A Current Sensor-less Maximum Power Point Tracking Method for PV

Transcription

1 A Current Sensor-less Maximum Power Point Tracking Method for PV System 1 Byunggyu Yu, 2 Ahmed G. Abo-Khalil 1, First Author, Corresponding Author Kongju National University, bgyuyu@kongju.ac.kr 2 Majmaah University on leave from Assiut University, a.abokhalil@mu.edu.sa Abstract This paper deals with a conventional single-phase, two-stage energy conversion system which is connected between PV array and electrical power system, employing a new, simple and effective MPPT algorithm. The basic operations are based on scanning of characteristics of PV array regularly to obtain maximum power point condition. At first, duty-ratio of boostconverter is set to zero where PV current is settled to zero. Then converter duty ratio is adjusted to unity in which PV current starts to increase while PV voltage decreases. In this time, PV voltage is measured and PV current is calculated and PV instantaneous power is calculated. The instantaneous calculated power is compared with previous value until maximum power point is obtained. The corresponding voltage is saved as a reference for maximum power point condition in normal operation. The main advantage of this method is eliminating current sensor. Meanwhile, this MPPT algorithm reduces power oscillations around peak power point which occurs with perturbation and observation algorithms. In addition, total cost will decrease by removing current sensor from PV side. Finally, simulation results confirm accuracy of proposed method. Keywords: Photovoltaic Generation, Power Converter, Maximum Power Point Tracking 1. Introduction In past few years, Photovoltaic (PV) generation has emerged as a one of most promising sources of large-scale renewable energy systems. The importance of PV generation comes from its advantages such as absence of fuel cost, little maintenance, no pollution, no noise and wear due to absence of moving parts. However, re are or issues with using of PV systems, which are high installation cost and low energy conversion efficiency. The voltage-power characteristic of PV array is a non-linear because of variation that caused by solar irradiation and temperature. Therefore, it is very important for a PV to operate at maximum power point to reduce cost of generated power to system installation cost. Many maximum power point tracking (MPPT) techniques for PV systems are well established in literature. The most commonly known are hill-climbing [1], fractional open-circuit voltage control [2], perturb and observe (P&O) [3], and incremental conductance [4], parasitic capacitance [5], constant voltage [6]. There are lesser known, but sometimes very appropriate, methods such as maximizing load current or voltage [7], fractional short-circuit current control [8], array reconfiguration [9], linear current control [10], fuzzy control [11], neural network [12], dc link capacitor droop control, pilot cells, current sweep, limit-cycle control, and several ors [13]. Generally, re are several methods which are commonly used to determine maximum power point. With P&O algorithm operating voltage V is perturbed with every MPPT cycle. As soon as maximum power point (MPP) is reached, V oscillates around ideal operating voltage V mpp. This causes a power loss which depends on step width of a single perturbation. If step width is large, MPPT algorithm will respond quickly to sudden changes in operating conditions with increasing losses under stable or slowly changing conditions. If step width is very small losses under stable or slowly changing conditions will be reduced, but system will only able to respond very slowly to rapid changes in temperature or illumination levels. The value of ideal step width is system- dependent and needs to be determined experimentally. Incremental conductance algorithm has an advantage over P&O method in that it can determine when MPPT reaches MPP, while output power in P&O method oscillates around MPP. Also, incremental conductance can track rapidly changing irradiance conditions with higher accuracy than P&O method. International Journal of Advancements in Computing Technology(IJACT) Volume 5, Number 11, July 2013 doi : /ijact.vol5.issue

2 Figure 1. An equivalent circuit model of PV array. Figure 2. PV voltage and current characteristics at a constant temperature. This paper deals with a conventional single-phase, two-stage energy conversionn system which is connected between PV array and electrical power system, employing a new, simple and effective MPPT algorithm. The basic operations are based on scanning of characteristics of PV array regularly to obtain maximum power point condition. At first, duty-ratio of boost- to unity in which PV current starts to increase while PV voltage decreases. In this time, PV converter is set to zero where PV current is settled to zero. Then converter duty ratio is adjusted voltage is measured and PV current is calculated and PV instantaneous power is calculated. The instantaneous calculated power is compared with previous value until maximum power point is obtained. The corresponding voltage is saved as a reference for maximum power point condition in normal operation. 2. PV array characteristics Solar cells are devices thatt convert photons into electrical potential in a PN junction, of which equivalent circuit is shown in Figure 1. Due to complex physical phenomena inside solar cell, manufacturers usually present a family of operating curves (V-I) as shown in Figure 2. These characteristics are obtained by measuring array volt-ampere or current, corresponding to maximum power point, for a different illumination values. From se characteristics, optimum voltage can be determined. It is clearly seen in Figure 2 that current increases as irradiance levels increase. The maximum power point increases with a steep positive slope proportional to illumination. The main parameters whichh influence illumination levels on a surface at a fixed tilt on earth are daily and seasonal solar path, presence of clouds, mist, smog and dust between surface and sunlight, and shade of object positioned such that illumination level is reduced, etc. The equation of PV outputt current is expressed as a function of array voltage I I sc - I e o q( V IRs ) KTk -1} - ( V IR )/R s sh (1) 359

3 where V and I represent PV output voltage and current, respectively; R s and R sh are series and shunt resistance of cell in Figure 1; q is electronic charge; I sc is light-generated current; I o is reverse Saturation current; K is Boltzman constant, and T k is temperature in Kelvin. Equation (1) can be written in anor form as [3] m K2V I I sc { 1 K1[ e 1]}- ( V IR s )/R sh (2) where coefficient K 1, K 2 and m are defined as K , m K 2 K 4 /( V oc ), K 4 ln(( K1 1) / K 1), K 3 ln[( Isc(1 K1) Impp) / K1I sc], m ln( K 3 / K4 ) / ln( V mpp / Voc ) I mp p is current at maximumm output power, V mpp is voltage at maximum power, I sc is short circuit current and V oc is open circuit voltage of array. Equation (2) is only applicable at one particular operating condition of illumination G and cell temperature. The parameter variations can be calculated by measuring variation of short-circuit current and open-circuit voltage in se conditions using parameters at normal illumination and cell temperature. Equation (2) is used for V-I characteristics for various illumination and fixed temperature (25[ C]) in Figure System control and estimation Figure 3 shows circuit configuration of proposed PV power conditioning circuit (PCS). The proposed PCS is composed of a dc dc boost converter to step-up PV voltage to operating DC link voltage and a dc ac inverter to invert dc link voltage to ac voltage with unity power factor correction. Without using dc current sensor for PV current, MPPT control is achieved by estimating PV current estimator. Figure 3. PV power circuit system consisted with boost converterr and full bridge inverter Figure 4. Typical structure of DC/DC boost converter 360

4 V i V o V i V o (a) (b) Figure 5. Equivalent circuits of DC/DC boost converter during switching on and off P max Open-Circuit duration Short-Circuit duration Normal duration Open-Circuit duration Short-Circuit duration Normal duration Time T oc T sc T op Toc T sc T op Figure 6. MPPT control sequence of proposed method 3.1. Boost converter and MPPT control algorithm The main advantages of boost converter in Figure 4 are higher efficiency and reduced component count and it converts unregulated voltage into desired regulated voltage by varying duty cycle at high switching frequency lowering size and cost of energy storage components. When a boost converter operates in continuous mode, current through inductor (I L ) never falls to zero. The output voltage can be calculated as follows, in case of an ideal converter (i.e. using components with an ideal behavior) operating in steady conditions. During On-state, switch S is closed, which makes input voltage appear across inductor, which causes a change in current (I L ) flowing through inductor during a time period (t) as shown in Figure 5(a). During Off-state, switch S is open, so inductor current flows through load as shown in Figure 5(b). Applying Kirchhoff s rules around loops and rearranging terms yields an intuitive result: Vo V 1 (3) 1 i D From above expression it can be seen that output voltage is always higher than input voltage (as duty cycle goes from 0 to 1), and that it increases with D, oretically to infinity as D approaches 1. If we considered operation of boost converter only during on-off states we can estimate converter current without a need to a current sensor. If D is set to zero for time longer than switching frequency -which is open circuit condition- output voltage in this case is equal to input voltage and inductor current is zero. On or hand if duty ratio is set to 1 which is short circuit condition- output voltage will be zero and inductor current can be calculated as: dil Vi ilr L (4) dt where R is inductor internal resistance. By solving differential equation in (4), instantaneous current can be expressed as: R V t i L il [1 e ] R (5) 361

5 D=0 i, V max =0 D=1 i= equation p(t)=v(t)*i( (t) V ma x=v cell (t) D=D-dDD if V max <V cel ll(t) If p(t+dt)<=p( (t) Yes V max =V max No Yes D=D End Figure 7. Flowchart for proposed MPPT algorithm Figure 8. Control circuit for PV system. The calculated current is used with PV measured voltage to track maximumm power point as follows: In beginning, boost converter duty-ratio is set to zero (open-circuit condition) where PV current is settled to zero. Then converter duty ratio is adjusted to unity (short-circuit condition) and PV current starts to increase while PV voltage decreases. In this time, PV voltage is measured and PV current is calculated using (5). Meanwhile, PV instantaneous power Ppv is calculated. The instantaneous calculated power is compared with previous value until maximum power point is obtained. The corresponding voltage V max is saved as a reference for maximum power point condition in normal operation. In next step, duty ratio is n decreased gradually until PV measured voltage is equal to stored V max. The converter duty ratio is n kept as same duty ratio for a predetermined time period Top before same sequence is repeated to update operating point in case of changing operating condition as shown in Figure 6. Figure 7 shows flow chart of proposed MPPT method Control of PV inverter The maximumm power-matching schemes require selected solar panel to have suitable output characteristics or configurations that can be matched with particular loads. It only approximates to location of MPP becausee y are basically associated with specific illumination and load conditions. Figure 5 shows a conceptual diagram that explains operational sequence for this control method. In beginning, duty-ratio of boost-converter is set to zero (open-circuit condition) where PV current is settled to zero. Then converter duty ratio is adjusted to unity (short-circuit condition) and PV current starts to increase while PV voltage decreases. In this time, PV voltage is measured and PV current is calculated using (5). Meanwhile, PV instantaneous power 362

6 P pv is calculated. The instantaneous calculated power is compared with previous value until maximum power point is obtained. The corresponding voltage V max is saved as a reference for maximum power point condition in normal operation. In next step, duty ratio is n decreased gradually until PV measured voltage is equal to stored V max. The converter duty ratio is n kept as same duty ratio for a predetermined time period Top before same sequence is repeated to update operating point in case of changing operating condition. Figure 7 shows flowchart of system operation. As mentioned previously, inverter controller has two main functions. One is synchronizing output current with grid voltage and or is that it controls dc link voltage. To achieve se two goals, inner current control loop and outer voltage control loop are used as shown in Figure 8. Figure 9. The simulation result of proposed MPPT method under a constant irradiation. Figure 10. The simulation results of proposed MPPT method under different irradiation levels. 363

7 4. Simulation results To verify effectiveness of proposed control method, proposed control scheme has been simulated using PSIM Program linked with C language. The simulation results of system following parameters, solar array 3kW, boost converter C 2 = 4700uF and L 1 = 3mH, single-phase inverter L 2 = 1mH and AC source 220 V. As shown in Figure 9, in beginning when illumination level is constant, converter duty ratio is set to zero which means inductor current is zero. In next step duty ratio is changed to unity which allows converter inductor current to increase as if PV array is short-circuited. Meanwhile, PV voltage is measured and converter inductor current is calculated. The product of calculated current and measured voltage is instantaneous power. The calculated instantaneous power is compared to previous instant and once maximum power point is obtained corresponding V max is stored. The next step is reducing duty ratio from unity to value which maintains PV voltage equal to V max. Figure 10 shows that output power changes according to illumination variation from 1000[W/m2] to 800[W/m2]. It is clear that MPPT algorithm finds maximum power point very fast and changes duty ratio accordingly to maintain maximum power extracting without oscillations. 5. Conclusion This paper presented two-stage energy conversion system to connect PV array to grid utility. Maximum power scanning algorithm is used regularly to determine optimum duty-ratio to extract MPP. It also presented inverter control scheme to deliver converter output power to utility grid at unity power. The simulation results have shown that advantage of this system is fast response, good transient performance, and high accuracy. 6. Acknowledgement This work was supported by research grant of Kongju National University in References [1] Y. Levron, D. Shmilovitz, Maximum power point tracking employing sliding mode control, IEEE Transactions on Circuits and Systems, vol. 60, no. 3, pp , [2] R. Kadri, J. Gaubert, G. Champenois, An improved maximum power point tracking for photovoltaic grid-connected inverter based on voltage oriented control, IEEE Transactions on Industrial Electronics, vol. 58, no. 1, pp , [3] P. Barrade, S. Delalay, A. Rufer, Direct connection of super-capacitors to photovoltaic panels with on-off maximum power point tracking, IEEE Transactions on Sustainable Energy, vol. 3, no. 2, pp , [4] N. Kasa, T. Iida, L. Chen, Flyback inverter controlled by sensorless current MPPT for photovoltaic power system, IEEE Transactions on Industrial Electronics, vol. 52, no. 4, pp , 2005 [5] J. Kwon, K. Nam, B. Kwon, Photovoltaic power conditioning system with line connection, IEEE Transactions on Industrial Electronics, vol. 53, no. 4, pp , [6] Y. Jiang, J. Abu Qahoug, Single-sensor multi-channel maximum power point tracking controller for photovoltaic solar systems, IET Power Electronics, vol. 5, no. 8, pp , [7] H. Patel, V. Agarwal, MPPT scheme for a PV-Fed single-phase single-stage grid connected inverter operating in CCM with only one current sensor, IEEE Transactions on Energy Conversion, vol. 24, no. 1, pp , [8] R. Wai, W. Wang, Grid-connected photovoltaic generation system, IEEE Transactions on Circuits and Systems, vol. 55, no. 3, pp ,