SOFT SWITCHING CONVERTER DESIGN WITH PV ARRAY TWO DIODE MODEL AND DIGITAL SIGNAL PROCESSOR BASED MPPT FOR SOLAR HYBRID APPLICATIONS

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1 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, SOFT SWITCHING CONVERTER DESIGN WITH PV ARRAY TWO DIODE MODEL AND DIGITAL SIGNAL PROCESSOR BASED MPPT FOR SOLAR HYBRID APPLICATIONS N.SenthilMurugan 1 Dr.C.Sharmeela 2 K.Saravanan 3 1 Assistant Professor, Dept. of Electrical & Electronics Engineering, Sri Venkateswara College of Engineering, Sriperumbudur , Tamilnadu, India. 2 Associate Professor, Department of Chemical Engineering, Anna University, Chennai -25,Tamilnadu, India. 3 Design Engineer, Power Electronics Division, MNC Chennai, Tamilnadu, India. { nsm@svce.ac.in, sharmeela20@yahoo.com, saravanank96@gmail.com} Abstract: This paper deals with the determination of the nonlinear I V parameters of the equation by adjusting the curve at three points: open circuit, maximum power, and short circuit. Given these three points, which are provided by all commercial array datasheets, the method finds the best I V equation for the two-diode photovoltaic (PV) model including the effect of the series, parallel resistances and recombination of charge carriers and confirms the maximum power of the model matches with the maximum power of the real array. The novel soft switching converter (ZVS-ZCS resonant action) is designed for maximum power point tracking (MPPT). The mathematical model of the PV device may be useful in the study of the dynamic analysis of converters, in the study of MPP tracking (MPPT) algorithms. The converter aims to get the regulated output voltage from photovoltaic (PV) arrays with low switching loss and conduction loss and efficiency of more than 92% can be easily achieved. DSP based MPPT algorithm adjusts solar array voltage (equal to battery voltage) with a digital compensator technique and discrete PI control to track the MPP with high tracking efficiency. Hence the proposed work gives a novel idea in the modern hybrid energy system. Keywords: Zero Voltage Switching (ZVS), Zero Current Switching (ZCS), Maximum power point tracking (MPPT), Solar Wind Hybrid systems (SWHS). a discrete PI control to track the MPP for the converter system is used in this paper to achieve the maximum power transfer and high efficiency for the solar energy system. The tracking efficiencies are on firmed by simulations and experimental results. If the solar energy system provides power to a load, the system often operates away from maximum power points of the solar array. Fig.1 shows the solar array I-V characteristics and the load curve, together with constant power curves (P = VI = const). It is observed that the delivered output power, which is represented by the operating point 1, is significantly smaller than the maximum output power, which is represented by point 2. In order to ensure a maximum power transfer, DC/DC converters are used to adjust the voltage at the load to the value of Vr = (Pm.R), r equivalent resistance of the load. In the districts where solar energy and wind energy are naturally complementary, the application of solar-wind hybrid generation systems (SWHS) can reduce the storage capacity of batteries and the total cost of the system compared with stand-alone PV or wind generation system.[1][2]. This paper presents a costeffect controller system for 1Kw to 5kW SWHS with low DC voltage input (18V DC or 48V DC) and high output AC sine wave voltage (220V AC). 1. INTRODUCTION In late years, the problem of energy crunch is more and more aggravating. Very much exploitation and research for new power energy are preceded around the world. In particular, the solar energy attracts lots of attention. In recent years, the development of power semiconductor technology results in easier conversion between AC and DC. Therefore, the use of solar energy is emphasized increasingly and regarded as an important resource of power energy in the next century.[1] The solar modules have a long lifetime (20 years or more) and their best production efficiency is approaching 20%. As the power supplied by solar arrays depends upon the insolation, temperature and array voltage, it's necessary to draw the maximum power of the solar array. A DSP based simple MPPT algorithm that adjusts the solar array / wind voltage with Fig. 1 The operation of the MPPT This control system includes features like: 1) PWM technique in charging control of batteries, which can make wind turbine and solar array operate at maximum power point so that the overall system efficiency can be improved greatly. 2) Constant voltage and limited current two-loop control of battery charge,

2 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, which can make batteries in float charging state, enhance the cycle rate and prolong the life of the batteries. 3) SPWM conversion with front-end high frequency DC-DC modules in parallel and special DSP control technique with high performance-cost, which can accomplish sine wave output voltage at the final stage to feed the grid with high reliability and high load efficiency. Fig. 2 Proposed Hybrid system block diagram Fig. 2 gives the introduction to overall hybrid system with soft switching converters and DSP based MPPT algorithm implementation which is a novel idea in modern renewable energy system. The impact of air pollution and global climate change are becoming increasingly important topics throughout the world and international organizations are fighting to reduce the carbon emissions produced by fossil fuel. [3] Energy conservation has become a priority. Table. I give, using hybrid systems how C0 2 emissions reduced drastically. Over a five year operating period, corresponding approximately to the life duration of the Ni-Cd battery and of the gen set in such a hybrid application, the reduction in the C0 2 emission for a 2 kw mobile site with this hybrid power system will be higher than 200 tons. This 200 tons should be compared with about only 2.7 tons of equivalent C0 2. The addition of a wind turbine or possibly solar panels will emphasize this reduction in C0 2 with an increase in cycling time. half-bridge LLC resonant converter and transformer. However, since the transformer has normally large air-gap, while the leakage inductance increases and magnetization inductance decreases. This causes low power conversion efficiency. To overcome such problems, a series resonant converter is widely used [4] However, the series resonant converter for the power supply generally operates with higher switching frequency than resonant frequency to achieve soft switching under continuous resonant current mode. In this case, the main switches can achieve zero voltage switching (ZVS), but it has disadvantage that the secondary side diode converter cannot achieve zero current switching. Furthermore, due to the higher switching frequency operation than resonant frequency, it has low voltage gain and high power loss since a large primary side circulating current flows [5]. Since the proposed power supply using LLC converter operates with lower switching frequency than the resonant frequency, it can achieve high voltage gain, which, in turn, offers low turns ratio for the transformer and high efficiency due to discontinuous resonant current. Fig. 3a. Simulated circuit of proposed PV system with boost converter Operational Details 2.5 KW 6 KW 12 KW 24 KW Daily Generator Operation Daily Battery Operation CO 2 savings Per year 8.25h 9h 9h 11.75h 15.75h 15h 15h 12.25h 58 Tons 56 Tons 58 Tons 42 Tons Fig. 3b. ZVS-ZCS resonant (LCC) circuit feeding the load Table. I Example of CO 2 savings with hybrid and gen set. 2. SOFT SWITCHING CONVERTER OPERATION In this paper, a hybrid power generation system using soft switching technique is proposed as shown in Fig 3a. The proposed system consists of a ZVS-ZCS boost converter, a Figure 3a gives the simplified circuit of the proposed PV system with boost converter to track the maximum power and Figure 3b ZVS-ZCS resonant (LCC) circuit feeding the load The PV model subsystem is constructed by all governing mathematical modeling equations and with different possible inputs like temperature, insolation etc,.

3 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, connected in series provide greater output voltages. If the array is composed of Np parallel connections of cells the PV and saturation currents may be expressed as Ipv = Ipv,cell Np, I0 = I0,cellNp. In (2), Rs is the equivalent series resistance of the array and Rp is the equivalent parallel resistance. B. Modeling the PV array. Table. II Specification of Half bridge converter A. Ideal PV Cell 3. MODELING OF PV DEVICES Fig. 4 shows the equivalent two diode model of the ideal PV cell. The basic equation from the theory of semiconductors [7] that mathematically describes the I V characteristic of the ideal PV cell is I=I pv, cell I 0, cell [exp (qv/αkt)-1]; Id = I 0, cell [exp (qv/kt)-1] (1) where Ipv,cell is the current generated by the incident light (it is directly proportional to the Sun irradiation), Id is the Shockley diode equation, I0,cell is the reverse saturation or leakage current of the diode, q is the electron charge ( C), k is the Boltzmann constant ( J/K), T (in Kelvin) is the temperature of the p n junction, and α is the diode ideality constant. Fig. 5 shows the I V curve originated from (1). The basic equation (1) of the elementary PV cell does not represent the I V characteristic of a practical PV array. Fig. 4 Two diode model of the ideal PV cell and its equivalent circuit with series and parallel resistances. Practical array requires the inclusion of additional parameters to the basic equation, I=I pv I 0 [exp (V+R s I/αV t )-1] (V + R s I) / R p (2) arrays are composed of several connected PV cells and the observation of the characteristics at the terminals of the PV where Ipv and I0 are the photovoltaic (PV) and saturation currents, respectively, of the array and Vt = NskT/q is the thermal voltage of the array with Ns cells connected in series. Cells connected in parallel increase the current and cells Assumption / Drawback of the proposed work so far by the different authors is that parallel resistance in single diode model is neglected and series resistances are assumed to be low [6]. Moreover Isc Ipv are considered but short circuit current and PV cell current are not same practically where as in our proposed work we have considered both resistances and framed the equation for two diode model and cell currents are considered practically. The assumption Isc Ipv is generally used in the modeling of PV devices because in practical devices the series resistance is low and the parallel resistance is high. The light-generated current of the PV cell depends linearly on the solar irradiation and is also influenced by the temperature according to the following equation I pv = (I pv, n + K 1 Δ T ) G/G n (3) where Ipv,n (in amperes) is the light-generated current at the nominal condition (usually 25 C and 1000 W/m2 ), ΔT = T Tn (T and Tn being the actual and nominal temperatures [in Kelvin], respectively), G (watts per square meters) is the irradiation on the device surface, and Gn is the nominal irradiation. The diode saturation current I0 and its dependence on the temperature may be expressed by as shown [10]. I 0 = I 0, n (Tn/T) 3 exp [qe g /αk (1/T n 1/T) + q 1 ΔT/αK] (4) where Eg is the bandgap energy of the semiconductor (Eg = 1.12 ev for the polycrystalline Si at 25 C [11] and I0,n is the nominal saturation current. I 0,n = I sc,n / (exp(v oc,n /αv t,n ) - 1) (5) with Vt,n being the thermal voltage of Ns series-connected cells at the nominal temperature Tn. The value of the diode constant α may be arbitrarily chosen. Usually, 1 α 1.5 and the choice depends on other parameters of the I V model. Some values for α are found based on empirical analyses. As α expresses the degree of ideality of the diode and it is totally empirical, any initial value of α can be chosen in order to adjust the model. The value of α can be later modified in order to improve the model fitting, if necessary. This constant affects the curvature of the I V curve and varying α can slightly improve the model accuracy. The PV model described in the equation (5) can be improved for two diode model as, I 0 = I sc, n + K 1 ΔT + q 1 ΔT / (exp ((V oc, n + K v ΔT) /αv t, ) - 1) (6) This modification aims to match the open-circuit voltages of the model with the experimental data for a very large range of temperatures.

4 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, MATLAB IMPLEMENTATION OF MODEL EQUATIONS The equation can be expressed as the following Vref(k+1) = Vref(k) ± C C is the amount of disturbance and the sign of C is determined by the power slope. In the voltage loop, the PI compensator is used to make the system stable. Therefore, the discretization of the compensator transfer function is required for system implementation. The transfer function of a traditional compensator is Y(S) / U(S) = Kp + Ki / S where Kp, is the proportional gain, and K i, is the integral gain. Rearranging equation in finite-difference form [Y(n+l)-Y(n)]/ T = Ki,U(n)+Kp[U(n+1) - U( n)/ T] where T is the sampling time. Taking the Z-transform of equation yields Y(Z) / U(Z) =Kp+KiT / Z-1 Equation can be expressed in state variable form as X(n+1)= AX(n)+BU(n) Y(n) = Cx(n)+Du(n) where A=l, B=KiT, C=l, D=Kp, and X(n) is the state variable. Fig. 7 shows the block diagram of the compensator for digital implementation. Fig. 5 Modeling blocks in MATLAB The PV module subsystem is considered based on the mathematical model and the governing empirical equations for different voltage and current variables including resistances of the two diode model. 5. MPPT CONTROL OF SWHS The basic block diagram of the MPPT control is shown in Fig. 6. The proposed control consists of two loops, the maximum power point tracking loop is used to set a corresponding Vref to the charger input, the regulating voltage loop is used to regulate the solar array output voltage according to Vref which is set in the MPPT loop. The functions of the two loops are performed by a DSP based controller. The controller senses the solar array current and voltage to calculate the solar array output power, power slope and Vref for maximum power control Fig.6 Basic block diagram of the control loop Fig.7 Implementation of the digital compensator 5.1 DSP Algorithm Fig. 8 shows the simplified MPPT control block diagram. In Fig. 8, D(j), Vref(j) and vcell(j) are respectively the converter switching duty ratio, the demanded cell voltage and the actual cell voltage in the jth MPPT controller cycle, where j = k, k+1. The MPPT controller calculates the new cell voltage set point based on the converter switching duty ratios and the measured cell voltages in the past and at present. The Proportional- Integral (PI) controller forces the cell voltage to follow the demanded cell voltage signal. In the practical design of the control software, the threshold ε j (j = 1,2,3 ), which is a small positive number close to zero, is used to determine whether the MPP has been reached and CV is used as a positive increment in the demanded cell voltage. The variable δ j ( j = 1, 2, 3 ) can be defined as: δ1 = V cell(k+1)-v cell(k) δ2 = D(k+1)-D(k) δ3 = D(k+1)+V cell(k+1) δ 2 / δ 1 When δ1 > ε1, the MPPT controller can be simplified as: V ref(k+1) = Vref (k) + Cv, δ3 > ε3 V ref(k+1) = Vref (k), δ3 > ε3 V ref(k+1) = Vref (k) - Cv, δ3 < -ε3

5 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, When δ1 < ε1, the MPPT controller can be simplified as: V ref(k+1) = Vref (k) + Cv, δ2 > ε2 V ref(k+1) = Vref (k), δ2 < ε2 V ref(k+1) = Vref (k) - Cv, δ2 < -ε2 Fig. 8 Simplified MPPT control block diagram The insolation changes much more rapidly than the temperature does. As the cell MPP voltages only vary slightly under different insolation levels, the PI controller runs at a much faster frequency than the MPPT to force the cell voltage to follow the demanded voltage. The MPPT controller executes once every 16 PI control cycles. The PI and the MPPT controllers in Fig. 9 are implemented by the digital signal processor, which is designed for 1.8V minimum supply voltage. As the cell voltage may fall below 1.8 V under rapid insolation changes, the switching control signals for the MPPT converter will be suspended by the software once the under voltage happens to allow the cell voltage to recover and the digital signal processor to function properly. Fig. 10 PV array V-I characteristics Figure 10 and 11 gives the PV array V-I characteristics and P-V curve respectively. Both the simulated characteristics are similar to the ideal characteristics of the PV array cells (refer Fig. 1), hence confirming the positive result of our proposed work. Fig 11 Power-Voltage curve of the PV array 7. PSPICE SIMULATION Fig. 9 MPPT device circuit diagram 6. PV ARRAY OUTPUT CHARACTERISTICS - MATLAB SIMULATION RESULTS The Figure 12 represents the simulated boost converter with coupled inductor using PSpice. Simulated results of the 33 W, 400 khz boost converter with auxiliary switch are presented in this section. The 33 W 400kHz boost converter with auxiliary switch is simulated in PSpice environment. Figure 13 show the pulse driving the main switch S. The main switch is turned on with a delay of 0.3 µs after the auxiliary switch is turned on. Figure 14 shows the resonant current output waveform conforming ZVS or low switching loss of converter. Figure. 15 gives the simulated circuit output voltage graph.

6 16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, CONCLUSION Fig. 12 Simulated boost converters with coupled inductor 8. SIMULATION INPUT/OUTPUT WAVEFORMS Fig.13 Gate pulse for main switch Fig 14 Resonant current waveform In our proposed work of two diode modeling of the PV array, we have considered both series and parallel resistances and framed the equation and solar cell currents are considered practically which overcomes the drawback of the existing work where parallel resistance in single diode model is neglected and series resistances are assumed to be low and Isc Ipv are considered same but short circuit current and PV cell current are not practically equivalent. The design of novel soft switching converter with maximum power point tracking algorithm for PV systems to achieve high efficiency and to deliver maximum power to the load was explained. An auxiliary switch dc dc boost converter using coupled inductor that achieves soft switching for both the main and auxiliary switches without increasing the main device current/voltage rating. Also a simple MPPT algorithm based on a DSP is presented to deliver the highest possible power to the load from the solar arrays DC-DC converter were used in the solar wind hybrid energy system to investigate the performance of the converters. The charging control of battery with respect to the final outputs yields its maximum life time. Simulation results are presented for a 1KW, 150 khz boost converter with excellent performance (efficiencies over 92% for ZVS-ZCS converters) and further analysis can be done on improving the converter efficiency with high power rating in MW and enhancement of proposed DSP algorithm to electrify rural areas. 10. REFERENCES 1.Youjie Ma, Deshu Cheng and Xuesung Zhou, Hybrid Modeling & Simulation for boost converter in Photo Voltaic system, IEEE Computer Society International Conference on Information and Computer Science, January 2009, pp S.M.Mousavi, S.H Fathi, Energy Management of Wind/PV and battery hybrid system, IEEE Proceedings on Industrial Electronics, August 2009, pp Joel Brunarie, George Myerscough, Delivering Cost Savings & Environmental Benefits with hybrid power, IEEE Transactions on Industrial Electronics, January 2008, pp M. Z. Youssef, H. Pinheiro, and Praveen K. Jain, Analysis and Modeling of a Self-Sustained Oscillation Analytical Technique, Proceedings of the IEEE International Energy and Telecommunications Conference, INTELEC, Oct pp Laszlo Huber, Kevin Hsu and Milan M. Jovanovic, 1.8-MHz, 48-V resonant VRM: Analysis, Design and Performance Evaluation IEEE transactions on Power Electronics Vol. 21 No 1,January pp M.Lopez, D.Morales, J.C Vannier, and D.Sadar nac, Influence of Power Converter losses evaluation in sizing of a Hybrid Energy system, IEEE Transactions on Power Electronics, March 2007, pp Fig. 15 Output DC voltage