Simulation Analyis of Maximum Power Control of Photo Voltaic Power System

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International Journal on Advanced Electrical and Electronics Engineering, (IJAEEE), ISSN (Print): 2278-8948, Volume-1, Issue-1, 2012

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Simulation Analyis of Maximum Power Control of Photo Voltaic Power System

Venkatesh Busa, Kiran Kumar Narsingoju & G.Vijay Kumar Department of EEE, Sri Chaitanya College of Engineering, Karimnagar, India

E-mail : [email protected], [email protected] & [email protected] .

Abstract - As primary energy source such as coal and oil has been dried up and has bad influence on environment, human urgently need renewable energy which is friendly to environment. So in the future, solar energy will be very important energy source. It is noticeable that the output characteristics of a PV array are influenced by the environment factors, and the conversion efficiency of PV arrays is very low. Photovoltaic array simulations can be used to find better methods to implement maximum power point tracking (MPPT) control for efficient solar power systems. Photovoltaic cells model of photovoltaic power generation system and basic control algorithm is discussed in this paper. MPPT control based on perturbation & observation (P&O) method and incremental conductance (IncCond) method is respectively simulated in Matlab/Simulink. The simulation analysis shows that P&O method is simple, but has considerable power loss. IncCond method has more precise control and faster response, but has correspondingly higher hardware requirement. In practice, in order to achieve maximum efficiency of photovoltaic power generation, a reasonable and economical control method should be chosen.

Keywords - Photovoltaic power generation; MPPT; P&O method; IncCond method; Matlab/Simulink.

I. INTRODUCTION

Photovoltaic power generation system is a main effective way of using solar energy, which can convert sunlight radiation directly into electricity through the photovoltaic effect, and has broad prospects for development with a series of advantages such as clean and pollution-free, noise-free, and renewable[1].

However, as the external environment such as sunlight intensity, temperature of PV panels etc., often change over time, PV arrays can not work at maximum output power point continuously. So electrical energy that PV array injects into the grid or load decreases, and energy conversion efficiency is also very low[2].In order to make photovoltaic arrays output more power in the same sunshine, temperature and other conditions, power electronic devices with maximum power point tracker (MPPT) control should be used in photovoltaic power generation system[3]. In this paper, firstly the equivalent model of photovoltaic cells is established, then photovoltaic arrays are constructed as the basis for building a Simulink model and completing the simulation. The output characteristics of photovoltaic cells are analyzed. Secondly, several commonly methods of the MPPT control is disserted in this paper.

Finally, modeling and simulation are used to compare

and analyze advantage and disadvantage of these MPPT control methods.

This research will help photovoltaic power generation system getting high photovoltaic conversion efficiency, working at the maximum output power, and utilizing solar energy more efficiently.

II. MODELING AND SIMULATION OF PV CELLS

A. Equivalent model of PV cells

The mathematical model of photovoltaic cells need to be established to study the characteristics of photovoltaic cells, Characteristics of photovoltaic cells can be obtained through the study of photovoltaic cells model[4]. PV Equivalent circuit model is shown in Figure 1

Figure 1. The equivalent circuit of PV cells

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10 B. Modeling and Simulation of the PV array

According to the output characteristic equation, the PV array model is established as shown in Figure 2.

Figure 2. Simulink model of PV array

P-V, V-I characteristic curves shown in Figure 3 are obtained through Simulink simulation of photovoltaic array model, where, left for the P-V curve, right for the V-I curve. From the characteristic curve, photovoltaic cells is neither constant voltage nor constant current source, and it is a nonlinear DC power supply.

Figure 3. P-V, V-I characteristic curve of PV array simulation

III. MAXIMUM POWER POINT TRACKER (MPPT) CONTROL

A. MPPT algorithm principle

PV array output characteristic curve is shown in Figure 4. Assume that curves1 and curve2 are output characteristic curves of the PV array under different light intensity[5]. Point A and point B are the corresponding maximum power output; and at certain point, the system runs at the point A .

When the light intensity changes, output characteristics curve of the PV array rises to curve2 from curve1. Meanwhile, if the load1 remains invariant, the system will run at the point A′ , which deviated from

the maximum power point of the corresponding light intensity. In order to continue to track the maximum power point, the system load characteristics should be changed to load2 from load1. Then the system is ensured for running on the new maximum power point B.

Similarly, if the light intensity changes, the output characteristics of photovoltaic arrays drops to curve1 from curve2, and the corresponding operating point changes from point B to point B′ , Then, load2 should be adjusted to load1 to ensure that the system is still operating at maximum power point under case of reduction of light intensity [6] .

B. Perturbation observation (P&O) method

Classical P&O algorithm is summarized as follows[7]: PV system controller change PV array output with a smaller step in each control cycle. The step size is generally fixed while mode can also be increased or decreased. Both PV array output voltage and output current can be the control object, so this process is called "perturbation"; then, by comparing PV array output power of the cycles before and after the perturbation.

Figure 4. MPPT algorithm analysis diagram

If power output is increased, “perturbation" will continue to work follow the direction in the previous cycle, else, if power output is decreased, “perturbation"

will change the direction. In this way, the actual operating point of PV array can move closer to the maximum power point, and ultimately back and forth to reach steady state in a relatively small area . Flow chart for P&O method given below.

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11 Fig5. flow chart for P&O algorithm

C. Incremental conductance (IncCond) method Incremental conductance method is one of common MPPT control algorithm[8]. P-V characteristic curve of the PV array indicated that its power - voltage curve is a single peak curve, and there exists a unique point of maximum power where the derivative of power to voltage is zero.

Due to P =V . I , at the maximum power point exists follow :

=0 +

= dV I dI dV dP

(1) That is dI / dV = -I /V ,This equation is the maximum power point conditions.

dI = I (k) - I (k -1) (2) dV = V (k) -V (k -1) (3) dP = P(k)-P(k -1) =V(k).I(k)-V(k -1). I(k -1) (4)

If dI / dV < −I /V , the PV operating point is at the right of maximum power point, then the output voltage should be reduced.

If dI / dV > −I /V ,he PV operating point is at the left of maximum power point, the output voltage should be increased.

Summarized as follows:

When max V >V, then dI / dV < −I /V ; When max V <V , then dI / dV > −I /V ; While max V = V, then dI / dV = −I /V .

Operating voltage can be adjusted with the relationship between dI / dV and I /V in order to achieve maximum power point tracker. flow chart for Inc conductance algorithm given below.

Fig6. flow chart for Inc conductance algorithm.

D. Modified Incremental conductance (IncCond) method

All modifications described above can be summarized in the flowchart:

It has the following five stages:

1. Calculation of the power and the increments in the power, voltage and current.

2. Determining the rate of change in the reference voltage by taking into account, first the step changes and then the six cases depending on the value of ∆I.

3. Core algorithm (P&O or InCond as shown in Figure 12 or Figure 22 respectively).

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12 4. Determining the irradiation slope, by comparing the

last three increments in the current and power, and adjusting ∆Vref .

5. Setting limits in the reference voltage so it is within the minimum and maximum input voltages of the converter.

Fig7: Flowchart of the proposed modified algorithm IV. PV SYSTEM MPPT CONTROL SIMULATION

The photovoltaic power generation system including: Photovoltaic modules, MPPT module, PWM module and Boost converter module[9]. The maximum power point tracker module is described mainly in this paper.

A. MPPT control simulation based on P&O method Photovoltaic system simulation model based on P&O method control is shown in Figure 8:

Figure 8. Simulation model based on P&O method

Design idea of this module is: next change or perturbation direction is decided by the change of power value. If the power value increases, the voltage perturbation will maintain the same direction, meanwhile, if the power value decreases, the voltage perturbation will turn into the opposite direction. The direction of the voltage perturbation is decided by whether Vk is greater than Vk-1 . Power change can be detected by comparing the previous power value and the current power value, which is used to calculate the control signal of PWM.

The simulation result is shown in Figure 9. The figure is the power waveform with time. It can be seen from the figure, the simulation waveform of MPPT control based on P&O method has overshoot in the beginning and shaking up and down regularly afterward until it reaches a stable equilibrium state. Because output power change of PV system has been decided from comparison between adjacent time in operation of the MPPT control based on P&O method. Only the output power values of adjacent time reach the same the control action will stop. This situation generally does not exist in practice.

Figure 9. Simulation results based on IncCond B. MPPT control simulation based on IncCond method

Photovoltaic system simulation model based on IncCond method control is shown in Figure 10:

In this MPPT module based on IncCond method , Output voltage and current of the PV array is the input of module, and a derivative module is used in this module.

Firstly it necessary to verify whether Vk is equal to Vk-1, then the current relationship or the relationship between Conductance change and instantaneous conductance negative value is judged respectively, and ultimately the direction of voltage perturbation is determined. Step size is still chosen as 0.01 here.

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13 The simulation result is shown in Figure 11. It can be seen from the figure that the PV system remains at a maximum output power point, rather than shocking near the maximum power point after achievement of maximum output power, which is consistent with the basic theory of IncCond method MPPT control described above.

Figure 10. Simulation model based on IncCond method

Maximum power point after achievement of maximum output power, which is consistent with the basic theory of IncCond method MPPT control described above.

The reason is that there is always a certain error in determining relationship between dI / dV and I /V ( dI / dV = −I /V ), while this error is not completely ignored to the software or computing precision. Therefore in a certain range, change of step size has not very significant effects on output power of PV system.

Figure 11. Simulation results based on IncCond

V. CONCLUSION

Photovoltaic cell simulation model is established in this paper, and output characteristics of photovoltaic array is studied and analyzed. Then two common MPPT control module based on two control algorithms is describes and a photovoltaic system MPPT control simulation model is built and simulated in Matlab/Simulink.

Comparison of this two methods result leads to the conclusion: both methods can realize the maximum power point tracker control. The P&O method is easier to achieve due to less demanding on hardware, but will have some power loss because PV array can only run in oscillation way around the maximum power point. The IncCond method has more precise control and smaller steady-state oscillation than P&O method, but hardware requirements are relatively higher.

In practice, various factors should be considered to choose a reasonable and economical control method to achieve maximum efficiency.

REFERENCES

[1] Zhao Chaohui. “Emerging Technology: Photo- Voltage Generation Status and Trends”. Journal of Shanghai Dianji University, 2008, 11(1): 104- 109.

[2] Zhao Zhengming, Liu Jianzheng. “Solar photovoltaic power generation and application”.

Beijing: Science Press, 2005.

[3] Wang Zhao’an, Huang Jun. “Power Electronics Technology”. Beijing: Mechanical Industry Press, 2007.

[4] Yu Huajun, Pan Junmin. “Simulation Analysis of Output Features and the Maximum Power Point Tracking of PV cells”. Computer Simulation, 2005, 22(6): 248-252.

[5] Mao Meiqin, Yu Shijie, Su Jianhui. “Versatile Matlab Simulation Model for Photovoltaic Array with MPPT Function”. Acta Simulata Systematica Sinica, 2005(5): 1248-1251.

[6] Wang Feng, Zhang Miao, Zhang Maodun.

“General Matlab Simulation Model of Photovoltaic Module Based on S-function”.

Journal of Guangdong University of Technology, 2008, 25(4): 69-72.

[7] Li Jing, Dou Wei, etc. “Research on MPPT Methods of Photovoltaic Power Generation System”. Acta Energiae Solaris Sinica, 2007, 28(3): 68-273 .

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14 [8] Zhou Dejia, Zhao Zhengming, etc. “Analysis

characteristics of photovoltaic arrays using simulation”. Journal of Tsinghua University (Science and Technology), 2007, 47(7): 1109- 1112,1117.

[9] Li Wei, Zhu Xinjian. “The Maximum Power Point Tracking Control of a Photovoltaic Power System”. Computer Simulation, 2006, 23(06):

239 -243. 1172.