Maximum power point tracking quadratic boost converter for

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Abstract – Providing maximum utilization of the photovoltaic system is one of the significant challenges. Therefore, converter technologies and maximum power.

ECAI 2016 - International Conference – 8th Edition Electronics, Computers and Artificial Intelligence 30 June -02 July, 2016, Ploiesti, ROMÂNIA

Maximum Power Point Tracking Quadratic Boost Converter for Photovoltaic Systems Necmi Altin, Ertan Ozturk Department of Electrical & Electronics Engineering, Faculty of Technology Gazi University Ankara, Turkey [email protected], [email protected] Abstract – Providing maximum utilization of the photovoltaic system is one of the significant challenges. Therefore, converter technologies and maximum power point tracking algorithms are important research topics. In this study, a quadratic boost converter with high conversion ratio is proposed. The proposed quadratic converter not only steps up the PV voltage which is commonly low, it also tracks the maximum power point of the PV system under varying operation conditions. The incremental conductance method is used as maximum power point tracking algorithm. Thus, productivity of the PV system is increased. It is seen from simulation results that, proposed system has fast tracking capability even under fast irradiation changes and less oscillation besides the features of high voltage conversion gain. Keywords – Quadratic Boost Converter; MPPT; Incremental Conductance; PV

I.

INTRODUCTION

Increasing world power demand, depletion of fossil fuels and increasing awareness on green word concept make renewable energy resources (RESs) popular, and therefore number of researches on RESs has been exponentially increased in last two decades. However, stochastic natures of RESs and variable power generation characteristics of these source makes power converter topologies important. Several DCDC, AC-DC and DC-AC converter topologies have been proposed for renewable energy systems [1]. Output voltage of the photovoltaic (PV) modules which are one of the most promising green energy sources varies with load level and natural conditions such as ambient temperature and solar irradiation. Same condition is also valid for fuel cells (FCs). Therefore, PV systems and FC systems require a DCDC converter for regulating their output voltage. The output voltage of the PV module and the FC stack are usually low and require to step-up. The conventional boost converters are commonly used to regulate and step-up the PV or the FC supplied DC voltage. However, step-up ability of the conventional DC-DC converter is related with the duty ratio, and to obtain high step-up ability, high duty ratios are required. Therefore, the practical voltage conversion gain of the conventional boost converter is limited [2-3]. Hence, different topologies have been investigated to obtain high step-up ability [3-5]. Initial studies have considered the cascaded connection of conventional 978-1-5090-2047-8/16/$31.00 ©2016 IEEE

DC-DC converters. However, number of the power processing stages and power switches are equal to number of cascaded converters and this increases the power losses and decreases the efficiency [6-7]. Recently, different DC-DC converter topologies with high voltage step-up ability have been proposed including conventional boost converter combined with switched capacitors [8], voltage multipliers and coupled inductors [9-10]. The quadratic boost converter (QBC) which is structurally similar to cascaded two boost converters has been proposed to provide high voltage conversion ratio. The QBC converter circuit is given in Figure 1. The output voltage of the QBC is given as a quadratic function of the duty cycle of switching signal [11]. The QBC has only one active switch, driver circuit requirement is removed and converter efficiency is improved. Therefore, the QBC is used in several applications where high voltage conversion ratio is required such as power factor correction applications and PV applications [12-13]. One of the important application of DC-DC converter in PV systems is regulating output voltage and power level to get maximum available power from PV modules. This action is called maximum power point tracking (MPPT). There is a nonlinear relation between the PV voltage and the PV current. Because of this nonlinear characteristic, there is a single point on the P-V curve of a PV module that the PV module power gets its maximum value. This operation point is called as Maximum Power Point (MPP) [14]. The MPP of the PV module changes with load level and environmental conditions such as irradiation and temperature, and therefore it should be tracked during the operation to obtain maximum power and maximum energy conversion efficiency for any operation conditions. Several methods have been proposed to track the MPP of the PV system. These methods can be analysis in two groups: directs methods and indirect methods. Indirect methods are usually easy to implement and provide fast response, but since the PV parameters such as voltage, current or solar irradiance are not measured online, the real maximum power point cannot be tracked. Direct methods are generally computational based methods, and they try to compute the real MPP in each control cycle. These methods can track the real MPPT but they may cause an oscillation on output power. The

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pilot cell, the constant voltage, the constant current, the look-up table can be refereed as indirect methods, and the perturb and observe (P&O), the incremental conductance (IC), the fuzzy logic, the neural network based methods are common and well-known direct MPPT methods [15-18]. Among them, the IC method has some advantages such as being a direct control method, it can adjust fast changing atmospheric conditions and there is less oscillation around MPP than the other direct methods [17-18]. In this study, a QBC with MPPT capability for PV systems is proposed. The low voltage generated by the PV system is step-up with a QBC which has high voltage conversion ratio, and required voltage level for inverters or other DC applications is obtained. In addition, the MPP of PV system is tracked with IC based MPPT method, and maximum energy is extracted from PV system in any operation conditions. The results obtained from MATLAB/Simulink simulations show that, proposed system has quadratic voltage conversion ratio and tracks the MPP of the PV system for different operation conditions.

(a)

(b) Figure 2. The quadratic boost converter a) When switch is ON; b) When switch is OFF

VC1 V0 I L1 Figure 1. The QBC circuit

II.

I L2

MODELLING OF THE QBC

The QBC shown in Figure 1 has only one active switch and analysis of the converter is performed according to switch condition. When the switch is ON, D2 diode is forward biases and D1 and D3 diodes are reverse biased. L1 and L2 are charged by supply voltage and C1 capacitor, respectively. When the switch is OFF, D2 diode is reverse biased, and D1 and D3 diodes are forward biased. C1 and C2 capacitors are charged by the supply voltage and inductors (L1 and L2). These operation conditions are shown in Figure 2.

Vin 1 D VC 2

(3)

Vin

1  D 2 Vin

R 1  D 4

Vin

R 1  D 3

(4)

(5)

(6)

here D is duty ratio. III.

PROPOSED QBC WITH MPPT CAPABILITY

In this study, maximum power point tracking quadratic boost converter for PV systems is designed. The designed system is depicted in Figure 3. As it is seen from figure, the system composes PV modules, the quadratic boost converter and the IC based MPPT algorithm.

By assuming that all components are ideal and supply voltage is constant DC voltage, equations given below can be written: TS

V L1

³v

L1 (t )dt

0

(1)

0

(2)

0

TS

VL 2

³v

L 2 (t )dt

0

here, TS is switching period. By using (1) and (2) expressions for each component voltage and current can be derived as given below:

VC1

Vin (3) 1 D

Figure 3. Bock diagram of the proposed system

The IC algorithm is one of the common MPPT methods. This method is more complex than the P&O method, however, the continuous perturbation requirement is removed and thus power and voltage oscillations appeared on P&O method are removed substantially. This method uses the slope of P-V curve of the PV system as depicted in Figure 4. The slope of

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Maximum Power Point Tracking Quadratic Boost Converter for Photovoltaic Systems the P-V curve is equal to zero when the operation point is equal to the MPP. If the slope of the P-V curve is negative, this represents that, operation point of the PV system is at the left side of the MPP. Similarly, if the slope of the P-V curve is positive, this represents that operation point of the PV system is at the right side of the MPP. This situation can be expressed analytically as given below [18]:

dP dV

d V ˜ I dV

I

dI dV V dV dV

I V

dI dV

dI 0Ÿ dV

I  V

D=0.3

400

300 V0

200

Vin

100

0 0.2

0.3

0.4

0.5

0.6

0.7

t (s)

0.8

(a)

(7)

500

D=0.5

V0 (V)

V0

400

Since the slope of P-V curve is equal to zero at the MPP, (8) can be written:

dP dV

500 V0 (V)

300 200

(8)

Vin

100 0

0.2

0.3

0.4

0.5

0.6

0.7

t (s)

0.8

(b) Figure 5. Input and output voltage waveforms of the proposed QBC a) For D=0.3, b) For D=0.5 Ir (W/m2)

1000 Ir (W/m2) 500 0

Figure 4. Schematic representation of the operation principle of the IC algorithm.

IV.

SIMULATION RESULTS

The proposed QBC and IC based MPPT algorithm are modeled and MATLAB/Simulink simulations are carried out. Both design of the QBC and performance of the MPPT method is investigated. Therefore, inputoutput voltage relations of the converter, response of the proposed system while the solar radiation and load are changing are tested. In Figure 5, input and output voltage of the proposed QBC converter for different duty ratio values such as 0.3 and 0.5 are given. It is seen that, while the input voltage value is 100V, the output voltage of the converter is equal to 200V and 400V for 0.3 and 0.5 duty ratio values, respectively. As one can easily see that, there is a quadratic relationship between the duty ratio and voltage conversion gain. In addition, both input and output variations are considered to check the performance of the proposed the proposed MPPT algorithm and the converter. As it is shown in Figure 6, step load changes are applied to the system. At t=0.3 s, the load resistance value is increased from 50 Ω to 100 Ω (load level of the converter is reduced to 50%), and at t=0.6 s, the load resistance value is reduced from 100 Ω to 50 Ω (load level of the converter is increased to 100%). In fact, 50% step load is applied and removed. As it can be easily seen from Figure 6 that, the proposed MPPT quadratic boost converter system tracks the MPP of the PV system and keeps the PV power at its maximum point by regulating its output voltage and power. The performance of the MPPT algorithm and the QBC converter is tested for varying solar irradiation conditions. As it is shown in Figure 7,

PPV (W)

1000 PPV (W) 500

0 V0 (V) 300 V0 (V) 200 100

0 I0 (V)

I0 (V) 4

2 0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7 t (s) 0.8

Figure 6. Response of the proposed system for load changes a) The solar radiation, b) The PV system power, c) The output voltage of the proposed converter, d) The load current

a step change in solar radiation from 1000 W/m2 to 250 W/m2 and from 250 W/m2 to 1000 W/m2 is applied at t=0.1 s and t=0.25 s, respectively. As seen from the figure, the MPPT quadratic boost converter tracks the variation in the operation conditions, and regulates its operation point to get maximum available power from the PV system. Furthermore, it is seen that the proposed system has high tracking speed and less oscillation. In addition, a ramp variation on the solar radiation (both increasing and decreasing solar irradiation conditions) is also applied and the performance of the proposed system is tested. It is seen that, the proposed MPPT quadratic boost converter can track the rapid changes in operation conditions. The proposed system with less oscillation,

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Necmi ALTIN, Ertan OZTURK Ir (W/m2)

1000 Ir (W/m2) 500

0 PPV (W) 1000 PPV (W) 500

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

t (s) 0.8

Figure 7. Response of the proposed system under variable solar radiation, a) The solar radiation (W/m2), b) The PV system power (W)

high tracking speed and accuracy has superior performance in both transient conditions and steady state conditions. V.

CONCLUSIONS

In this study, a maximum power point tracking quadratic boost converter is proposed. Since there is a quadratic function between the output voltage and the duty ratio values of the converter, this converter is very suitable for PV systems, where the voltage level is usually low and is required to step-up. In addition, the incremental conductance MPPT method is applied in control of the proposed QBC. Thus, the proposed system can track the MPP of the PV modules and provides more efficient operation. It is seen from MATLAB/Simulink simulation results that the proposed system has higher voltage conversion gain than the conventional boost converters. Moreover, the proposed converter with IC algorithm has high tracking speed and less oscillations, and it is suitable for tracking MPP of PV system even under rapidly changing atmospheric conditions.

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ACKNOWLEDGMENT This research has been supported by European Union Ministry of Turkey, National Agency of Turkey within the Project Code: 2015-1-TR01-KA203021342 entitled Innovative European Studies on Renewable Energy Systems.

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