High Gain DC-DC Converter with Enhanced Adaptive MPPT for PV Applications

The increasing demand for electricity has pushed more effort to focus on renewable energy sources to satisfy the consumer. The renewable energy sources are playing a major role in the generation of electricity. Out of all the renewable energy sources, solar has emerged as one of the best sources of energy since it is clean, inexhaustible and eco-friendly. However, the voltage generated by the solar cell is not sufficient for any consumer load and it is also variable. Therefore, it is necessary to implement DC-DC converters for regulating and improving the output voltage of the solar panel. In order to extract the maximum output from the PV (Photovoltaic) panel, a comparative analysis of various MPPT (Maximum Power Point Tracking) algorithms is proposed in this paper. The proposed enhanced adaptive P&O (Perturb and Observe) algorithm is modeled and implemented with a high gain DC-DC converter. The converter investigated in this paper consists of a single power electronic switch (MOSFET) for its operation, which leads to reduction of switching and conduction losses. The proposed converter has less ripple content and a high conversion ratio. A simulation study of the proposed power electronic converter powered by PV source is carried out in MATLAB/SIMULINK and the results are validated using an experimental setup.


INTRODUCTION
T he PV energy generation has become the most predominant form of energy. This is due to the fact, that the energy generated by solar panel can be directly converted into direct current. Furthermore, the solar systems produce clean power, with no pollutant emissions, helping in such a way to strive against the global warming [1]. The output generated from the PV panel mainly depends upon the junction temperature and the solar insolation level.
The major problem in solar systems is that the operating characteristics of the load and the PV array are mismatched. To obtain the optimum condition when it is directly connected to the load, a curve between the panel voltage and current is drawn and a point of intersection is determined. MPPT is then employed in the solar system to obtain the maximum power from the PV array and delivers it to the load. Maximum power is transferred to the load by varying the impedance on the load side and also matching it with the peak power at the instant of changing the duty cycle [2]. In this respect, several MPPT techniques are proposed. P&O is the simplest MPPT technique when compared to the conventional types and also exhibits very fast convergence to achieve the MPP (Maximum Power Point). But, the negative side of this method is that, when a perturbation occurs, the algorithm will force the operating point to continuously move in the region of the MPP [3]. Based on the level of perturbation, the power loss gets increased. The next drawback is that the P&O algorithm loses its tracking direction with respect to the irradiance level [4]. When the tracking direction gets deviated, the algorithm becomes mystified and it gets diverged from the MPP [5].
Another major drawback is the tracking of global peak under partial shading condition is poor. To overcome all these problems, an enhanced adaptive technique has been proposed in this paper.
Though maximum power is obtained from the PV panel, the regulation of output voltage is essential since it is variable in nature. Therefore, a power electronic converter has to be interfaced with the solar system. Apart from regulation, the voltage obtained from the PV should match the load side specifications. These issues have triggered a severe demand for the use of high efficient DC-DC converters [6].
To achieve high voltage gain, conventional boost converters with high duty ratio can be used. But, this leads to conduction losses and also results in reverse recovery problems. To overcome these problems, various researchers in the field of power electronic converters have come up with new modified topologies of high gain boost converters. The high gain converters that employ switched capacitors produce high transient current, thus reducing the life of the switched capacitor [7]. This drawback can be eliminated by the integration of voltage doubler circuit along with the switched inductor topology in order to achieve high voltage with reduction of stress across the switches on the power electronic devices [8].
But this scheme requires high number of switching devices, which makes the circuit configuration to be complex and the cost is also high. Therefore, quadratic boost converter is incorporated with the minimum number of switches. This requires transformer with large turns ratio which results in high leakage inductance, more voltage and current spikes on the power switches [9]. To overcome these problems, a single switch high gain boost converter by integrating a transformer-less quadratic boost converter with a conservative model of boost converter is proposed in this paper.

PV MODELING AND CHARACTERISTICS
A solar cell is basically a p-n junction which converts the light source into electricity through photovoltaic effect.
When exposed to the sunlight, an electron-hole pair is created which is proportional to the incident radiation. A higher range of voltage and current can be attained by incorporating multiple cells [10]. The representation of PV cell is shown in Fig. 1. [13] The series resistance and the shunt resistance correspond to the internal losses and the leakage current respectively.

FIG. 1. EQUIVALENT CIRCUIT OF PV CELL
The output current is a function of solar radiation, temperature and various other parameters.
The PV modeling is done using the following Equations

Enhanced Adaptive Perturb and Observe Algorithm
The Under partial shading condition, local peaks occur at certain conditions. When the shading level increases gradually with respect to the number of series modules, the local peaks position gets shifted towards right.
Therefore, it is necessary to shift the predicted points to the right along with the increasing shading level. The scanning points are set to V 1 ,V 2 ,……V Ns . The value of current is determined by choosing the point which is close to the short circuit current and it is used to compute the irradiance level. Then it is shifted to V 2 and the value of I 2 is noted. In the same manner, the EAPO service the change of irradiance level and shifts the peak points [16]. After scanning all the predicted peak points, the scanned power will be compared. By comparing the obtained values, the global peak point is determined.
The EAPO algorithm is demonstrated using a flow chart and is shown in Fig. 2.

HIGH GAIN DC-DC BOOST CONVERTER
Normally, classical boost converters are employed for PV applications, but they suffer from severe voltage and current stress when operated at large duty ratios to achieve high gain. Moreover, it produces high ripple both at the input and output side. This can be overcome with the help of a high gain DC-DC converter. Several high gain converter topologies are discussed by various researchers [17][18][19][20][21] and this paper focuses on a topology which combines the conventional boost and quadratic boost to achieve high gain and efficiency. The circuit diagram for the proposed high gain DC-DC converter is shown in Fig. 3.
The operation of the converter is explained in two modes. The value of the voltage in inductor L 1 is nothing but the input voltage V g , and the voltage in inductor L 2 is equal to the difference of the capacitor voltages C 1 and C 2 . The value of current in capacitors C 1 and C 2 is equal to that of the current flowing through inductor L 2 and L 1 respectively. At this condition the charging of capacitor C 2 takes place.

Mode
The value of inductor, 2 2fsΔ where, D is the duty ratio, V in is the input voltage, f s is the switching frequency, I L1 is the current through inductor L 1 ,I L2 is the current through inductor L 2 The current across the inductor L 1 is, where, I o is the output current The current across the inductor L 2 is, The voltage across the inductor is, (11) where, V in is the input voltage The voltage across the inductor is Where, V c1 and V c2 are the voltages across capacitors C 1 and C 2 respectively.
The value of the capacitor C 1 is,  ΔVc1fs The value of the capacitor C 2 and C 3 is, Where , The voltage across the capacitors are, Where, D' = (1-D), D is the duty ratio, V in is the input voltage, V o is the output voltage.
The voltage gain for the converter is given by, Using the above equations, the simulation parameters are designed for the high gain converter and it is shown in Table 1.

SIMULATION RESULTS
The PV modelling is done using MATLAB/SIMULINK and the results are shown in Fig. 4

High Gain DC-DC Converter with Enhanced Adaptive MPPT for PV Applications
Power  Table 2.
From  Fig. 8 shows the generated gate pulses after the implementation of enhanced adaptive MPPT algorithm.
The simulation of high gain DC-DC converter is carried out using MATLAB/SIMULINK for an input voltage of 37.8V. The Simulink model is shown in Fig. 9.
The waveforms for voltage, current and power are shown in Figs. 10-12 respectively.  Fig. 11, it is found that the output voltage ripple is in the range of 0.018V which is a low ripple value. Fig. 12 shows the waveform for the output current. The current is maintained at 1.656A with less ripple content.
The waveform is shown in Fig. 13 Fig. 13, it is clearly indicated that the input current ripple is in the range of 0.03A which is a low ripple value.
From Fig. 14, it is found that the maximum power has been achieved. It is in the range of 242.2 W and the required power rating is 250W.
The voltage stress across the main devices for the proposed topology is shown in Fig. 15.

VOLTAGE STRESS ACROSS THE SWITCHING DEVICES
A graph drawn between the percentage of efficiency with respect to the load for the high gain converter topology is shown in Fig. 16.
From Fig.16, it is clear that the proposed converter gives an efficiency of 95.6% at full load.
The high gain converter investigated in this paper is compared with some other topologies of boost converter based on the parameters such as output voltage ripple, input current ripple, duty ratio, voltage gain and voltage and current stress across the main switch. The comparison of parameters is shown in Table 3.
From Table 3, it is clear that the proposed high gain converter is efficient in terms of ripple voltage, ripple current, duty ratio and voltage gain when compared to the other boost converter topologies.
The high gain topologies are also compared based on the number of devices used, control complexity and filter components. The comparison is shown in Table 4.    Fig. 19, and it is found that high conversion gain can be achieved even at low duty ratio.
For the required power rating, high voltage conversion has been achieved at a duty ratio of 0.4 The full load efficiency for all the discussed topologies is shown in Table 6. From