Average and Detailed Modeling Approaches Emphasizing Subsystems in a Hybrid Mobile Refrigeration

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1 Average and Deailed Modeling Approaches Emphasizing Subsysems in a Hybrid Mobile Refrigeraion Yue Cao, Suden Member, IEEE, Philip T. Krein, Fellow, IEEE Absrac Averaging-based and deailed dynamic models of hybrid elecric power sysems are presened, wih emphasis on elecric machines and drives. The objecive is o compare echnical and pracical aspecs associaed wih hybridizaion of refrigeraion unis in delivery rucks. Challenges unique o his hybrid applicaion, including he hermal sysem inerface and drive-cycle response, are inroduced. The sysem opology is presened, and modeling approaches for each major subsysem, including an ac machine, he baery se, and converers, are discussed. An average modeling echnique is fas enough o allow sysem-level power and efficiency o be evaluaed over a long ime inerval. Compared o he average model, a deailed model including ransien response and harmonics gives more accurae power loss esimaes a he cos of slower simulaion speed. The wo models, inerfaced wih he hermal sysem, are examined wih simulaion sudies in MATLAB/Simulink, showing he feaures of each model. Experimenal seup and resuls are presened o validae he models. Index Terms Mobile refrigeraion uni (MRU), hybrid power sysems, elecric drives, average model, dynamic models I. INTRODUCTION roduce delivery rucks ha require diesel-powered mobile Prefrigeraion unis (MRU) o mainain food freshness, like ha in Fig. 1, consume significan energy [1-3]. The main engine may need o idle indefiniely when he ruck is sopped for a delivery, or a separae small diesel engine may be dedicaed o he refrigeraion uni. I is becoming more necessary o comply wih new laws and increasing cusomer demands for more environmenally friendly and fuel-efficien delivery vehicles [4]. This paper presens sysem-level models based on fas simulaion ha seeks o inegrae elecric drive and oher subsysems for hybridizaion of MRUs based in par on he developmens in [5]. Models based on averaging and based on more deailed dynamics are provided. The reamen focuses on elecric machine dynamics including orque and speed responses o various delivery cycles and hermal loadings. The dynamic model includes swiching volage and curren waveforms. I serves a validaion check for average model accuracy and provides guidelines for hardware design. Figure 1. A ypical produce delivery ruck wih MRU Source: FormerWMDriver on flickr.com II. HYBRID POWER SYSTEM FOR MRU A high-level sysem configuraion of a proposed hybrid MRU is depiced in Fig. 2. In conras o a more convenional sysem wih dedicaed small engine, his version akes in power from he main ruck engine via an ac generaor, and also has he opion o connec o he grid. Afer power conversion and conrol, several elecric moors drive compressors, fans, and blowers, comprising he hermal sysem. Heaers are needed o cover a full range of climae condiions, since an MRU is no specialized: he same ruck migh deliver frui one day and frozen mea he nex. Fig. 3 shows he hybrid power sysem: he main engine shaf drives an ac generaor, feeding an acive recifier ha sabilizes is oupu dc bus, hen an inverer operaes an ac inducion machine o drive he compressor; he baery connecs a he dc bus, possibly hrough a separae dc-dc converer. Addiional small moors o run fans and blowers are no shown. The differences beween mobile and saionary refrigeraion unis include 1) frequen and drasic emperaure changes due o loading and unloading of producs, 2) a wide variey of ruck operaing profiles and environmens depending on road condiions, delivery schedules, and oudoor emperaures, and 3) lack of access o consisen and reliable power sources. Commands & vehicle CAN Mobile refrigeraion uni Engine/exernal Power Hybrid Power Sysem Thermal Sysem Power Supply Sysem Conrol uni Sensors Compressor This work was suppored in par by Thermo King, a division of Ingersoll- Rand, and by he Grainger Cener for Elecric Machinery and Elecromechanics a he Universiy of Illinois a Urbana-Champaign. Yue Cao is wih he Deparmen of Elecrical and Compuer Engineering, Universiy of Illinois, Urbana, IL 6181, USA ( yuecao2@illinois.edu). Philip. T. Krein is wih he Deparmen of Elecrical and Compuer Engineering, Universiy of Illinois, Urbana, IL 6181, USA ( krein@illinois.edu). Grid Sandby Alernaor Power elecronics, swiches & proecion Uni baery Fans Blowers Heaers Aux loads Figure 2. Sysem configuraion of he proposed MRU /13/$ IEEE 1132

2 Funcions f 1 and f 2 are look-up ables from curren esing [9]. The coefficiens in (4) found in [5] are from curve fiing of experimenal daa of V, C, and R versus SOC. Single-cell daa (curren, SOC) were exraced from measuremens of he Panasonic CGR1865A 3.7 V, 2.2 A-h Li-ion baeries [1]. The baery erminal volage is hen calculaed as V = Voc I c ( Rseries + Rs + Rm + Rh ) (5) sc sc sc s m h Figure 3. Hybrid power sysem srucure in he simulaion model III. AVERAGE MODELING APPROACH The average modeling approach seeks o model energy flows and power balances, including power losses in each subsysem (moor, converer, baery, ec.). Conducion and swiching devices losses are modeled based on in equivalen seady sae condiions. Mos losses have a direc relaionship wih he associaed currens. Calculaed oupu currens from one subsysem are passed as inpu currens o he nex subsysem. Energy flows can be examined in eiher direcion, wih inpu and oupu currens changing roles. Machine mechanical losses mus also be included. These can be inferred from machine daa shees and basic ess. Since he ac generaor is coupled o he drive engine, is operaing speed is linked o a dynamic vehicle drive cycle, and ranges from 175 o 5 RPM. MRUs experience more complicaed operaing sequences han auomobiles, so convenional drive cycles have limied value. For simulaion sudies repored here, drive cycle daa were obained from an indusry sudy ha recorded ypical ruck driving condiions [6]. The ac generaor is modeled as a 3-phase permanen magne synchronous machine (PMSM) wih coninuous oupu raing of 17.3 kw. The ac moor ha drives he compressor is modeled as a 3-phase 12 HP 46 V 6 Hz inducion machine (IM). The IM and PMSM models employ convenional perphase seady-sae equivalen circuis [7]. The recifier, inverer, and any dc-dc converers are power elecronic elemens wih swiching loss and conducion losses in he IGBTs. These can be esimaed as [8] 2 2I rms V on 2 Pcond = + IrmsR (1) ds π and 2 2IrmsV bus on + off Pswich = f (2) swich π 2 The conducion loss can be modeled by means of an ideal swich in series wih a forward volage drop (V on ) and a series resisor (R ds ). In he swiching loss calculaion, f swich is he inverer swiching frequency. Times on and off are he IGBT swiching rise and fall delay imes, respecively, which are found in device daashees. V bus is he main dc bus volage. The baery model is based on he circui in Fig. 4, in which volage source, resisors and capaciors depend nonlinearly on he baery s sae of charge (SOC) 6 6 k ln( V, C, R) = a + a1 ln( SOC) a6 ln ( SOC) = a k ln ( SOC) (3) k= SOC is modeled as in [5] as (4) SOC( ) = SOCiniial + f1[ ich arge ( )] ich arge ( ) d + f 2[ idisch arg e ( )] idischarg e ( ) d Figure 4. Lihium-ion baery model circui Noice ha he model includes explici dynamics on ime scales of seconds, minues, and hours. IV. DETAILED MODELING APPROACH A deailed dynamic model was consruced in Simulink/SimPowerSysems. This modeling approach differs from he average model in several areas. The converer semiconducor devices are reaed as swiches wih parasiic losses. The elecric machines (PMSM, IM) are modeled as differenial equaions Vqs = Rsiqs + dϕqs / d + ωϕ qs Vds = Rsids + dϕds / d + ωϕ ds V ' qr = R' r i' qr + dϕ' qr / d + ( ω ωr ) ϕ' dr (6) V ' dr = R' r i' dr + dϕ' dr / d + ( ω ωr ) ϕ' qr Te = 1.5p( ϕdsiqs ϕqsids ) dωm / d = (1/ 2H )( Te Fωm Tm ) ha produce ransien waveforms [11]. Filers are also included beween componens. The dc-dc converer is feedback conrolled o sabilize he dc bus volage. A scalar vols-per-herz (V/f) conrol (Fig. 5) is implemened in he IM drive (Fig. 6). The IM and PMSM are conrolled o respond robusly o a wide range of orque and speed commands from he hermal sysem, and o engine dynamics associaed wih he vehicle driving cycle. Figure 5. IM V/f closed-loop conrol diagram 1133

3 Figure 6. dc-ac inverer circui ied o he ac moor Figure 8. Power levels a he inverer, moor, and compressor V. SIMULATION STUDY A comprehensive simulaion mus be run for he inegraed hermal-elecrical sysem. I is imporan o evaluae each subsysem s performance under real-life scenarios. The scenario includes he vehicle drive cycle, dynamic hermal loading demand based on he desired ruck conainer emperaure and ambien emperaure, door open and close evens, connecion o shore power, ec. The energy flow is developed in reverse order: based on hermal requiremens, he compressor has known speed and orque requiremens. These in urn are supplied by he inducion machine, and o his machine by he inverer. The inverer draws power from he generaor or he baeries. Generaor operaion is linked o engine RPM during he vehicle drive cycle, siing idle a minimum speed when he vehicle sops. For a complee sudy, simulaion is o analyze a delivery cycle lasing eigh hours or more. Sysem-level simulaion mus be faser han real ime o make his useful. The average model oleraes a wide range of sampling imes up o.1 s o accommodae differen hermal or oher elecrical inerfacing requiremens. The deailed model has a maximum sampling ime of 5 µs. The average model akes hree minues o simulae one hour of a delivery run, while he deailed model operaes only slighly faser han real ime and is no as useful for for-cycle sudies. To demonsrae he capabiliies of he average model, a sysem simulaion from 2, s o 29, s, corresponding o 1: PM o 3:3 PM during a ypical day s delivery schedule, was prepared. The moor operaes when here is demand from he hermal sysem. The baery is charged, if below a prese SOC value, when he generaor runs, and i is discharged when he moor is on while he ruck is no moving. Figs. 7-9 show he power levels a each componen in he hybrid power sysem. Noice ha he ime unis reflec an inerval nearly hree hours long. Figure 9. Baery charging and discharging power From he deailed model, Fig. 1 shows dynamic response of roor speed, saor curren, and elecromagneic orque of he IM during a compressor sar. The simulaion sars a sysem ime 54 s, and he inerval shown is less han 1.5 s. Noe ha i akes abou.5 s for he roor o ramp up o he desired speed, 172 RPM. Afer anoher.5 s, a speed of 178 RPM is commanded, and furher ransien response is observed. Fig. 11 shows ha he dc bus volage is held o 7 V hroughou he run, reflecing he capabiliies of an acive recifier and a dc-dc converer o inerface he baeries. The inverer PWM modulaion index is ramped up gradually o he desired value in his V/f sraegy. Deails of he highfrequency swiching IM saor volage can be observed. Figure 1. IM roor speed, saor curren, and orque during ransien Figure 7. Power levels a he generaor, recifier, and converer Figure 11. Inverer dc bus volage, oupu volage, and modulaion index 1134

4 Similarly deailed waveforms on he PMSM side can be obained. Fig. 12 shows he generaed volage and curren during he same ime inerval, and Fig. 13 illusraes he oupu orque and speed from he generaor. The speed is ied o he ruck drive cycle. In addiion, Fourier specra of he IM and PMSM saor currens are analyzed in Fig. 14. Figure 12. PMSM erminal volage and saor curren The deailed model also serves as a check for he average model. Fig. 15 plos en efficiency values on he engine power supply efficiency curve, and Table 1 liss he deailed daa for hese en poins. The wo efficiency curves are wihin 5% of each oher. Generally, he deailed model efficiency is somewha lower since i is more refined. Table 1. Simulaed power and efficiency daa for he deailed model Generaor RPM Moor RPM Torque (Nm) Inpu Power (W) Oupu Power (W) Efficiency VI. EXPERIMENTAL VALIDATION Tess of he generaor and he ac-dc recifier were conduced separaely [12]. A hree-phase PMSM, he one modeled in Simulink, was used in he experimenal ess. This device was used previously for an elecric air condiioning uni in a ciy bus. Differen loads were applied for each of various speed seings, and he corresponding powers and efficiencies were obained hrough direc measuremen, as shown in Fig. 16. Figure 13. PMSM generaed orque and roor speed Figure 14. Fourier specra for IM curren (lef) and PMSM curren (righ) Figure 15. Simulaed efficiency comparison for average and deailed models Figure 16. PMSM efficiency versus power a differen speeds, from [12] couresy of Thermo King Corporaion, used by permission. A hardware es bed for he variable frequency ac moor drive, including he dc-ac inverer and he moor is shown in Fig. 17. The sysem operaes a abou 1/6 scale compared o he vehicle sysem. A hree-phase 23 V 2 HP 4-pole IM was chosen, and a modular dc-ac inverer was used. The conrol box of he inverer is based on a TI-2812 DSP and commanded by MATLAB/Simulink [13]. The orque and speed profiles were referenced from he hermal sysem demand. The measured efficiency was on average abou 5% lower han he simulaed efficiency, as shown in Fig. 18. This 1135

5 is expeced since he es machine used for his experimen has a raed efficiency abou 5% lower han he simulaed machine. Figs. 19 and 2 show he IM roor speed and saor phase curren during he iniial one second of sar-up. Modular inverer Measuremen panel Dyno and moor Figure 17. Experimen seup for he power sysem esing. Figure 18. Measured efficiency compared o simulaed efficiency Figure 19. Measured IM roor speed during ransien VII. CONCLUSION A hybrid power sysem for mobile refrigeraion has been modeled using an average approach and a deailed approach. The model has been validaed hrough successful simulaion and experimenal work. Wih he average model, a complee hermal-elecric sysem simulaion has been buil and serves as a basis for fuure hybrid sysem opimizaion sudies. The complee simulaion operaes abou weny imes faser han real ime. The deailed model can be used for hardware design, and i has he developmen poenial in a few direcions, including 1) advanced exising or novel moor drive conrol echniques for highly efficien and precise inernal loading requiremen, and 2) mulilevel converer based moor drive for improving efficiency and reducing moor sress, especially under high power requiremens. I runs slighly faser han real ime, bu is complexiy and fas sampling imes make i less suied for inegraion wih hermal subsysems. REFERENCES [1] T. van Keulen, e al., Design, implemenaion, and experimenal validaion of opimal power spli conrol for hybrid elecric rucks, Conrol Engineering Pracice, vol. 2, no. 5, pp , May 212. [2] T. Hofman, e al., Hybrid componen specificaion opimizaion for a medium-duy hybrid elecric ruck, Inernaional Journal of Heavy Vehicle Sysems, vol. 15, no. 2-4, pp , 28. [3] L. He, e al., Impac of vehicle usage on consumer choice of hybrid elecric vehicles, Transporaion Research Par D: Transpor and Environmen, vol. 17, no. 3, pp , May 212. [4] C. V. Kulkarni, Modeling and he performance analysis of ransporaion refrigeraion unis wih alernae power sysems, Ph.D. disseraion, Mechanical and Aeronauical Engineering, Univ. of California, Davis, CA, 27. [5] Y. Cao and P. T. Krein, An average modeling approach for mobile refrigeraion hybrid power sysems wih improved baery simulaion, o appear in Proc. IEEE Transporaion Elecrificaion Conf., June 213. [6] R. A. Jackey, A simple, effecive lead-acid baery modeling process for elecrical sysem componen selecion, The Mahworks Inc., Novi, MI, Rep , 27. [7] A. E. Fizgerald, C. Kingsley, and S. D. Umans, Elecric Machinery, 6 h ed. NY: McGraw Hill, 23, pp [8] P. T. Krein, Elemens of Power Elecronics. New York: Oxford Universiy Press, 1998, pp [9] R. C. Kroeze and P. T. Krein, Elecrical baery model for use in dynamic elecric vehicle simulaions, in Proc. IEEE Power Elecronics Specialiss Conf., 28, pp [1] R. C. Kroeze, Elecrical baery model for use in dynamic elecric vehicle simulaions, M.S. hesis, Elecrical and Compuer Engineering, Univ. of Illinois, Urbana, IL, 28. [11] P. C. Krause, O. Wasynczuk, and S. D. Sudhoff, Analysis of Elecric Machinery and Drive Sysems, 2 nd ed. Edison, NJ: Wiley-IEEE Press, 22, pp [12] P. Kadanik, e al., Transpor hybridizaion sudy sponsor updae Thermo King and Ingersoll Rand, unpublished, 212. [13] J. Kimball, e al., Modular inverer for advanced conrol applicaions, Grainger Cener for Elecric Machinery and Elecromechanics, Urbana, IL, CEME-TR-26-1, 26. Figure 2. Measured IM saor curren during ransien 1136