SINGLE-SWITCH VOLTAGE EQUALIZER USING MULTI-STACKED BUCK-BOOST CONVERTERS FOR PARTIALLY-SHADED PHOTOVOLTAIC MODULES

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1 SINGLE-SWITCH VOLTAGE EQUALIZER USING MULTI-STACKED BUCK-BOOST CONVERTERS FOR PARTIALLY-SHADED PHOTOVOLTAIC MODULES Author-1 MuppallaPavani PG Student N.I.E, Author-2 Ramesh Matta Asst.prof N.I.E,Macherla. Abstract This paper proposes a solar photovoltaic (SPV) array fed water pumping system utilizing a zeta converter as an intermediate DC-DC converter in order to extract the maximum available power from the SPV array. Controlling the zeta converter in an intelligent manner through the incremental conductance maximum power point tracking (INC-MPPT) algorithm offers the soft starting of the brushless DC (BLDC) motor employed to drive a centrifugal water pump coupled to its shaft. Soft starting i.e. the reduced current starting inhibits the harmful effect of the high starting current on the windings of the BLDC motor. A fundamental frequency switching of the voltage source inverter (VSI) is accomplished by the electronic commutation of the BLDC motor, thereby avoiding the VSI losses occurred owing to the high frequency switching. A new design approach for the low valued DC link capacitor of VSI is proposed. The proposed water pumping system is designed and modeled such that the performance is not affected even under the dynamic conditions. Suitability of the proposed system under dynamic conditions is demonstrated by the simulation results using MATLAB/Simulink software. Keywords SPV array, Zeta converter, INC-MPPT, BLDC motor, Electronic commutation. I. INTRODUCTION Drastic reduction in the cost of power electronic devices and annihilation of the fossil fuels in near future invite to use the solar photovoltaic (SPV) generated electrical energy for various applications as far as possible. Water pumping, a standalone application of the SPV array generated electricity is receiving wide attention now a days for irrigation in the fields, household applications and industrial usage. Although the several researches have been carried out in the area of SPV array fed water pumping, combining various DC-DC converters and motor drives, the zeta converter in association with the permanent magnet brushless DC (BLDC) motor is still unexplored to develop such kind of system. However, the zeta converter has been used in some other SPV based applications [1-4]. The merits of both the BLDC motor and zeta converter can contribute to develop a favorable SPV array fed water pumping system possessing the potential of operating satisfactorily under the dynamically changing atmospheric conditions. The BLDC motor has high reliability, high efficiency, high torque/inertia ratio, improved cooling, low radio frequency interference and noise and requires practically no maintenance [5-6]. On the other hand, a zeta converter exhibits following advantages over the conventional buck, boost, buck-boost converter and Cuk converter when employed in SPV based applications. Belonging to the family of buck-boost converters, the zeta converter can be operated either to increase or to decrease the output voltage. This property offers a boundless region for maximum power point tracking (MPPT) of the SPV array [7]. The MPPT can be performed with simple buck and boost converter if the MPP occurs within the prescribed limits. The aforementioned property also facilitates the soft starting of the BLDC motor unlike a boost converter which habitually step up the voltage level at its output, not ensuring the soft starting. Unlike a simple buck-boost converter, the zeta converter has a continuous output current. The output inductor makes the current continuous and ripple free. However, a small ripple filter may be required at the input to smoothen the input current. Although consisting of the same number of components as the Cuk converter, the zeta converter operates as non-inverting buck-boost converter unlike an inverting buck-boost and Cuk converter. This property obviates the requirement of associated circuits for negative voltage sensing hence reduces the complexity and probability of slow down the system response [8]. The merits of the zeta converter mentioned above are favorable for the proposed SPV array fed water pumping system. An incremental conductance (INC) MPPT algorithm [9-10] is used to operate the zeta converter such that the SPV array always operates at its MPP and the BLDC motor experience a reduced current at the starting. A three phase voltage source inverter (VSI) is operated by fundamental frequency switching for the electronic commutation of BLDC motor [6]. Simulation results using MATLAB/Simulink software is examined to demonstrate the starting, dynamics and steady state behavior of the proposed water pumping system subjected to the random variation in the solar irradiance. The SPV array is designed such that the proposed system always exhibits satisfactory performance regardless of the solar irradiance level or its variation. This paper is organized as follows. Configuration and operation of the proposed system are illustrated in section II and section III respectively. Section IV presents the design of the various stages of the proposed system. The control techniques used are briefly described in section V. Finally, the performance of the proposed system is evaluated using the simulated results in section VI followed by the concluding remarks in section VII.

2 II. CONFIGURATION OF THE PROPOSED SYSTEM The structure of the proposed SPV array fed BLDC motor driven water pumping system employing a zeta converter is shown in Fig. 1. As shown in Fig. 1, the proposed system consists of (left to right) the SPV array, the zeta converter, the VSI, the BLDC motor and the centrifugal water pump. The BLDC motor has an inbuilt encoder. The pulse generator is used to operate the zeta converter. The step by step operation of the proposed system is reported in the following section in detail. III. OPERATION OF THE PROPOSED SYSTEM The SPV array generates the electrical power demanded by the motor-pump system. This electrical power is fed to the motor-pump system via the zeta converter and the VSI. SPV array appears as the power source for the zeta converter as shown in Fig. 1. Ideally, the same amount of power is transferred at the output of zeta converter which appears as the input source for the VSI. In practice, due to the various losses associated with a DC-DC converter [11], slightly less amount of the power is transferred to feed the VSI. The pulse generator generates, through INC-MPPT algorithm, the switching pulse for the IGBT (Insulated Gate Bipolar Transistor) switch of the zeta converter. The INC-MPPT algorithm takes the voltage and current variables as feedback from SPV array and returns an optimum value of duty cycle. Further, the pulse generator generates actual switching pulse by comparing the duty cycle with the high frequency carrier wave. In this way, the maximum power extraction and hence the efficiency optimization of the SPV array is accomplished. On the other hand, VSI converting the DC power output from the zeta converter into the AC power feeds the BLDC motor to drive the centrifugal pump coupled to its shaft. The VSI is operated by the fundamental frequency switching availed by the so called electronic commutation of BLDC motor assisted by its built-in encoder. The high frequency switching losses are thereby eliminated, contributing in the effective and increased efficiency operation of the proposed water pumping system. IV. DESIGN OF THE PROPOSED SYSTEM The various operating stages shown in Fig. 1 are intellectually designed in order to develop an effective water pumping system, capable of operating under uncertain conditions. A BLDC motor of 2.89 kw power rating and the SPV array of 3.4 kw maximum power capacity under standard test conditions (STC) are selected to design the proposed system. The detailed design of the various stages such as the SPV array, the zeta converter and the centrifugal pump are described as follows. A. Design of SPV Array As per the discussion in section III, the practical converters are associated with the various power losses. In addition, the performance of the BLDC motor -pump is influenced by the mechanical and electrical losses associated with them. To compensate these losses, the size of SPV array is selected with slightly more maximum power capacity to ensure the satisfactory operation regardless of the power losses. Therefore the SPV array of maximum power capacity of P mpp = 3.4 kw under STC (STC: 1000W/m², 25 C, AM 1.5), slightly more than demanded by the motor-pump is selected and its parameters are designed accordingly. Sunmodule Plus SW 280 mono [12] SPV module made by SolarWorld is selected to design the SPV array of an appropriate size. Electrical specifications of this module are listed in Table I and the numbers of modules required to connect in series/parallel are estimated by selecting the voltage of the SPV array at MPP under STC as, V mpp = V. The current of the SPV array at MPP, I mpp is hence estimated as, I mpp = P mpp /V mpp = 3.4/ = A (1) The numbers of modules required to connect in series are as, N s = V mpp /V m = 187.2/31.2 = 6 (2) The numbers of modules required to connect in parallel are as, N p = I mpp /I m = 18.16/9.07 = 2 (3) Fig.1 Configuration of proposed SPV array-zeta converter fed BLDC motor drive for water pumping system.

3 TABLE I. ELECTRICAL SPECIFICATIONS OF SUNMODULE PLUS SW 280 supply (AC) voltage is reflected on the DC side as a dominant MONO SPV MODULE harmonic in the three phase supply system [13]. Here, the Peak power, P m (Watt) 280 fundamental frequencies of the output voltage of the VSI are Open circuit voltage, V o (V) 39.5 estimated corresponding to the rated speed and the minimum Short circuit current, I s (A) 31.2 speed of the BLDC motor essentially required to pump the Voltage at MPP, V m (A) 9.71 water. These two frequencies are further used to estimate the Current at MPP, I m (A) 9.07 values of their corresponding capacitors. Out of the two Number of cells connected in series, N ss 60 estimated capacitors, larger one is selected to assure the satisfactory operation of the proposed system even under the Connecting 6 and 2 modules respectively in series and duration of minimum solar irradiance level. parallel, the SPV array of required size is designed for the The fundamental output voltage frequency of the VSI proposed system and its detailed data are given in Appendix corresponding to the rated speed of BLDC motor, ω rated is A. estimated as, B. Design of Zeta Converter ω 2 π 3000*6 The zeta converter is the next stage to the SPV array. Its rated rated 120 2π * rad/sec. (9) design consists of the estimation of the various components such as input inductor, L 1, output inductor, L 2 and intermediate The fundamental output voltage frequency of the VSI capacitor, C 1. These components are so designed that the zeta corresponding to the minimum speed of the BLDC motor converter always operated in continuous conduction mode essentially required to pump the water (N = 1100 rpm), ω min is resulting in the reduced stress on them. Estimation of the duty estimated as, cycle, D initiates the design of the zeta converter which is estimated as [6], ωmin = 2πfmin 2π NP 2π * 1100* rad/sec. (10) V dc D 200 V V 0.52 (4) where f rated and f min are the fundamental output voltage dc mpp frequencies of the VSI corresponding to the rated speed and the minimum speed of the BLDC motor essentially required to where V dc is an average value of output voltage of the zeta pump the water respectively, in Hz; N rated is rated speed of the converter (DC link voltage of the VSI) equal to the DC BLDC motor; P is the numbers of poles in the BLDC motor. voltage rating of the BLDC motor. The value of DC link capacitor of the VSI corresponding to An average current flowing through the DC link of the VSI, I dc is estimated as, ω rated is as, I C dc 17 I dc = P mpp /V dc = 3400/200 = 17 A (5) 2,rated 6* ωrated * µf (11) V dc 6*942*200*0.1 Then L 1, L 2 and C 1 are estimated as [6], DV mpp 0.52* * mh (6) L 1 f sw I L *18.16*0.06 L (1 D)V dc (1 0.52)* * mh (7) 2 f sw I L *17*0.06 C 1 DI dc f 0.52*17 22 µf (8) sw V C *200*0.1 where f sw is the switching frequency of IGBT switch of the zeta converter; I L1 is the amount of permitted ripple in the current flowing through L 1, same as I mpp ; I L2 is the amount of permitted ripple in the current flowing through L 2, same as I dc ; V C1 is the amount of permitted ripple in the voltage across C 1, same as V dc. Detailed data of the zeta converter is given in Appendix B. C. Estimation of DC Link Capacitor of VSI A new design approach for the estimation of DC link capacitor of the VSI is presented in this sub-section. This approach is based on a fact that 6 th harmonic component of the 2 π f N P rated Similarly, the value of DC link capacitor of the VSI corresponding to ω min is as, C I dc 17 2,min 410 µf (12) 6* ω * V min dc 6*345.57*200*0.1 where V dc is the amount of permitted ripple in the voltage across the DC link capacitor, C 2. Finally, C 2 = 410 µf is selected to design the DC link capacitor. D. Design of Centrifugal Pump To estimate the proportionality constant, K for the selected centrifugal water pump, its torque-speed characteristics [14] is used as, T Kω 2 (13) L r where T L is the load torque offered by the centrifugal pump which is equal to the electromagnetic torque developed by the BLDC motor under steady state for stable operation and ω r is the mechanical speed of the rotor in rad/sec. Since the rated torque, T L and the rated speed, N rated of the selected BLDC

4 motor is 9.2 Nm and 3000 rpm respectively, the proportionality constant, K is estimated using (13) as, K T L * (14) ω r 2π * The centrifugal pump with this data is selected for the proposed system and its detailed data are given in Appendix C. V. CONTROL OF THE PROPOSED SYSTEM The proposed system is controlled at two stages. These two control techniques namely, MPPT and electronic commutation are discussed in brief as follows. A. INC-MPPT Algorithm An efficient and commonly used INC-MPPT technique [9] in various SPV array based applications is utilized in order to optimize the power available from the SPV array and to facilitate the soft starting of the BLDC motor. Selecting an optimum value of perturbation size ( D = 0.001) not only avoids the oscillations around the MPP but provides the soft starting of the BLDC motor also. An intellectual agreement between the tracking time and the perturbation size is held to fulfill the objectives. B. Electronic Commutation The BLDC motor is controlled by the VSI operated through the electronic commutation of BLDC motor. 6 switching pulses are generated as per the various possible combinations of 3 Hall -effect signals. These 3 Hall-effect signals are produced by the inbuilt encoder according to the rotor position. A particular combination of the Hall-effect signal is produced for specific range of rotor position [6]. The electronic commutation provides fundamental frequency switching of the VSI, hence the losses associated with the high frequency switching is completely eliminated. TETRA 115TR9.2, a BLDC motor of motor power company [15] with inbuilt encoder is selected for the proposed system and its detailed data are given in Appendix C. VI. RESULTS AND DISCUSSION Performance evaluation of the proposed SPV array fed BLDC motor driven water pumping system employing zeta converter is carried out using simulated results in MATLAB/Simulink. The proposed system is designed, modelled and simulated considering the random and instant variation in solar irradiance level and its suitability is demonstrated by testing the starting, steady state and dynamic behaviour. Fig.2 presents the starting, steady state and dynamic performance of the proposed water pumping system. To demonstrate the suitability of the proposed system under dynamic condition, solar irradiance level is varied as indicted in Table II. Behaviour of the various stages such as the SPV array, zeta converter and BLDC motor-pump are depicted on an individual basis in the following sub-sections. A. Performance of SPV Array The performance of the maximized power SPV array used to feed the water pumping system is shown in Fig. 2(a). The solar irradiance level, S is varied following the sequence indicated in Table II. Other variables such as the SPV array voltage, v pv, SPV array current, i pv and the SPV array power, P pv are varied accordingly. The presented results manifest that the maximum power available from the SPV array is extracted regardless of the irradiance level and its dynamic variation. Since it is desired to achieve the soft starting of the BLDC motor, the MPP is tracked appropriately at the starting. It is clear from Fig. 2(a) that the INC-MPPT algorithm is allowed, at the starting, to take more time for maximum power extraction by selecting an optimum value of perturbation size in order to achieve the soft starting of BLDC motor. Oscillation around the MPP is also reduced by properly selecting the optimum value of perturbation size. Under steady state condition, at the standard value of solar irradiance i.e W/m 2, all the variables possess their rated values while they possess the minimum values at minimum solar irradiance i.e. 200 W/m 2. B. Performance of Zeta Converter Fig. 2(b) clarifies the various performances of the zeta converter. All the variables viz. the current flowing through the input inductor, i L1, the voltage across the intermediate capacitor, v C1, the current flowing through the output inductor, i L2 and the voltage at the output (DC link voltage of the VSI), v dc comply the variation in the solar irradiance level. Regardless of the irradiance level, the zeta converter is always operated in continuous conduction mode. Unlike a simple buck-boost converter, the zeta converter has positive polarity voltage at its output as shown in Fig. 2(b) which reaches, under steady state, the rated DC voltage of the BLDC motor at 1000 W/m 2 of solar irradiance level. Moreover, at the solar irradiance level of 200 W/m 2, it provides a DC voltage level to the BLDC motor sufficient to make attained more than the required speed to pump the water. Small amount of ripples in the zeta converter variables are observed caused by permitting the ripples up to an extent in order to reduce the size of the components. C. Performance of BLDC Motor-Pump Performance of the BLDC motor-pump is shown in Fig. 2(c). Following points are clearly observed from the presented simulation results. The motor pump variables viz. the back EMF, e a, the stator current, i sa, the rotor speed, N, the electromagnetic torque, T e and the pump load torque, T L are abide by the variation in solar irradiance. At the starting, the rate of rise of stator current is decreased as an evidence of soft starting of the BLDC motor.

5 The motor-pump variables reach their rated values under steady state at 1000 W/m 2, standard value of solar irradiance. However, it should be highlighted that the motor always attains a higher speed than minimum speed required to pump the water i.e rpm (even at 200 W/m 2 ) regardless of the solar irradiance level. The electromagnetic torque developed by BLDC motor is same as torque required by the centrifugal pump. This torque balance between the BLDC motor and the centrifugal pump irrespective of the solar irradiance variation verifies the stable operation of the proposed system. A small and acceptable pulsation in the electromagnetic torque is observed because of the electronic commutation and reflection of the ripples present in the DC link current of VSI. Fast and precise response of the BLDC motor subjected to the dynamic variation in solar irradiance is undoubtedly ascertained by the simulated results. (c) Fig.2 Performances of the proposed SPV array based Zeta converter fed BLDC motor drive for water pumping system (a) SPV array variables, (b) Zeta converter variables, and (c) BLDC motor-pump variables. TABLE II. VARIATION IN SOLAR IRRADIANCE LEVEL (a) Solar Irradiance Level, S (W/m 2 ) Duration (Sec.) VII. CONCLUSIONS The SPV array-zeta converter fed VSI-BLDC motor-pump for water pumping has been proposed and its suitability has been demonstrated by simulated results using MATLAB/Simulink and its sim-power-system toolbox. First, the proposed system has been designed logically to fulfil the various desired objectives and then modelled and simulated to examine the various performances under starting, dynamic and steady state conditions. The performance evaluation has justified the combination of zeta converter and BLDC motor drive for SPV array based water pumping. The system under study availed the various desired functions such as MPP extraction of the SPV array, soft starting of the BLDC motor, fundamental frequency switching of the VSI resulting in a reduced switching losses, reduced stress on IGBT switch and the components of zeta converter by operating it in continuous (b) conduction mode and stable operation. Moreover, the

6 proposed system has operated successfully even under the minimum solar irradiance. APPENDIX A Parameters of solar PV array: Open circuit voltage, V oc = 237 V; Short circuit current, I sc = A; Maximum power, P mpp = 3.4 kw; Voltage at MPP, V mpp = V; Current at MPP, I mpp = A; Numbers of cells connected in series in a module, N ss = 60; Numbers of modules connected in series, N s = 6; Numbers of modules connected in parallel, N p = 2. APPENDIX B Parameters for Zeta converter: Switching frequency, f sw = 20 khz; Input inductor, L 1 = 5 mh; Intermediate capacitor, C 1 = 22 μf; Output inductor, L 2 = 5mH; DC link Capacitor, C 2 = 410 μf. APPENDIX C Parameters for BLDC Motor-Pump: Stator phase/phase resistance, R s = 0.36 Ω ; Stator phase/phase inductance, L s = 1.3 mh; Torque constant, K t = 0.49 Nm/A peak ; Voltage constant, K e = 51 V peak L-L/krpm; Rated current, I srated = 18.9 A; Rated torque, T rated = 9.2 Nm; Rated speed, N rated = V DC; Rated power, P rated = 2.89 kw; No. of poles, P = 6; Moment of inertia, J = 17.5 kg.cm 2 ; Proportionality constant, K = 9.32*10-5. ACKNOWLEDGMENT Authors are very thankful to Department of Science and Technology (DST), Govt. of India, for supporting this work under Grant Number: RP REFERENCES [1] M. Uno and A. Kukita, Single-Switch Voltage Equalizer Using Multi- Stacked Buck-Boost Converters for Partially-Shaded Photovoltaic Modules, IEEE Transactions on Power Electronics, no. 99, [2] R. Arulmurugan and N. Suthanthiravanitha, Model and Design of A Fuzzy-Based Hopfield NN Tracking Controller for Standalone PV Applications, Electr. Power Syst. Res. (2014). Available: [3] S. Satapathy, K.M. Dash and B.C. Babu, Variable Step Size MPPT Algorithm for Photo Voltaic Array Using Zeta Converter - A Comparative Analysis, Students Conference on Engineering and Systems (SCES), pp.1-6, April [4] A. Trejos, C.A. Ramos-Paja and S. Serna, Compensation of DC-Link Voltage Oscillations in Grid-Connected PV Systems Based on High Order DC/DC Converters, IEEE International Symposium on Alternative Energies and Energy Quality (SIFAE), pp.1-6, Oct [5] G. K. Dubey, Fundamentals of Electrical Drives, 2 nd ed. New Delhi, India: Narosa Publishing House Pvt. Ltd., [6] B. Singh and V. Bist, A Single Sensor Based PFC Zeta Converter Fed BLDC Motor Drive for Fan Applications, Fifth IEEE Power India Conference, pp.1-6, Dec [7] R.F. Coelho, W.M. dos Santos and D.C. Martins, Influence of Power Converters on PV Maximum Power Point Tracking Efficiency, 10th IEEE/IAS International Conference on Industry Applications (INDUSCON), pp.1-8, 5-7 Nov [8] Dylan D.C. Lu and Quang Ngoc Nguyen, A Photovoltaic Panel Emulator Using A Buck-Boost DC/DC Converter and A Low Cost Micro-Controller, Solar Energy, vol. 86, issue 5, pp , May [9] Zhou Xuesong, Song Daichun, Ma Youjie and Cheng Deshu, The Simulation and Design for MPPT of PV System Based on Incremental Conductance Method, WASE International Conference on Information Engineering (ICIE), vol.2, pp , Aug [10] Ali Reza Reisi, Mohammad Hassan Moradi and Shahriar Jamasb, Classification and Comparison of Maximum Power Point Tracking Techniques for Photovoltaic System: A review, Renewable and Sustainable Energy Reviews, vol. 19, pp , March [11] A. Shahin, A. Payman, J.-P. Martin, S. Pierfederici and F. Meibody- Tabar, Approximate Novel Loss Formulae Estimation for Optimization of Power Controller of DC/DC Converter, 36th Annual Conference on IEEE Industrial Electronics Society, pp , 7-10 Nov [12] Sunmodule Plus SW 280 mono, Performance Under Standard Test Conditions [Online]. Available: content/uploads/2013/07/sunfields-solarworld_sw _mono_en.pdf [13] K.H. Ahmed, M. S. Hamad, S.J. Finney and B.W. Williams, DC-Side Shunt Active Power Filter for Line Commutated Rectifiers to Mitigate the Output Voltage Harmonics, IEEE Energy Conversion Congress and Exposition (ECCE), pp , Sept [14] W.V. Jones, Motor Selection Made Easy: Choosing the Right Motor for Centrifugal Pump Applications, IEEE Industry Applications Magazine, vol.19, no.6, pp.36-45, Nov.-Dec [15] TETRA 142TR12, Brushless Servomotors [Online]. Available: R_ENG.pdf

7 Author-1 Miss.MuppallaPavaniwas born in Vinukonda,Guntur(Dist), Andhra Pradesh,India.On 5 th -May-1992.She has completed Bachelor of Technology in Electrical and Electronics Engineering fromkrishnaveni Engineering College For Women Kesanupalli, Narasaraopeta(Mandal),Guntur(Dist) A.P, India, Jawaharlal Nehru Technological University, Kakinada,Presently she is pursuing her Masters of Technology in Newtons Institute Of Engineering, Guntur, A.P, India. Specialization in Electrical Machines and Drives. id :-pavanimuppalla6@gmail.com Contact No : Author-2 Mr.RameshMatta obtained his Bachelor of Technology in Electrical and Electronics Engineering from Newtons Institute of Engineering, Guntur, A.P, India, Jawaharlal Nehru Technological University Kakinada. He completed his Master of Technology in specialization in Power systems from K.L.University, Green fields, Vaddeswaram, Guntur dist A.P, India. He is currently working as an Associate professor in Electrical and Electronics Engineering Department in Newtons Institute of Engineering, Guntur, A.P, India. His area of interest includes Multi Level Inverters. powerquality.renewable energy Resource. Id:-mattaramesh275@gmail.com Contact No :