http://dx.doi.og/10.5755/j01.eee.0.5.7111 ELEKTRONIKA IR ELEKTROTECHNIKA ISSN 139 115 VOL. 0 NO. 5 014 PV Supplied Single Stage MPPT Invete fo Induction Moto Actuated Ventilation Systems S. Ozdemi 1 N. Altin I. Sefa G. Bal 1 Atatuk Vocational School Gazi Univesity Cubuk Ankaa Tukey Depatment of Electical & Electonics Engineeing Faculty of Technology Gazi Univesity 06500 Besevle Ankaa Tukey naltin@gazi.edu.t 1 Abstact Ventilation and efigeation is indispensable pocess fo human life. Paticulaly it has vital impotance in deset climate conditions and ual aeas whee the gid powe may not be available. Although the cost is still highe than taditional enegy esouces enewable enegy may be the ight choice fo these applications. In this pape a DSP based vaiable speed dive fo induction motos to actuate a ventilation system supplied photovoltaic aays is poposed. The maximum powe point of the photovoltaic aays is tacked by poposed vaiable speed dive. A vaiable stepped incemental conductance and constant voltage methods ae combined to obtain fast tacking algoithm which geneates voltage and fequency efeences fo thee phase thee level invete. Also the switching fequency of the invete is kept constant to impove the quality of cuent wavefom at low powe levels. Expeimental esults show that the poposed system has fast eaction and is suitable fo apidly changing iadiation level. Since the additional convete equiement is eliminated by using single stage invete the total system efficiency is inceased. Index Tems PV ventilation V/f single stage invete MPPT. I. INTRODUCTION The inteest in enewable enegy souces is inceasing day afte day paallel with the wold enegy demand. Since sola enegy is common nealy all aound the wold; it is one of the most pomising enewable enegy souces in the wold. The wold installed sola enegy capacity was 98.9 MW in 009 and it is 46.9 % highe than pevious yea [1] []. Since the unit cost of enegy geneated fom these souces is moe expensive than taditional souces enegy efficiency has been in the focus of attention of eseaches. Today efficiencies of the commecial available photovoltaic (PV) modules ae between 1 % and 19 %. The pefomance of the PV module is elated with vaious factos such as tempeatue exposue to sunlight popeties of sunlight dit dust and etc. [3]. PV modules geneate a specific powe fo a constant tempeatue and iadiation level. The vaiations in iadiation and tempeatue levels change the obtainable maximum powe (MP) level. So level of load powe should be aanged accoding to the PV panel powe vesus voltage (P-V) cuve to povide maximum Manuscipt eceived Septembe 15 013; accepted Januay 9 014. benefit fom investment. This can be ealized though a maximum powe point tacking (MPPT) algoithm [4] [5]. Since vaiations in enegy supply and load powe demand match duing a day PV supplied electical powe is suitable fo ventilation and ai conditioning applications [6]. On the othe hand PV can be used fo both cooling and heating of the buildings. Namely standad oof tiles can be many degees wame than the ambient of outside ai. A positive pessue mechanical ventilation system can be designed both fo ventilating and peheating the house via a small fan. Howeve an auxiliay electic souce equiement is disadvantage of this system. PV module can be used to supply the fan to ovecome this dawback [7]. Unlike most of the loads used in the daily life a PV panel poduces DC voltage which has some disadvantages in use. These ae the mechanical effects of a bush and the collecto such as electical ac lowe efficiency of the DC motos and highe cost of the DC system equipment such as fuse chille fan etc. So PV powe should be conveted to AC by static invetes. Additionally almost 70 % of the whole electic enegy geneated is used by the electic motos in industialized counties. Futhemoe ove than 60 % of whole electic enegy tansfomed to mechanical enegy is used up by induction moto diven pumps and fans. Theefoe enegy saving and efficiency ae impotant in these kind of applications [8] [9]. Nevetheless flow and pessue of these systems ae often egulated by mechanical devices such as thottle valves and simila. The mechanical contol deceases the efficiency and inceases the pessue in pipes and valves as well as inceasing electical losses. Despite the fact that the most extended method of electical contol is still based only on the soft stating without any speed contol afte the stating these disadvantages can be emoved by contolling the moto speed [9] [10]. Diffeent speed contol methods such as scala contol vecto contol diect toque contol and etc. have aleady been poposed fo induction motos [11] [1]. The scala contol method called as V/f contol is vey common in both open and closed loop contolled industial applications. In these applications invete switching fequency usually vaies with speed of dives and the associated eo bands and also it is quite low as compaed to the sampling fequency of system. But the vaiable switching fequency makes vey complicate designing of invete output filte 116
[13]. PV supplied moto dive applications have been poposed in past studies. In most of these studies MPPT pocess is caied out by additional DC/DC convete [14] [15]. Since the DC/DC and DC/AC powe convesion schemes ae used these systems ae called two-stage invetes. The DC/DC convete pefoms MPPT and invete convets DC geneated by PV to AC. While using two-stage invete simplifies the contol it deceases the total efficiency of the system and inceases the cost. PV systems cost can be minimized by deceasing the numbe of powe convesion stages and the numbe of components involved in each stage. In these systems DC/AC convesion and MPPT pocesses ae pefomed by same convetes meanwhile this causes an incease in complexity. Futhemoe single-stage invetes ae 4 % 10 % moe efficient than the two stage configuations [16]. In this study a DSP based vaiable speed dive fo PV supplied induction moto is poposed to geneate the mechanical enegy fo ventilation efigeation and aspiation applications. Opeation point of the induction moto is detemined by the MPPT algoithm accoding to PV aay paametes to obtain maximum efficiency. Combination of vaiable stepped incemental conductance (IC) method and constant voltage (CV) method is used as MPPT algoithm. In addition the switching fequency is kept constant fo all fequency values of output voltage so same effects obtained fo all opeation egion and cuent wavefom distotions ae pevented at low fequencies. Expeimental esults showed that the poposed MPPT algoithm has fast stating esponse of the CV method and stability and accuacy of the IC method. Also vaiable step opeation capability of IC method deceases the oscillations at steady state. Since the system is designed as single stage invete the total system efficiency of the dive has been inceased and measued as 94.7 %. II. V/F CONTROL OF INDUCTION MOTOR Thee phase squiel cage induction motos ae low cost high efficient obust eliable and low maintenance equied motos and they ae widely used in industial application with these advantages [11] [1] [17]. Nevetheless speed vaiation with load level and lack of vaiable speed opeation capability ae the main dawbacks of these motos [1]. The state equations which ae expessed as non-linea diffeential equations in the synchonous efeence fame ae given below fo a symmetical squiel cage induction moto [18] whee: X F X U (1) d f 3 M 3 M 1 p imd iqs p imqids TL dt J J J J () dimd M imd s imq i ds dt L L (3) dimq M imq s imd i qs (4) dt L L whee: T X imd imq (5) T U ids iqs s. (6) whee and s ae oto and synchonous speeds espectively; p is the pole numbe of the induction moto; M is mutual inductance and J is moment of inetia; i md and i mq ae oto d q-magnetizing cuents; T L is load toque; is equivalent oto esistance; L is oto inductance. When a linea optimal contol is applied a lineaization of the above moto model is expessed in state space as a deteminist linea system: X AX BU (7) F A X X d (8) F B. X U d (9) whee X d and U d ae the desied state and contol vaiables. If the oto magnetizing cuent i mq is popely tuned to zeo the field oientation with the oto flux may be ideal. Then (10) (1) ae obtained unde steady state [18]: d M d imd ids L (10) d d M iqs s d L imd (11) d 3 d d T pmidmiqs. (1) If the stato powe loses ae neglected toque (T) by using stato supply voltage and flux fo steady state ae given below [17]: R 3 P vs T... S f R R Xls X S l v S (13) 1 S.. f (14) It is seen that the toque and the magnitude of the stato flux of the induction moto ae diectly elated with the atio between the stato voltage and fequency (V/f). In industial applications induction motos ae used with diffeent type of loads such as constant toque constant powe and vaiable toque. Speed powe and toque vaiations of these loads ae given in Fig. 1. The V/f contol is a low cost and simple method to contol the speed of the induction moto in wide speed anges. In this method toque esponse is elated with the electical time constant of the moto and tuning equiement of the contol paametes is emoved. It can be said that V/f 117
contol is effective solution fo simple vaiable speed applications like fans pumps [15] [16] [19]. chaacteistic of the pump and geneally detemined as k= 4. Big values of k coespond to chaacteistics of the pump with highe static pessue component. Fig. 1. Types of the load. The centifugal fans and pumps ae vaiable toque loads and they have geat enegy saving potential. The Affinity Laws descibes the elationship between the speed and othe fan o pump vaiables. It also govens these loads. Accoding to law 1 change in flow popotional to change in speed; the change in head (pessue) popotional to the change in speed squaed; the change in powe popotional to the change in speed cubed as given in (15) (17): Q1 N1 (15) Q N H1 N1 H N 3 P1 N1. P N (16) (17) whee Q is volumetic flow H is head (pessue) P is powe and N is speed (pm). Accoding to (15) (17) if the flow is contolled by educing the speed of the fan o pump a elatively small speed change will esult in a lage eduction in powe consumption. The powe consumption of a fan o pump system is elated with the flow contol method as given in Fig.. By eplacing dampes which ae used to egulate the flow of fans with vaiable fequency dive (VFD) fo induction moto moe efficient opeation can be obtained fo same flow. A novel contol algoithm fo VFDs educes the input powe of an induction moto used to dive a vaiable toque load as fan o pump. In addition noise and vibation of the fan ae also deceased with speed. In V/f contol value of the V/f atio affects the system pefomance and elated with type of the system. The egulation law fo the pump and fan units can be witten as in (18) V constant (18) k 1 f whee k is the facto depending on the kind of mechanical Fig.. Powe consumptions of the systems accoding to contol techniques. III. NPC INVERTERS Multi-level invetes that intoduced by Nabae have some advantages such as educed filte equiement at the same switching fequency; educed voltage and/o cuent hamonics and educed electomagnetic intefeence (EMI) [7]. Recent pogess in micopocesso technologies make easie of contolling multi-level invetes. They have been stated to be used commonly especially in high powe applications. Howeve these invetes ae equied moe components such as powe semiconducto switches capacitos gate dives etc. and thei cicuit stuctues ae moe complex [3] [5] [6] [8]. In addition multi-level invetes have an advantage of achieving high voltage and high powe levels. Thee ae diffeent types of multi-level invetes such as the neutal point clamped (NPC) invete the cascaded H- bidge (CHB) invete and the flying capacito (FC) invete [3] [8]. These topologies have diffeent advantages and disadvantages such as difficulty of balancing of two capacitos voltages in NPC invete bigge capacito equiement and inceased size of FC topology isolated voltage equiement of CHB. In addition isolated DC voltage souce equiements can be emoved by using NPC invetes. In last decade switch modules fo a leg of the NPC invete have become commecial available and this makes easie design and implementation of NPC invete. The geneal diagam of the NPC invete is shown in Fig. 3. Fig. 3. Equivalent cicuit of PV supplied NPC IM dive. The switches opeate with only half of the DC bus voltage fo all positions (P O and N) in NPC invete [10]. 118
Vaiation of the output voltage value of invete one leg with espect to switch position is given in Table I. TABLE I. OUTPUT VOLTAGES OF INVERTER ONE LEG ACCORDING TO SWITCHES STATUS. Switch Position S1 S S3 S4 Output voltage P (Positive) 1 1 0 0 1/V O (Medium) 0 1 1 0 0 N (Negative) 0 0 1 1-1/V IV. MPPT METHODS Thee is a nonlinea elationship between PV output powe level and a vaiety of factos such as tempeatue sola iadiation popeties of sunlight dit dust and etc. These factos affect the PV cell pefomance [3] [0]. The equivalent cicuit of the PV is shown in Fig. 4. voltage iadiation tempeatue etc. values and some mathematical equations o pe-pepaed tables fo evaluating the powe convete contol signal to detemine the appopiate opeating point of the PV system. Constant cuent (CC) Constant Voltage (CV) Pilot Cell (PC) and Look-up Table (LUT) ae examples of the offline methods. Although offline methods have fast esponse time they ae affected fom envionmental conditions and aging of the PV modules. Futhemoe they find appoximate MPP and they ae effective fo only specified panel. Fig. 4. The equivalent cicuit of PV. (a) Accoding to this equivalent model (19) () can be witten fo output voltage of PV V c by neglecting I d and I p cuents [1]: NTI I Voc TI.ln Rs TI. I Isc TI Isc TI V ITI Rs TI.Isc TI (1 ) ln Voc TI R s I T I (19) Voc T I Vmp T I T I (0) T I ln N T I (1) Imp T I ln I sc T I Rs T I. Isc T I Vmp T I. 1 Rs T I. Imp T I Isc T I Voc T I T I (). V T I oc Expoting maximum obtainable enegy to the load and obtaining maximum efficiency can only be povided via MPPT methods. The MPPT method is used to contol powe convete opeation point accoding to the load and the PV system popeties. Daily vaiation of iadiation level and tempeatue measued fom -axis plane; obtained P-V cuves and MP cuve which is composed of maximum powe points (MPPs) of these P-V cuves ae seen in Fig. 5. The powe convete should tack this MP cuve which vaies duing the daytime to obtain maximum efficiency []. MPPT methods can be categoized as online (diect o tue seeking) and offline (indiect o quasi seeking) methods [3] [4]. The main facto in this classification elates to the tue MPPT opeation. The offline methods use PV cuent mp (a) Fig. 5. Iadiation and ambient tempeatue levels (a); MPPT cuve (b). On the othe hand online methods check the PV system if it is opeating at its MPP o not all the time. The Petub & Obseve (P&O) method and the IC method ae the most common online methods. The IC method checks the vaiation of output powe accoding to PV voltage (dp/dv) and defines the point of dp/dv = 0 as MPP point. If dp/dv < 0 this means opeating point is on left side of the P-V cuve and if dp/dv > 0 opeating point is on ight side of the P-V cuve and next contol signal of the powe convete is detemined accoding to this value. Unlike offline methods online methods find the eal MPP; thus envionmental effects and panel dependency ae emoved. V. PROPOSED SYSTEM In this study thee phase vaiable speed dive fo PV supplied induction moto actuated ventilation efigeation and aspiation applications is poposed. The pinciple scheme of the poposed system is depicted in Fig. 6. MAGNA-POWER TDS III 600-8 Photovoltaic Powe Pofile Emulato (PPPE) which can opeate as PV modules 119
is used as DC voltage souce. As seen fom Fig. 7 the PPPE inteface pogam is used to define the commecial available PV module SHARP NA-901WP by using I mp V mp V oc I sc T n T op I n I op tempeatue and iadiation coefficients values which ae given in datasheet. The electical chaacteistics of a module at 5 C 8 seial connected PV modules at 5 C and 8 seial connected PV modules at 39 C ae given in Table II. The total powe of the PV system is 656.1 W. Fig. 6. Poposed system. is used as speed contolle. In this study V/f egulation law given in (18) is used and k is detemined as. Since speed esponse accuacy is not main concen geneally open-loop speed contol is used in heating ventilation and ai conditioning (HVAC) fan o blowe applications. In this case slip of the moto neglected and actual speed of the moto is found by assumption that the moto will oughly follow its synchonous speed. Also in PV supplied HVAC applications desied moto speed is detemined by MPPT algoithm to obtain MP that the PV modules can geneate. In othe wods in the PV supplied HVAC applications main concen is tacking the MPP of the PV system not speed efeence. So an openloop speed contolle is used in this application. The V/f speed contolle detemines the voltage and fequency values fo PWM geneato accoding to the speed efeence. In Fig. 8 the flowchat of the poposed MPPT algoithm is depicted. Initially CV method which is one of the off-line methods is used to achieve a faste stat up time. When the PV voltage educes to MPP voltage (0.80 of V OC) a vaiable stepped IC method is opeated. Consequently the fast stat up esponse of the CV method with educed oscillation and fast tacking speed of IC method ae integated by the poposed MPPT algoithm. The step size of the IC method is changed depending on absolute value of dp/dv to impove the pefomance of the poposed method which is contolled via ezdsp 8335 contolle boad. Fig. 7. Inteface pogam of the PPPE. TABLE II. THE ELECTRICAL CHARACTERISTICS OF THE PV MODULE. Numbe of PV T (C) I ef (W/m ) V mp (V) I mp (A) V oc (V) I sc (A) P (W) 1 5 1000 53.5 1.98 66.6.0 105.9 8 5 1000 48 1.98 53.8.0 847.4 8 39 800 410.08 1.6 510.4 1.78 656.1 In this study combination of IC and CV method is used as MPPT algoithm. This algoithm detemines the speed efeence accoding to PV opeation point to obtain maximum enegy convesion efficiency. The V/f contol method which is commonly used in vaiable speed induction moto applications fo both open and closed-loop contolle Fig. 8. Flowchat of the poposed MPPT algoithm. VI. EXPERIMENTAL RESULTS The expeimental setup of the poposed system is shown in Fig. 9. The test ig of system coves Magna Powe PPPE ezdsp8335 contolle boad voltage souce invete and backwad cuved centifugal fan. The paametes of the induction moto used to actuate the fan ae given in Table III. The PPPE is pogammed to emulate 8 seies connected SHARP NA-901WP PV modules. Also the 10
ELEKTRONIKA IR ELEKTROTECHNIKA ISSN 139 115 VOL. 0 NO. 5 014 system was tested with diffeent iadiation tempeatue levels. educed even unde low speed conditions. TABLE III. PARAMETERS OF INDUCTION MOTOR. Paamete P (W) ULL (V) n (pm) f (Hz) Value 550 0 1415 50 The statup of the induction moto is shown in Fig. 11. Initially the efeence speed is defined by CV method and the V/f speed contolle povides soft stating and the inush cuent of the moto is limited. Afte stat-up the IC method detemines the efeence speed accoding to opeation point of the PV system. The speed contolle detemines the equied voltage level and the fequency accoding to efeence speed and then contols the PWM geneato. The poposed system tacks the MPP of the PV system and it has fast esponse. In addition efficiency of the dive system is measued as 94.7 %. Fig. 9. Expeimental setup. The line voltages and cuents of the induction moto ae plotted in Fig. 10. Since the switching fequency of the invete is kept constant the line voltages and cuents ae in sinusoidal wavefom and thei hamonic components ae (a) (b) Fig. 10. Thee phase line voltages of induction moto (a); Thee phase moto cuents (b). Fig. 11. Vaiable speed opeation of poposed system. 11
VII. CONCLUSIONS In this study a thee phase induction moto dive supplied fom PV is poposed to geneate the mechanical enegy fo applications such as ventilation efigeation and aspiation. The poposed dive is capable of online MPPT of the PV panel without any additional convete. Combination of CV and vaiable stepped IC method is used as the MPPT algoithm. The step size of the IC method is vaied accoding to absolute value of dp/dv. So fast stating esponse of the constant voltage and stability and accuacy of the IC method ae integated. The MPPT algoithm geneates speed efeence and V/f speed contolle contols the invete output voltage and fequency. Since the switching fequency is kept constant fo all opeation conditions; the quality of the output voltage wavefom is impoved especially at low speed. Both vaiable speed dive and MPPT functions ae caied out by the thee-level voltage souce invete. The efficiency of the poposed dive is measued as 94.7 %. 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