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- 1. IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 9, SEPTEMBER 2015 4809 A Single-Stage Photovoltaic System for a Dual-Inverter-Fed Open-End Winding Induction Motor Drive for Pumping Applications Sachin Jain, Athiesh Kumar Thopukara, Ramsha Karampuri, and V. T. Somasekhar, Member, IEEE Abstract—This paper presents an integrated solution for a pho- tovoltaic (PV)-fed water-pump drive system, which uses an open- end winding induction motor (OEWIM). The dual-inverter-fed OEWIM drive achieves the functionality of a three-level inverter and requires low value dc-bus voltage. This helps in an optimal arrangement of PV modules, which could avoid large strings and helps in improving the PV performance with wide bandwidth of operating voltage. It also reduces the voltage rating of the dc-link capacitors and switching devices used in the system. The proposed control strategy achieves an integration of both maximum power point tracking and V/f control for the efﬁcient utilization of the PV panels and the motor. The proposed control scheme requires the sensing of PV voltage and current only. Thus, the system re- quires less number of sensors. All the analytical, simulation, and experimental results of this work under different environmental conditions are presented in this paper. Index Terms—Centrifugal pump, dual-inverter, maximum power point tracking (MPPT), open-end winding induction mo- tor (OEWIM), photovoltaic (PV) cell. I. INTRODUCTION ELECTRICAL motors constitute more than 40% of total electric power consumption [1]. Modernization of human society and growing applications of electric motors have expo- nentially increased the demand for electrical energy. This forces an increase in the power generation capacity. However, due to ecological concerns, restriction and constraints are imposed on increasing the generation capacity of conventional sources. So, contemporary research is focused toward an effective utiliza- tion of nonconventional energy sources. Among the available nonconventional sources, photovoltaic (PV) technology seems to be the most promising and attractive. This can be attributed to declining cost of PV modules, free energy source, zero main- tenance, and noise free operation. Thus, employment of a PV source for powering electric motor could be a good solution especially for water-pumps, electric fans, submersible pumps, etc. Such loads can have the option of optimally using PV power whenever Sun power is available [2]. Further, when such loads are used in the stand-alone system like water pumping application in domestic, agricultural, and Manuscript received May 23, 2014; revised August 25, 2014; accepted Octo- ber 14, 2014. Date of publication October 30, 2014; date of current version April 15, 2015. Recommended for publication by Associate Editor C. A. Canesin. The authors are with the Department of Electrical Engineering, Na- tional Institute of Technology, Warangal 506004, India (e-mail: jsachin@ nitw.ac.in; athiesh.44@gmail.com; ramsha_k@nitw.ac.in; vtsomasekhar@ rediffmail.com). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TPEL.2014.2365516 industrial sectors, solar PV powered system could be a good solution. It could meet the requirement during critical situation, i.e., during summer especially in tropical countries like India. This encourages the use of electric motor-pump with better performance and efﬁciency with the PV system [3]. Some possible solutions given for PV-fed water-pump were based on usage of dc motor either directly coupled [4] or via a dc–dc converter [5] with the PV source. However, the require- ment of continuous maintenance and higher cost restricts the use of dc motors for their application in PV water pumping sys- tems [3]. Thus, there is a need of such a solution that uses the PV power effectively, while using a low cost, low maintenance, reliable, and robust motor for pumping application. The most suitable motor for such an application is an induction motor (IM). Some of the initial proposals for a PV-fed IM, based on the two-stage system, were given by various authors in the past. Most of them have used a dc–dc boost converter in the ﬁrst stage and the second stage comprises of a dc–ac inverter. Boost converter ampliﬁes and operates the low value PV input voltage near maximum power point (MPP) while the inverter gives the required ac voltage to the IM. Also, the control techniques are based on either independent frequency control [2], or a V/f con- trol [6]. Recently, a proposal based on closed-loop speed control to improve the performance and efﬁciency of the system was given by Alves Vitorino et al. [7], in which the authors have presented a sensor-less speed control technique. However, two- stage power conversion, high voltage rating of semiconductor devices, and more number of sensors increase the power loss and cost of the system. A typical PV pumping system with two power conditioning stage is shown in Fig. 1(a). As discussed earlier, this system results in poor performance and lesser efﬁciency. Therefore, a single-stage system with simpler control, as shown in Fig. 1(b), could be a better choice [8]–[13]. One of such a system was proposed by Muljadi [14], in which the authors have used a six- step quasi-square wave inverter, which can take care of dc–ac inversion as well as maximum power point tracking (MPPT) for the PV source. It is a better proposition since the author has given an integrated single-stage power conversion solution. But the drawback of this system is that the six-step quasi-square inverter deteriorates the motor performance and hence the efﬁciency. It also require ﬁlters, which are bulky and expensive. Further, this system requires a higher voltage rating for the input dc-link capacitor and semiconductor devices. All these may increase the cost, weight, size, and power loss of the system. 0885-8993 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.
- 2. 4810 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 9, SEPTEMBER 2015 Fig. 1. Block diagram of PV-powered centrifugal pump. (a) Conventional two stage. (b) Conventional single stage. (c) Proposed single stage. Recently, Makhlouf et al. [15] have suggested the closed-loop vector control for the PV pumping system using a single-stage multilevel pulse width modulation (PWM) converter. While this system results in low motor losses, it is more complex with more number of control variables and sensors. Most recently, Perp´etuo Corrˆea et al. [16] have published a paper with a stand- alone PV pumping system. It is also a single-stage system, wherein the authors have proposed MPPT and minimum losses point tracking methods of control. However, this system requires more number of sensors, and results in low bandwidth for PV operating voltage. Thus, from the previous discussion, it is evident there is a requirement for a low-cost, low-voltage single-stage power con- version PV water pumping system with wide bandwidth of PV operating voltage. Also, the three-level/multilevel inverter with a low dc-bus voltage could be a better solution as the perfor- mance of this system improves with the increased number of levels/steps in the inverter output voltage. Also, many investi- gations on open-end winding induction motor (OEWIM) with multilevel inverters are documented in the literature [17]–[21]. Further various PWM techniques and control schemes [22]–[27] for OEWIM drive have been analyzed by various researchers. In short, OEWIM promises to provide effective solutions for drive application as compared to common neutral IM [20]. Apart from high reliability and redundancy [23], [27], OEWIM has many good features as discussed in the next paragraph. One of the most important feature of OEWIM is it uses two two-level voltage-source inverters (VSIs) to achieve three-level inversion [28]. It also lowers voltage rating of semiconduc- tor devices and input dc-bus electrolytic capacitor [17]. This can be attributed to the fact that dc-bus voltage is designed based on maximum value of phase voltage instead of line-to-line voltage. The decoupled PWM technique increases the apparent switching frequency of switching devices, reducing the ripple in the motor phase current. Unlike the conventional three-phase neutral point-clamped three-level inverter, wherein the dc-link capacitor is constrained to carry load current leading to large ﬂuctuations of voltage of the dc-neutral point, the dc-link ca- pacitor of OEWIM carries only the ripple current, thus resulting a negligible voltage ﬂuctuations [23]. This increases the life and reliability of the dc-bus capacitor or in other words the relia- bility of the inverter system, which is of paramount interest for systems connected to the PV source. Thus, OEWIM coupled with the PV source could be a good proposition. This paper presents one such solution for a simple water-pump application as depicted in Fig. 1(c). The proposed system has the following features: 1) it is an economical system as it uses a single power con- ditioning stage; 2) it has inherent low dc-bus voltage requirement with V/f control integrated with MPPT and uses three-level (dc– ac) inversion for better performance of motor; 3) low input dc-bus voltage requirement reduces the voltage rating of dc-link capacitor and increases bandwidth of PV operating voltage, it also reduces the voltage rating of the semiconductor devices used in the inverter; 4) it optimally uses the PV source for all environmental con- ditions by operating it at MPP, also, it employs V/f control integrated in the MPPT algorithm which improves the per- formance of the motor and requires less number of sensors for its operation; 5) three-level output with the decoupled sample-averaged zero-sequence elimination (DSAZE) algorithm [24] fur- ther improves the performance with reduced motor current ripple. Rest of this paper is divided into four sections. Section II gives details of modeling of the proposed system. Section III describes the operation and analysis of the proposed system. Section IV describes the control strategy and algorithm proposed. Section V depicts the simulation and experimental results obtained. It also gives the detailed cost analysis and comparison for the proposed system with the existing system(s). II. MODELING OF THE PROPOSED SYSTEM A proposed conﬁguration of the solar PV-powered pumping system is shown in Fig. 2, which comprises of: 1) solar PV array; 2) dual-inverter namely Inverters I and II; 3) three-phase OEWIM with pump load; and 4) controller block which consists of MPPT and DSAZE PWM algorithm. These components are described in detail in the following sections. A. PV Source Model The PV source was modeled by using PV cell current–voltage characteristic equation as follows [29]: ipvcell = iL − i0 e q (v p v c e ll + i p v c e ll R s ) n k T − 1 (1)
- 3. JAIN et al.: SINGLE-STAGE PHOTOVOLTAIC SYSTEM FOR A DUAL-INVERTER-FED OPEN-END WINDING INDUCTION MOTOR DRIVE 4811 Fig. 2. Schematic circuit diagram of the proposed system. where ipvcell is PV cell current, iL is photocurrent, i0 is diode saturation current, n is diode quality or ideality factor, k is Boltz- mann constant, q is electron charge, T is panel operating tem- perature in Kelvin, Rs is PV cell series resistance, and vpvcell is PV cell voltage (V). The output of PV source is connected to inverter with dc-bus capacitance Cpv . By applying KCL at input of the inverter [from Fig. 2] ipv = ic + iinv ⇒ ipv = Cpv dvpv dt + iinv . (2) Integral solution of (2) is the voltage vpv across capacitance Cpv , which is used by the PV model to calculate the PV source current. The inverter current iinv is the current drawn by In- verters I and II. Further dual inverter has two series connected equal value capacitors across the dc link. These capacitors share equal voltage with respect to the common point “o” as shown in Fig. 2. B. Modular Three-Level Dual-Inverter Model Dual inverter used in the proposed conﬁguration is modeled using switching functions [30]. To model dual inverter, switch- ing function SW (where W can be A, B, C, A’, B’ or C’ depending on the phase and number of inverter) requires the logic gener- ated from the PWM controller. It has value 1 and –1 which represents turn ON of top and bottom switch, respectively, for the given leg or phase of the inverter. The modular dual inverter shown in Fig. 2 consists of six poles (a, b, c, a’, b’, and c’) and 12 switches (two switches per pole). The value of the pole voltages in a particular phase can be ±Vpv /2 depending on the switch (whether top or bottom) is turned ON. If top switch of phase “a”, S1 is turned ON, the pole voltage vao is +Vpv /2 and when bottom switch of phase “a”, S4 is turned ON, then the pole voltage vao is −Vpv /2. Thus, pole voltage of Inverter I can be given as vao = SA Vpv 2 ; vbo = SB Vpv 2 ; vco = SC Vpv 2 . (3) Fig. 3. OEWIM model. Similarly, pole voltage of Inverter II can be given as va o = SA Vpv 2 ; vb o = SB Vpv 2 ; vc o = SC Vpv 2 . (4) The motor phase voltage vaa is given by vaa = Vpv 2 2 3 (SA − SA ) − 1 3 ((SB − SB ) + (SC − SC )) . (5) Similarly, the other phase voltages vbb , vcc of the inverter output can be derived for the system. Further, the input inverter current iinv can also be derived using switching functions. Cur- rent ﬂowing through Inverter I is given by iinv1 = 1 2 (SA + 1)iaa + 1 2 (SB + 1)ibb + 1 2 (SC + 1)icc . (6) Current ﬂowing through the Inverter II is given by iinv2 = 1 2 (SA + 1)(−iaa ) + 1 2 (SB + 1)(−ibb ) + 1 2 (SC + 1)(−icc ). (7) The net current ﬂowing through the dual inverter is iinv = 1 2 iaa (SA − SA ) + 1 2 ibb (SB − SB ) + 1 2 icc (SC − SC ). (8) Thus, the previous values of phase voltage and current can be used by the Simulink model of OEWIM, which is discussed in the next section. C. OEWIM Model An OEWIM [28] is obtained by opening the neutral point of the star-connected stator windings of a normal three-phase IM. The winding diagram of three-phase OEWIM is shown in Fig. 3. For modeling and analysis, the decoupled form of OEWIM is considered. For transforming the stator (φ = θ) and the ro- tor parameters (φ = β) to decoupled form, the transformation
- 4. 4812 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 9, SEPTEMBER 2015 matrix used is given as follows: ⎡ ⎢ ⎣ xqy xdy x0y ⎤ ⎥ ⎦ = 2 3 ⎡ ⎢ ⎣ cos φ cos (φ − 2π/3) cos (φ + 2π/3) sin φ sin (φ − 2π/3) sin (φ + 2π/3) 1/2 1/2 1/2 ⎤ ⎥ ⎦ ⎡ ⎢ ⎣ xaa xbb xcc ⎤ ⎥ ⎦ (9) whereθ is theanglebetweenthestator as-axis andthequadrature (q) axis, β is the angle between rotor ar-axis and the q-axis, also β = θ − θr , θr is the angle between rotor ar-axis and stator as- axis (see Fig. 3), parameter x can be either voltage “v” or current “i” or ﬂux linkage “λ” and subscript parameter y can be “s” or “r.” The subscript “s” denotes the parameters of stator and the subscript “r” denotes the parameters of rotor. The dynamic d–q model of an OEWIM is described by vqs = Rsiqs + ω(Llsids + Lm (ids + idr )) + pλqs (10) vds = Rsids − ω(Llsiqs + Lm (iqs + iqr )) + pλds (11) vqr = Rr iqr + (ω − ωr )(Llr idr + Lm (ids + idr )) + pλqr (12) vdr = Rr idr − (ω − ωr )(Llr iqr + Lm (iqs + iqr )) + pλdr (13) where Rr is rotor resistance, Rs is stator resistance, Lls is sta- tor leakage inductance, L’lr is rotor leakage inductance, Lm is mutual inductance between stator and rotor winding, ω is the synchronous speed, ωr is the electrical speed of motor, and p denotes the time derivative. Also, here v qr = v dr = 0, since rotor bars are short-circuited. The expression for the electromagnetic torque Tem is given by Tem = 3 2 P 2 Lm [iqsidr − idsiqr ] (14) where P is the number of poles. The mechanical equation governing the OEWIM-pump drive is expressed as follows: Tem = J dωrmech dt + Bω2 rmech + TL (15) where J is motor inertia (kg · m2 ), B is centrifugal load torque coefﬁcient, TL is load torque (N · m), and ωrmech is instantaneous angular velocity of motor shaft (rad/s). III. OPERATION AND ANALYSIS OF THE PROPOSED SYSTEM The proposed dual inverter is operated by using the decou- pled PWM strategy. It incorporates simple V/f control for the efﬁcient operation of system below the rated speed. The pro- posed PWM strategy requires the information of magnitude and angle of the reference voltage vector. The magnitude is calcu- lated and controlled by the MPPT algorithm and the angle “α” is the function of time and fundamental frequency of reference Fig. 4. Demonstration and comparison of dc-bus voltage requirement for H- bridge and dual-inverter systems. (a) Schematic circuit diagram of the two-level H-bridge inverter with input dc voltage (PV voltage) of “Vpv .” (b) Space vector locations of voltage vector obtained from two-level inverter. (c) Schematic circuit diagram of dual-inverter-fed OEWIM drive with input dc voltage (PV voltage) of “Vpv /2.” (d) Space vector locations of voltage vector obtained from three-level dual-inverter scheme. modulating waveform. The reference voltage vector |vsr|ࢬα is further divided into two decoupled components |vsr|/2 ࢬ α and |vsr|/2 ࢬ (180 + α). The decoupled components are then given as the reference vector for Inverters I and II, respectively, as shown in Fig. 2, for generation of required output voltage. Thus, using decoupled PWM conﬁguration has the beneﬁt of double output voltage. A. Low Input DC-Bus Voltage Requirement of Dual Inverter for OEWIM-Pump Drive To analytically verify the low input dc-bus voltage require- ment, a comparison between two-level and dual-inverter-fed OEWIM is done. Both the inverters are compared for generat- ing the same output voltage vector with different values of input PV source voltage. The low input voltage requirement of dual inverter for an OEWIM drive is demonstrated in Fig. 4. Let the PV source voltage required to generate the rated in- stantaneous IM phase voltage van is Vpv as shown in Fig. 4(a). From Fig. 4(a) and (b), the inverter output voltage, van , vbn , and vcn can be obtained using the inverter pole voltage, vao, vbo , and vco; and switching functions SA , SB , and SC as follows: van = vao − vno = 2 3 SA − 1 3 (SB + SC ) Vpv 2 (16) where vno is common-mode voltage given by vno = 1 3 (vao + vbo + vco) = 1 3 (SA + SB + SC ) Vpv 2 . (17) Thus, the space vector location of reference voltage vector −→ OA [see Fig. 4(b)] can be generated by the switching functions SA = 1, SB = −1, and SC = −1. Substituting these values into (17) will result in the phase voltage, van as 2Vpv /3. Now, consider the three-phase, three-level dual inverter con- nected to an OEWIM as shown in Fig. 4(c). Let the input PV source voltage is Vpv /2, which is half of the voltage taken for two-level H-bridge inverter. Now, the dual inverter output phase
- 5. JAIN et al.: SINGLE-STAGE PHOTOVOLTAIC SYSTEM FOR A DUAL-INVERTER-FED OPEN-END WINDING INDUCTION MOTOR DRIVE 4813 voltage −−→ OG [see Fig. 4(d)] is given as vaa = vao − va o − voo (18) where voo is the common mode voltage [see Fig. 4(c)] which is given by voo = 1 3 Vpv 4 [(SA − SA ) + (SB − SB ) + (SC − SC )]. (19) Therefore, vaa is given by vaa = 2 3 (SA − SA )− 1 3 [(SB − SB ) + (SC − SC )] Vpv 4 . (20) So, to generate voltage vector −−→ OG [see Fig. 4(d)], the switch- ing functions required are SA = 1, SB = −1, and SC = −1; SA = −1, SB = 1, and SC = 1. Substituting these values in (20), results in the phase voltage of magnitude 2Vpv /3 corre- sponding to phase aa’. Hence, to generate the phase voltage of 2Vpv /3, the PV source voltage required in case of two-level inverter is Vpv and in case of dual inverter connected to OEWIM is Vpv /2. However, the DSAZE PWM technique [22] needs excess 15% of dc-link voltage to generate the rated motor phase voltage. IV. CONTROL STRATEGY AND MPPT ALGORITHM The solar PV-powered-fed dual inverter connected to OEWIM-pump drive is operated using a simple control strategy, which simultaneously accomplishes MPPT and DSAZE PWM integrated together. This integrated algorithm generates the re- quired PWM control signals for the modular dual inverter. The ﬂowchart of control algorithm is shown in Fig. 5. The MPPT part of algorithm facilitates motor-pump drive to extract maximum available power from the PV source, thereby assuring effective utilization of the PV source. The DSAZE PWM part of the algo- rithm incorporates V/f control. It maintains constant rated ﬂux in the motor which retains the maximum torque capability of the machine for the given PV power. Thus, in pump drive applica- tion where torque is proportional to square of speed, maintaining the maximum torque further helps in maximum utilization of the input source. A. MPPT Algorithm One of the simplest, most popular, and commercially used methods of MPPT, namely, the Hill Climbing algorithm [31] is employed in the proposed system. The algorithm ﬁrst senses the voltage (vpv ) and current (ipv ) of PV array for calculating the power (ppv ). The calculated and sensed PV power and voltage are then compared with their previous value to determine the slope of the operating point. The considered slope then deter- mines the correction for modulation index (ma). With respect to sign of the slope (negative or positive), the value of ma is modi- ﬁed (incremented or decremented) as described in the ﬂowchart shown in Fig. 5 until the operating point reaches near MPP. The calculated value of ma is then used by the DSAZE PWM algorithm. Fig. 5. Flowchart of the MPPT, DSAZE algorithm used in the proposed scheme. B. DSAZE PWM Algorithm The magnitude of the reference voltage vector generated by the MPPT output and angle “α” is decomposed into the in- stantaneous three-phase reference voltage vas, vbs, and vcs for Inverter I. The gating time Tgj (j = a, b, c) for top switches of Inverter I (see Fig. 2) is then obtained by the switching algo- rithm [32]. This algorithm [32] requires instantaneous values of the reference phase voltage for calculating the effective time (time for which all the active vectors are switched) or turned ON time for top switches. The position of effective time period can be adjusted in such a way that the offset time is equal to Ts/2 within a switching period. This feature is exploited by the DSAZE PWM technique to eliminate the zero sequence voltage within a sampling period. As DSAZE is a center-spaced PWM technique for the dual- inverter system, the ripple content in current is less and hence results in the improvement of developed torque. The other
- 6. 4814 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 9, SEPTEMBER 2015 Fig. 6. Simulation results showing waveforms at PV source side. advantage of the DSAZE PWM algorithm for OEWIM drive is that it requires less memory and computing time for process- ing. As both the reference voltage vectors for Inverters I and II are in the phase opposition, the gating time Tgj (j = a, b, c) for the top switches of Inverter II are directly obtained using the gating time for Inverter I as follows: Tgj = Ts − Tgj (21) where j = a, b, c and Ts is the inverter switching time period(s). Complement of the respective gating signals for Inverters I and II are generated for bottom switches for both inverters. V. SIMULATION AND EXPERIMENTAL RESULTS WITH COST ANALYSIS A. Simulation Results The single-stage PV-powered OEWIM drive for pump appli- cation, shown in Fig. 2, is simulated using MATLAB/Simulink. A 3.6 kW (20 × 3) PV array feeding power to a 4 kW OEWIM-pump drive is considered for simulation. The Solarex MSX60 PV panel parameters under standard test conditions are Voc = 21 V, Isc = 3.74A, Vm = 17.1 V, Im = 3.5 A, and Pm = 59.9 W. A dc-bus capacitor (Cpv ) of value 1000 μF is used at the output of PV source. A four-pole, 400 V, 1430 r/min IM is used. Further the important parameters of the mo- tor are Rs = 1.405 W, Rr = 1.395 W, xls = xlr = 1.8344 W, and xm = 54.0982 W. Simulations are performed by consider- ing 96 samples per cycle of applied fundamental voltage, irre- spective of the modulation index. This means that the switching frequency is a variable quantity. The switching frequency varies from 1.28 kHz (corresponding to a modulation index, ma = 0.2 with fundamental frequency f = 13.33 Hz) to 4.8 kHz (corre- sponding to a modulation index, ma ≥ 0.75 with fundamental frequency f = 50 Hz). Fig. 6 shows the simulation results of the proposed system under different environmental conditions for PV source-side parameters. The increasing and decreasing nature of PV power with respect to insolation (G) and temperature (T) can be ob- served with the waveforms of PV power and ma . This veriﬁes the MPP tracking, further small oscillations in the value of ma near Fig. 7. Simulation results showing waveforms at motor-pump side. MPP and small ripple content in a PV power conﬁrms the oper- ation near optimum voltage. Also, another useful observation in the simulation results is that the operating voltage of PV array passes through optimum voltage for every step increase in inso- lation and temperature. This can be justiﬁed with the matching values of peak power value during transient tracking and steady state near MPP as given in the PV power subplot. Further, it can be noted that PV voltage waveform shows a sudden rise and fall in the value with the step increase in insolation and temperature. This can be attributed to charging and discharging of PV capac- itor CP V with excess or deﬁcit PV power, respectively, during transient condition. Fig. 7 shows the motor-side parameters for different environ- mental conditions. Variation on ma with respect to PV power can also be seen here in the waveform of torque, speed, and mechanical power output. It can be observed that torque, speed, and mechanical power output follow ma or the PV power indi- rectly. Also, it can be easily observed that slip power too follows ma. Thus, slip power loss increases and decreases with increase and decrease of PV power, respectively. This may help in keep- ing small variation in efﬁciency which can be observed from last subplot of Fig. 7. Also, another important feature that re- late results of Figs. 6 and 7 is motor phase voltage plot. The transient variations in PV voltage Vpv during step increase in insolation can be depicted with a peak value of motor phase voltage waveform. Another typical observation between Figs. 7 and 8 is the rip- ple contents in the torque waveform and motor phase current follows ma. It can be observed that torque ripple is more dur- ing lower insolation and decreases as insolation increases. This
- 7. JAIN et al.: SINGLE-STAGE PHOTOVOLTAIC SYSTEM FOR A DUAL-INVERTER-FED OPEN-END WINDING INDUCTION MOTOR DRIVE 4815 Fig. 8. Harmonic spectrum of motor phase current (iaa ) at steady state ob- tained from simulation under different environmental conditions: (a) at 0.1 insolation and 25 °C; (b) at 0.6 insolation and 35 °C; (c) at 1.0 insolation and 55 °C. can be attributed to the fact that the increase and decrease of ripple content in the motor phase current will result in the in- crease and decrease of the torque ripple, respectively. Further, it can be noted that the amplitude of motor phase current also follows ma and total harmonic distortion (THD) of motor phase current decreases with increase in insolation. Thus, in short, system performance improves with increase in PV power. This can be observed in Table I, where the simulation results are summarized. B. Experimental Results To verify simulation results, a prototype of the proposed sys- tem is developed. To emulate PV characteristics, a dc power supply with ﬁxed series resistance is used. The power sup- ply is operated in constant voltage mode with variable cur- rent limit. The three-phase OEWIM with speciﬁcations 440 V, TABLE I SUMMARY OF SIMULATION RESULTS FOR THE PROPOSED SYSTEM G Suns T (°C) PV power (W) O/P power (W) η (%) Speed (r/min) Torque (N·m) Slip (%) Slip power (W) Current THD (%) 0.1 25 329 240 72.95 352 6.5 4.74 12 11.94 0.3 30 1040 895 86.06 740 11.6 3.62 33.5 8.58 0.6 35 2133 1902 89.17 1026 17.7 3.8 75.25 5.06 0.8 45 2763 2465 89.21 1139 20.65 4.24 109.25 4.28 0.9 50 3072 2735 89.03 1187 22 4.46 128 3.97 1 55 3365 2990 88.86 1229 23.25 4.7 147.5 3.77 Fig. 9. DSAZE PWM modulating waveforms for leg a and a’ of Inverters I and II: (a) from simulation (Vpv ≈ 320 V and ma = 0.64), (b) from experimental setup (Vpv ≈ 110 V and ma = 0.5) (x-axis: simulation time (s); y-axis: gate time (s)). Fig. 10. α–β plot of three-phase vo1tage input to OEWIM obtained from: (a) simulation (Vpv ≈ 320 V and ma = 0.64) and (b) experimental setup (Vpv ≈ 110 V and ma = 0.5). 1.5 A, 1440 r/min, and 1 hp is used with fan type of cen- trifugal load which guarantee the operation for water-pump load proﬁle. The motor is fed from the dual inverter which uses Semikron insulated-gate bipolar transistor (IGBT) mod- ule (SKM75GB128DN). Gate pulses required by IGBT module is generated from dSPACE 2201 and is given via a driver IC (HCPL 316J). The control algorithm implemented in dSPACE includes the MPPT and V/f control with DSAZE PWM for the OEWIM coupled to a centrifugal torque load. This control al- gorithm can also be implemented using a low-cost Freescale semiconductor made MC56F8013 signal controller. For track- ing MPP of the source, a voltage sensor (LV25-P, 600 V rms, 10/25 mA) to sense the terminal voltage and a current sensor (Tektronix A622, 0–70 A rms, 100 mV/A) to sense the PV cur- rent are used. The sensed voltage and current are given to the computing system through the ADC’s of dSPACE. The comput- ing system internally generates the value of ma required for the
- 8. 4816 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 9, SEPTEMBER 2015 Fig. 11. Experimental results for the proposed system on the dc source side, i.e., source voltage, current, power, and the modulation index: (a) under starting condition, (b) under running condition, and (c) for various emulated environmental conditions. DSAZE PWM algorithm to calculate Tgj and Tgj for Inverters I and II, respectively (see Fig. 5). Fig. 9 gives the waveform for Tga and Tga obtained from simulation and experimental setup. The complementary nature of Tga and Tga can be observed in the waveforms. This veri- ﬁes the utilization of the decoupled PWM strategy. Further, as proved decoupled PWM also reduces the required value of dc- bus voltage which can be depicted from Fig. 10. In this ﬁgure, the plots of phase voltage vector in αβ plane obtained from simulation and experimental results were shown. Requirement of low dc-bus voltage can be easily veriﬁed with the given input values of ma and input PV voltage of the inverter as mentioned in Fig. 10(a) and (b). The synchronization of change in ma after one complete cycle can be depicted from zoom-in part of Fig. 10(a). If this synchronization between MPPT and V/f control is disturbed in the ﬁeld, then it may result in a poor per- formance of the system. This can be taken care of by assuring proper synchronization between MPPT and V/f control under all ﬁeld conditions. The other drawback in the ﬁeld may be the inverter failure. If an inverter fails, a rewiring would enable the healthy inverter to deliver partial load. In other words, the fault tolerance with OEWIM is more compared to the conventional two-level VSI drive. Also, the power cable required to connect the OEWIM and inverter is double compared to the conventional two-level VSI drive. However, this drawback in the ﬁeld can be taken care by minimizing the distance between inverter and the motor. Fig. 11 gives the experimental results for the parameters at the input side of inverter. Increasing value of modulation index and input power conﬁrms the MPP tracking which can be seen in Fig. 11(a). Oscillations near MPP can be observed in Fig. 11(b). More number of oscillations in ma near MPP can be attributed to sharp power–voltage characteristics of input source and the algorithm used. However, operation near MPP can be observed in the waveform of source power. Also, to emulate various en- vironmental conditions in the PV system, the current limit of programmable power supply is changed from 0.3 to 0.5 A and again back to 0.3 A. This would nearly emulate the increasing and decreasing insolation in the PV array system. Fig. 11(c) shows experimental results obtained from developed test setup for this condition. The increasing and decreasing value of ma Fig. 12. Experimental results for the proposed system at motor-pump side, i.e., motor phase voltage and current: (a) under starting condition and (b) under running condition. Fig. 13. Motor aa’—phase current and its harmonic spectrum: (a) under starting condition and (b) under running condition. and source power shows MPP tracking for various environmen- tal conditions. As fan type load has approximately similar char- acteristics to centrifugal water pump load, so nearly the same performance is accepted with actual water pump load systems. Fig. 12 shows the experimental results for the parameters at the inverter output. Implementation of V/f control can be veri- ﬁed by variable frequency operation under starting and running conditions. At starting when the value of ma is low, the operating frequency is kept low and near MPP when ma value becomes high frequency of phase voltage also increases. The motor “aa” phase current waveform along with its harmonic spectrum under starting and running conditions is shown in Fig. 13(a) and (b), respectively. The harmonic spectrum under starting and running conditions is comparative with the harmonic spectrum of cur- rent at low and high insolation (see Fig. 8), respectively. Another important thing is that the typical pattern of ripple in the cur- rent waveform shown in Fig. 13 is matching with the simulation results given in Fig. 8.
- 9. JAIN et al.: SINGLE-STAGE PHOTOVOLTAIC SYSTEM FOR A DUAL-INVERTER-FED OPEN-END WINDING INDUCTION MOTOR DRIVE 4817 TABLE II COST COMPARISON BETWEEN THE PROPOSED SYSTEM WITH TWO BASIC TOPOLOGIES AVAILABLE IN THE LITERATURE FOR PUMP APPLICATION Item / part no. Using conventional induction motor Using OEWIM 2-stage 2-level [3] 1-stage 2-level [14] 1-stage 3-level [proposed] Controller requirements PWMs PWMs PWMs 4 3 6 ADCs ADCs ADCs 3 2 2 Flash / RAM Flash / RAM Flash / RAM Medium Less Less Controller MC56F8013 MC56F8013 MC56F8013 $ 4 $ 4 $ 4 Voltage Sensors/ LV 25-P 2 1 1 $ 120 $ 60 $ 60 Current Sensors/ LTSR 15-NP 1 1 1 $ 23.68 $ 23.68 $ 23.68 Miscellaneous $ 20 $ 10 $ 20 Sub-total $167.68 $97.68 $107.68 INVERTER DC- link capacitors (2200uH, 400V)/ ALS30A222MF400 4 4 2 $ 155.84 $ 155.84 $ 77.92 Semiconductor Switches∗ (IGBT’s) 6 (900V, 28A)/ IRG4PF50WPBF 6 (900V, 28A)/ IRG4PF50WPBF 12 (400V, 20A)/ IRGB14C40LPBF $ 31.68 $ 31.68 $ 39.36 Driver circuit $ 26 $ 26 $ 52 Sub-total $213.52 $213.52 $169.28 FRONT-END CONVERTER Inductor/ DLFL-0147–12D5 1 NA NA $ 25.6 — — Capacitor/ ALS30A222MF400 3 NA NA $116.88 — — Semiconductor switch∗/ IRG4PH40KDPBF 1 NA NA $ 5.44 — — Diode/ D22–25–12 1 NA NA $ 6.16 — — Driver circuit $ 5 NA NA Sub-total $ 159.08 — — TOTAL $540.28 $311.20 $276.96 ∗ Switches are chosen with respect to the availability of the devices with nearest rating. C. Cost Analysis A brief cost analysis has been carried out (see Table II) for the three topologies presented in Fig. 1. The cost values presented in Table II are only for the inverter and converter. The IM-pump and the PV panel costs are assumed to be nearly similar (since the same power rating is considered) for all the three topologies. When compared to a two-stage system, the proposed one is a low cost because of the nonrequirement of an extra dc–dc converter. Also, the lower cost of the proposed system compared to the conventional single-stage two level is because of the requisition of low-voltage dc-link capacitors and switching devices. VI. CONCLUSION In this paper, an integrated single-stage solution of PV-fed pump drive is presented. The proposed system has the feature of low dc-bus voltage requirement, MPPT integrated with V/f, three-level inverter operation and low cost. Analytical proof for low dc-bus voltage requirement in the proposed system is pre- sented. Implementation of V/f with MPPT can be veriﬁed with simulation and experimental results. Thus, a high-performance integrated solution is proposed. Low cost of the proposed sys- tem can be attributed to the requirement of low voltage dc-link bus capacitor, low voltage rating switches, and less number of sensors for the integrated control operation which is presented in Table II. Thus, in all, this paper presents one of the effective and simple solutions for PV power-fed water-pump application. REFERENCES [1] D. Maheswaran, K. K. Jembu Kailas, V. Rangaraj, and W. Adithya Kumar, “Energy efﬁciency in electrical systems,” in Proc. IEEE Int. Conf. Power Elect. Drives Energy Syst., Dec. 2012, pp. 1–6. [2] Y. Yao, P. Bustamante, and R. S. Ramshaw, “Improvement of induction motor drive systems supplied by photovoltaic arrays with frequency con- trol,” IEEE Trans. Energy Convers., vol. 9, no. 2, pp. 256–262, Jun. 1994. [3] J. V. Mapurunga Caracas, G. de Carvalho Farias, L. F. 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[26] J. Ewanchuk, J. Salmon, and C. Chapelsky, “A method for supply voltage boosting in an open-ended induction machine using a dual inverter system with a ﬂoating capacitor bridge,” IEEE Trans. Power Electron., vol. 28, no. 3, pp. 1348–1357, Mar. 2013. [27] Y. Wang, D. Panda, T. A. Lipo, and D. Pan, “Open-winding power con- version systems fed by half-controlled converters,” IEEE Trans. Power Electron., vol. 28, no. 5, pp. 2427–2436, May 2013. [28] H. Stemmler and P. Guggenbach, “Conﬁgurations of high-power voltage source inverter drives,” in Proc. 5th Eur. Conf. Power Electron. Appl., 1993, pp. 7–12. [29] G. Walker, “Evaluating MPPT converter topologies using a MATLAB PV model,” J. Elect. Electron. Eng., vol. 21, no. 1, pp. 49–56, 2001. [30] B.-k. Lee and M. Ehsami, “A simpliﬁed functional simulation model for three-phase voltage source inverter using switching function concept,” IEEE Trans. Ind. Electron., vol. 48, no. 2, pp. 309–321, Apr. 2001. [31] S. Jain and V. Agarwal, “Comparison of the performance of maximum power point tracking schemes applied to single-stage grid-connected pho- tovoltaic systems,” IET Electr. Power Appl., vol. 1, no. 5, pp. 753–762, 2007. [32] J..-S. Kim and S.-K. Sul, “A novel voltage modulation technique of the space vector PWM,” in Proc. Int. Power Eng. Conf., 1995, pp. 742–747. Sachin Jain is currently working as an Assistant Pro- fessor at the National Institute of Technology (NIT- W),Warangal, India. Before joining NIT-W, he was working as an Senior Design Engineer in the R&D Department of the Solar Energy Business Group of Schneider Electric at Bangalore. His research inter- ests include power electronics applications in non- conventional energy conditioning, power quality, and distributed generation. Athieshkumar T received the B.Tech. and M.Tech degrees from the VR Siddhartha Engineering Col- lege, and the National Institute of Technology (NITW), Warangal, India, respectively. He is currently working as an Engineer in VEM Technologies, Hyderabad. His areaof interests in- clude photovoltaic pumping systems, integration of renewable energy sources to power electronic con- verters, and switching-mode power supplies. Ramsha Karampuri received the B.Tech. and M.Tech degrees from S.R.I.T, J.N.T. University, and V.N.I.T., Nagpur, India, respectively. He is currently working toward the Ph.D. degree in electrical engi- neering from the National Institute of Technology (NITW), Warangal, India. His research interests include power electronics and drives, and application of power electronics to non-conventional energy conditioning. V. T. Somasekhar(M’11) received the Graduate de- gree from the Regional Engineering College Waran- gal (currently, the National Institute of Technology), Warangal, India, in 1988, the Postgraduate degree from the Indian Institute of Technology, Bombay, In- dia, in 1990, and the Doctoral degree from the Indian Institute of Science, Bangalore, India, in 2003. From 1990 to 1993, he was a Research and De- velopment Engineer with Perpetual Power Technolo- gies, Bangalore, and a Senior Engineer with Kirloskar Electric Company, Ltd., Mysore, India. Since 1993, he has been with the National Institute of Technology,Warangal, India, where he is currently a Professor. His research interests include multilevel inversion with open-end winding induction motors, ac drives, and pulse width-modulation strategies.

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