen manager elecronics a ebm-paps Mulfingen GmbH & Co. KG, Germany. Armin Hauer, advanced echnology manager a ebm-paps Inc. Farmingon, CT, member Ashrae, member TC 1.11, TC 1.9, TC 5.1 For more informaion, visi 18 appliancedesign July 2015 Moors.indd 18 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. (see Mike Olson aricle in he bibliography) 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.

3 Figure 1: Topology of a 3ph PDS MOTORS EMC filer Converer DC link Inverer 3ph AC power source DC link Moor inpu Figure 2: Power line curren unmiigaed Figure 3: 3ph PDS wih 2% line reacor Line reacor (exernally) EMC filer 3ph AC power source DC link Moor inpu Figure 4: Power line curren wih line reacor Converer DC link Inverer Moor Moor Power line curren harmonics The converer wih a passive recifier bridge and is highly capaciive DC link circui draws non-sinusoidal, non-linear power line inpu curren. Disored inpu curren oscilloscope races of a ypical 6 kw PDS are shown in figure 2. Three quaniies describe power line inerference commonly: Displacemen power facor cos 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. The cos 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 would be <1.0. True power facor is he quoien of effecive power in was over apparen power in volamps. 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 and cos are idenical. Toal harmonic curren disorion THD i is he raio of he geomeric sum of all curren harmonics up o he forieh order over he fundamenal curren expressed in percen. Nonlinear loads quie commonly produce THD i > 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 reacors 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. Figures 3 and 4 relae o he aforemenioned 6 kw EC moor example wih an addiional 2% line reacor. DC link filer choke The filer of figures 5 and 6 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 ha is based on common power supply sysems. However, appliancedesign July Moors.indd 19

4 MOTORS Figure 5: 3ph PDS wih DC link filer choke EMC filer Converer DC link Inverer sensiive applicaions such as offgrid 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 R sce. If his siuaion leads o very low THDi specificaions for elecrical loads hen comprehensive measures mus be aken o miigae harmonic curren emissions. AC line inpu passive harmonics filer These filers (figures 7 and 8) are voluminous and expensive bu hey miigae harmonic curren emissions well. Acive converer 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. (Figures 9 and 10). Par load operaion 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, a concern can arise: The power harmonics analyzer may display a and a THD i ha appears worse han adverised: When ha occurs, hen he PDS likely operaes a par load. I does consume reduced power jus as prediced and he curren draw of he fundamenal frequency does drop accordingly. Ye he reducions of he higher order harmonic currens are less pronounced. (Figure 11) Remember ha boh and he THD i are raios. The absolue values of boh he numeraors and he denominaors grow smaller, bu disproporionally. Therefore, and THD i values rarely maer for par load operaion. Sandby operaion 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. (Figures 12, 13, 14) 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. 3ph AC power source DC link Moor inpu Figure 6: Power line curren wih DC link filer choke Figure 7: 3ph PDS wih passive harmonics filer Passive harmonics filer (exernally) EMC filer 3ph AC power source DC link Moor inpu Converer DC link Inverer Figure 8: Power line curren wih passive harmonics filer Moor Moor 20 appliancedesign July Moors.indd 20

5 Figure 9: 3ph PDS wih acive converer MOTORS EMC filer Acive converer DC link Inverer 3ph AC power source DC link Moor inpu Figure 10: Power line curren wih acive converer Figure 11: Harmonics of hree-phase PDS for par load operaion See original XLS file Figure 12: 3ph PDS curren harmonics a nominal load Moor 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 singlephase 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. 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. 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 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. Summary An elecrical sysems design process should always include a judgmen abou THD i. Nonlinear curren draw causes nonlinear 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 appliancedesign July Moors.indd 21

6 MOTORS Figure 13: Oscillogram of 3ph PDS sandby operaion increase heir size and weigh, may reduce heir maximum oupu, and herefore jeopardizes he cos savings poenial from variable speed sysems. References, Bibliography, Noes European Commiee for Elecroechnical Sandardizaion CENELEC draf sandard CLC x: Energy efficiency and Eco-design requiremens for Power drive sysems, moor sarers, Power elecronics and heir driven applicaions hp:// files/eup-lo-30-task-1-5-jun Draf.pdf Inernaional Elecroechnical Commission draf sandard IEC x (1s ediion) Efficiency of frequency converers, power drive sysems and complee drive sysems Canadian Sandards Associaion CSA sandard C Tes mehods for measuring power consumpion in low and idle power modes of commercial and indusrial equipmen. Inernaional Elecroechnical Commission sandard IEC Limis for harmonic curren emissions (equipmen inpu curren 16 A per phase) Inernaional Elecroechnical Commission sandard IEC Limis for harmonic currens produced by equipmen conneced o public low-volage sysems wih inpu curren > 16 A and 75 A per phase Air Condiioning and Refrigeraion Insiue sandard AHRI Performance Raing of Variable Frequency Drives Olson, M.R Harmonic Disorion: Realiy vs. Percepion Nov HPAC Engineering Magazine hp:// hpac.com/moors-drives/harmonic-disorion-realiy-percepion Ashrae Handbook HVAC Sysems And Equipmen chaper 45. Moors, Moor Conrols, and Variable-Speed Drives 2011 Ashrae Handbook HVAC Applicaions chaper 56. Elecrical Consideraions Figure 14: 3ph PDS curren harmonics for sandby operaion Figure 15: Third harmonic neural problem wih 1ph PDS 4- wire 3ph source 60 Hz Neural 180Hz 1ph PDS 1ph PDS 1ph PDS 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 22 appliancedesign July Moors.indd 22 6/18/15 10:06 AM