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MODELING AND SIMULATION OF SOLAR
PHOTOVOLTAIC APPLICATION BASED MULTILEVEL
INVERTER WITH REDUCED COUNT TOPOLOGY FOR
STAND-ALONE SYSTEM

Amarnath, R.K. Nema, Deepak Verma
Department of Electrical Engineering
Maulana Azad National Institute of Technology, Bhopal 462003,
Madhya Pradesh, India

ABSTRACT

As the solar market is blooming and forecasted to continue this trend in the coming years. The efficiency and reliability of PV based system has always been a contention among researchers. Therefore, multilevel inverters are gaining more assiduity as it has multitude of benefits. It offers high power capability along with low output harmonics. The main disadvantage of MLI is its complexity and requirement of large number of power devices and passive components. This paper presents a topology that achieves 37.5% reduction in number of passive components and power devices for five-level inverter. This topology is basically based on H-bridge with bi-directional auxiliary switch. This paper includes a stand-alone PV system in which designing and simulation of Boost converter connected with multilevel inverter for ac load is presented. Perturb and observe MPPT algorithm has been implemented to extract maximum power. The premier objective is to obtain Voltage with less harmonic distortion economically. Multicarrier Sinusoidal PWM techniques have been implemented and analysed for modulation scheme. The Proposed system is simulated n MATLAB/Simulink platform.

KEYWORDS

MLI;P&O MPPT algorithm;MC-SPWM; Boost converter

1. INTRODUCTION

Increasing energy demands due to population growth and technology advancement, there is tremendous stress on our environment. Majority of electrical production are provided by fossil fuels like coal, natural gas and oil which contribute highly to the increasing CO2 production. Photovoltaic power generation is a technology that directly transforms unlimited, unpolluted and free solar energy into electric energy. Renewable energy resources include solar, wind, biomass, geothermal and tidal. These energy resources are not only clean, sustainable but also available in a large amount. However, there are some disadvantages with these sources like unreliability in supply, resource location and cost which prevents them from entering in mainstream energy sources. Solar Energy is becoming quite significant among sustainable energy system since itoffers many advantages such as no fuel cost, no pollution, requirement of little maintenance and no noise. But solar PV system is accused of having relatively low conversion efficiency in order of (12%-15%), high installation cost, and energy fluctuation and location dependency. The output power of PV array is dependent on temperature as well as solar radiation or insolation [1]. So with the variation in environmental temperature or insolation, PV power and maximum power point also varies. The non-linear characteristics of PV panel and underutilization (because of shading or low insolation) do affect the efficiency of a PV array. For tracking of a maximum operating point under a different environmental condition, we use controlled converter with maximum power point tracking algorithm. To make it use as standalone it must be converted from dc to ac with high efficiency and less total harmonic distortion. Therefore, multilevel inverter can be a good option for this.

High power applications are in demand now-a-days. Many industrial applications equire medium voltage and high power level [2]. Therefore, multilevel inverter can be a good alternative for applications like PV panel, wind and fuel cell. Conventionally, there are three topologies for MLI. But now-a-days researchers are working on reduced count topology because conventional topologies require a lot of power electronic switches and capacitors. The rating of these power electronics switches depend on the dc voltage source to which it is connected. Multilevel inverter has lots of advantages over onventional inverter. The voltage generated by MLI contains veryless harmonics. Common mode voltage produced by MLI is very less which reduces stress on motor bearings. Input current of MLI contains less distortion. In addition to these MLI also workson low switching frequency which causes less switching loss and high efficiency.

2. PROPOSED SYSTEM

This system consists of two stage stand-alone PV system in which voltage is stepped up by boost converter and then is converted to AC with the help of MLI. The P&O MPPT algorithm is implemented to control the duty cycle of boost converter. The output voltage obtained by MLI consists of five levels with multicarrier sinusoidal pulse width modulation scheme. The block diagram of the stand-alone solar PV system is shown in figure 1.

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3. SOLAR PV ARRAY

3.1 OVERVIEW

The proportional single diode model of a viable PV cell is shown in figure….It is also termed as five parameter model(n, Rsh, Rs, Io, Iph). RS is acquainted as with consider the voltage drops and inward misfortunes in because of stream of current. Rshtakes into record the spillage current to the ground when diode is backward one-sided.

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Where,
n is ideality factor of diode
RS is series resistance
Rshis parallel resistance
Iph is photon current which is directly proportional to solar insolation
ID is diode current

3.2 SPECIFICATION OF SOLAR PV ARRAY

UE Solar ZHM175 is the Solar Module that has been taken for Analysis and its data sheet isshown in Table 1 [3]-[4].

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The array is taken to get maximum power of 110kW approximately and is connected as:
Number of Parallel Strings = 70
Number of Series Connected modules per String = 9
I-V and P-V characteristics of the PV array taken are shown in Figure 3-6

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4. BOOST CONVERTER

4.1. OVERVIEW

Boost converter is a class of switch-mode power supply which is used to step up DC voltage,having two semiconductor switches and two energy storing element as shown in
figure7.When switch S is turned on, current flows through inductor and energy is stored. When Switch S is turned off, stored energy provides an induced voltage that is esponsible for stepping up of voltage.

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4.2. DESIGNING OF BOOST CONVERTER

While designing we have considered ideal boost converter. Selection of Inductor is done on the basis of estimated Inductor current ripple and can be calculated as [5]

7                           1

Where, ∆I_(L )is the estimated inductor current ripple and can be calculated as

8         2

Above calculated value of inductor must be more than the critical value of inductor (L_C)
otherwise boost converter will not be able to operate in current continuous mode. Critical Inductance can be calculated as

93

Selection of output capacitor is done on the basis of output voltage ripple and can be calculates as

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5. MPPT TECHNIQUE

Several MPPT techniques together with their implementation are explained in the literature [6]-[10]. Researchers always face problem while choosing an MPPT technique for a particular application. Unfortunately, only a few techniques were available in this field including Curve fitting, Fractional Short Circuit Current, Fractional Open Circuit Volta Among all the MPPT methods, Perturb & Observe (P&O) and Incremental Conductance (IC) are most generally used because of simple implementation and less tracking time to track the maximum power point. Under suddenly changing weather conditions Incremental Conductance method can be a good choice. However, instead of more efficiency the complexity of the algorithm is very high compared to the former one and hence the cost of execution increases. So we have to extenuate with a trade-off between ramification and efficiency. Therefore, keeping in view the complexity and easy implementation P&O method is used here to track maximum power.

5.1. PERTURB & OBSERVE ALGORITHM

Perturb and observe (P&O) method is mostly used out of other method as it is simple and less expensive [11]. This algorithm is based on the sign of slope of P-V curve of solar module. In this algorithm, voltage is perturbed and slope (dP / dV) is checked weather it is positive, negative or zero. If the slope is zero then that point is MPP and if slope is negative then voltage is perturbed in reverse direction else voltage perturbation is continued in same direction until we reach peak point. The algorithm for P&O is shown in Figure8.

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6. MLI TOPOLOGY

6.1. CIRCUIT CONFIGURATION

The proposed MLI consists of a basic cell which produces five level of output voltage. The circuit diagram is shown in the figure9.

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Proposed topology contains minimum number of switches for generating same number of output levels as compared to conventional MLI. This topology basically uses one voltage sources Vs (here voltage source represent stepped up DC-output voltage from PV arrays) and four power devices (Q1, Q2, Q3, Q4) and one auxiliary switch (S1) with four diodes.

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6.2. WORKING

The table 4 below gives the switching states of basic cell of proposed MLI. Here logic ‘1’ is
considered as ‘ON’ state and logic ‘0’ is considered as ‘OFF’ state of the switches.

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Reactive load has been considered as load in this case. At the Output terminal of boost converter, two capacitors are connected in parallel so that no voltage swing is produced in normal operation.

7. MODULATION SCHEME

Appropriate Modulation Scheme is always required to generate and control high quality Output of MLI. Various techniques for modulation have been discussed in literature like Multi-carrier Pulse width Modulation and space Vector modulation [13].Here, due to simplicity and easy implementation MCPWM scheme has been discussed.

To generate five levels output four carrier signals are required. Different way to generate multicarrier signals based on level shift has been shown in figures 6.

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7.1. PHASE DISPOSITION (PD-PWM)

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In this scheme all the carrier signals are in phase and level Shifted. The zero reference is placed at the middle of the carrier set.

7.2. PHASE OPPOSITION DISPOSITION (POD-PWM)

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In this scheme all the carrier signal above zero reference are in same phase and all the carrier signals below zero reference are shifted by 180 degree.

7.3. ALTERNATE PHASE OPPOSITION DISPOSITION (APOD-PWM)

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8. SIMULATION RESULTS

The following results are obtained with constant insolation of 1000 W/m^2 and Temperature of 25℃.The switching frequency of boost converter and inverter is taken 5 KHz and 2 KHz respectively. Resistance of 100 ohm is taken as load.

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9. CONCLUSION

The paper presents a comprehensive analysis and simulation in MATLAB Simulink. The P-V and V-I curve is plotted for different temperature and insolation.The P&O MPPT algorithm is also implemented to extract maximum power with direct duty ratio control. Different curves of PV voltage, PV current, Boost converter outputs are obtained. Output voltage with five levels is obtained as expected with the bi-directional switch. The THD in output voltage of system is 22.23% without any filter. A significant reduction in number of switches is obtained. The aforementioned topology can also be used for asymmetric source configuration. Number of levels can also be increased by connecting this topology in cascade manner, as the number of levels increase it ameliorate the power quality and can moderate the need of filter.

10. ACKNOWLEDGEMENT

The author would like to thank everyone, just everyone!

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AUTHORS

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