Tech. Seminar 2.docx

Advanced seven level transformer-less multilevel inverter topology 2018-2019

Chapter-1 INTRODUCTION In this modern trend of renewable energy sources, the demand for developing the PV system is increasing day by day because of excellent characteristics in main aspects compared to the other renewable sources. In present days PV installations are increasing exponentially mainly with support of government to develop the green energy concept. One of the foremost vital varieties of PV installation is that the grid connected electrical converter configurations. These grid connected PV systems is classified from two main points: PV cell and electrical converter configurations Fig. 1.1. The PV cell should be classified into five groups: string, multi-string, AC-cell, AC-module and centralized technology. All approaches have some benefits and drawbacks; and will compromise numerous attributes like harmonic generation, complexity, efficiency, flexibility, dependableness, safety, modularity and price. However, for residential applications, the most appropriate configuration appears to be the string or multi-string technologies where one or more strings of PV cells are connected to a single inverter and there will be no losses by using this type of configuration compared to centralized technology. Moreover, Maximum Power Point Tracking (MPPT) technique is possible at all conditions independently and overall system efficiency is more.

Fig.1.1 PV System Categorization Dept. of EEE, Dr. AIT

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

There are various methods to implement string and multi-string technologies. Generally, these modules consist of a solar array and a DC-DC converter with MPPT technique. Then build up voltage from DC-DC converter is converted to AC by means of inverter. The main issue of PV system is the number of power electronic components in the overall system is more. Hence, the whole system is in complex nature and cost is not effective because of semiconductor components. The only possibility of reducing the components is designing the inverter effectively in all aspects; so researchers are concentrated in reducing the device count in multilevel inverters. This paper presents the most advanced multilevel Packed U Cell (PUC) concept in medium voltage and high power applications. Among the existing topologies like Neutral Point-Clamped topology (NPC); Flying Capacitor topology (FC); and Classic Cascaded Hbridges, in these topologies abounding drawbacks are found if voltage levels are increased. The main problem associated with these topologies is more number of components while increasing the levels and transformer also required, it results in more price and difficult in implementation. So researchers are focused in creating the new ideas in multilevel converters with more benefits in each and every aspect. In perspective of all the merits, a transformerless converter arrangement is outlined in this paper which is called Packed U Cell (PUC) . It accomplishes high power conversion quality by reducing the device count and low switching disturbances with respective to decreased in cost, circuit complexity at higher voltage levels there by avoids in bulky installations compared to existing topologies.

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

Chapter-2 DESIGN OF PV POWER SYSTEM This model implemented in Matlab/Simulink and it comprised as follows 1. Solar modelling 2. Boost DC-DC converter 3. MPPT Technique 4. Packed U Cell Inverter The proposed concept is shown in Fig. 2.1

Fig. 2.1 PV Power System Block Diagram

2.1 Solar Module Solar cell basically comprised of PN-Junction semiconductor diode. The light energy which is extracted from sun can be directly converted to electricity through photovoltaic effect. Naturally PV system exhibits non-linear characteristics of P-V and I-V by varying

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Advanced seven level transformer-less multilevel inverter topology 2018-2019 with respective to radiant intensity and temperature. The single diode modelling of PV -Cell is done which is as follows and the equivalent circuit is as shown in Fig. 2.1.

Fig. 2.1.1 Equivalent circuit of single diode modelling By applying the Kirchhoff’s Current Law Ish = Id + Ish + I

(1)

I = Iph – Id – Ish

(2)

Ish =( V + IRS) / Rsh

(3)

Id = Io ( e vsh/vT – 1)

(4)

By placing the values of Ish and Id in equation (2) then PV cell current is

2.2 MPPT Technique The entire PV module depends on MPP (Maximum Power Point) to get maximum efficiency and to track maximum power output. Now a days there are so many MPPT techniques available, P&O technique is most widely used because of their merits. According to temperature and irradiance conditions the maximum power point curve is obtained. The model characteristics curve of PV & IV is shown in Fig. 2.2.1 The P&O algorithm operates periodically perturbing i.e. incrementing or decrementing. How change of power is happening in the strategy of P&O technique is followed by Dept. of EEE, Dr. AIT

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

This algorithm encompasses a terribly less time complexness, however once it reaches nearer to the maximum power point, it perturbs on both the directions no end. An acceptable error limit is to be set or a wait operate are often added once MPP is reached therefore increasing the time complexness of the algorithmic rule.

Fig.2.2.1 Model characteristics of PV & IV curve

2.3 Boost DC-DC Converter The output of solar panel is passed to the boost DC-DC converter to step-up the voltage to the required input voltage of inverter. In application of step-up voltage converters boost converter is most widely used because it ensures excellent efficiency with economical in cost characteristics, schematic design is shown in Fig. 2.3.1

Fig. 2.3.1 Schematic design of Boost Converter Generally in ideal condition the relation between input and voltage is stated as

Vo = Vin /(1-D) Dept. of EEE, Dr. AIT

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Advanced seven level transformer-less multilevel inverter topology 2018-2019 Where Vo and Vin are the output and input voltages of boost converter and D is the duty cycle. The basic behaviour is 

When switch is in off condition, it results in current decreases.



When switch is in on condition, it results in current increases.

A MOSFET based boost converters are designed to increase the voltage levels of 81V and 27V to 153.1V and 49.1V dc. Here firing pulses are produced by P&O MPPT technique.

2.4 Design of PACKED U CELL Topology It contains of packed u cells (PUC). Each U cell has an arrangement of two switches and one capacitor. It offers high-energy conversion quality using a small amount of capacitors and power devices, and appropriately they have low production cost. It is very simple in terms of interconnection of components. In this topology number of levels can be recognized by using the following equation:

2n+1 – 1 = Number of levels where n=1, 2, 3, ......

(8)

No of capacitors can be recognized by using the following equation:

N = 2Nc + 1 – 1

(9)

Where I is the no of voltage levels, Nc is the number of capacitors, similarly the quantity of voltage levels N with individual to the quantity of switches Nsw given by following equation:

N = 2(Nsw/2) – 1

(10)

In fact, that above equations shows the advantages of this topology not only utilizing single DC source but also the reduced number of power switches used to generate the desired voltage levels. For the 7 level PUC topology six active switches and two sources are required compared to the other topologies and the comparison Table 1 give the clear performance of this topology.

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

The main applications of this topology are PV applications, Motor drives etc. It offers better power quality in terms of achievable number of voltage levels, against other multilevel topologies and reliability of this system is more.

2.5 Evaluation of seven level packed u cell The PUC type inverter consists six semiconductor devices Sa, Sb, Sc and Sa', Sb', Sc'. With every switch consists of an IGBT and diode which is connected in anti-parallel direction. Here single arm is formed using three switches and with this arrangement eight possible combination can be obtained to form a single phase PUC inverter. The dc source V 1 is directly provided from the supply, and the source V2 which is provided by the converter limiting circuit to the preferred voltage level that generates the definite number of voltage levels around the load. It should be described Sa', Sb', Sc' and are working in complementary of Sa, Sb and Sc. So each brace of (Sa, Sa'), (Sb, Sb') and (Sc, Sc') cannot conduct at the same time. The switching voltage sequence can be given in Table 2. From the table the voltages are as V1, V1-V2, V2, 0, 0, -V2, V2-V1, - V1 .

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

Here IGBT switches are used because it is a sort of transistor which works with greater amount of power transfer and contains a higher switching speed with high efficient. 6 IGBT switches are utilized in seven level PUC topology and it can be divided into two legs, hence three switches from one leg which is as shown in fig.2.5.1

Fig.2.5.1 Seven level topology

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

Chapter-3 SIMULATION ANALYSIS In this section the proposed PV system was simulated using MATLAB/Simulink software. The power system consists of two PV modules with different ratings, boost dc-dc converters with P&O based MPPT technique and 7-level Packed U Cell topology. The simulation parameters of PV modules are shown in Table 3.

The characteristics of PV modules are obtained with varying irradiance and constant temperature of 0.5kW/m2 & 1kW/m2 at 25oc are chosen. Fig. 3.1, 3.2, 3.3, 3.4 shows the PV & IV characteristics of PV Module 1 & Module 2.

Fig.3.1 PV Characteristics of Module-1

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Fig.3.2 IV Characteristics of Module-1

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

Fig.3.3 PV Characteristics of Module-2

Fig.3.4 IV Characteristics of Module-2

Fig. 3.5 and 3.6 shows the Maximum Power Point tracking for two different irradiations at t=0.05 sec the irradiance is varied from 0.5kW/m2 to 1kW/m2 which leads to output power rise correspondingly.

Fig. 3.5 MPPT of PV module-1 for a step change in irradiation from 500W/m2 and 1000W/m2

Fig. 3.6 MPPT of PV module-2 for a step change in irradiation from 500W/m2 and 2 1000W/m Dept. of EEE, Dr. AIT

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Advanced seven level transformer-less multilevel inverter topology 2018-2019 Here the PV system generates the voltage of 81V and 27V as shown in Fig. 3.7 and 3.8 respectively, so this voltage is not enough; hence by taking the reference of solar panel the boost converter design is done to raise the voltage up to 153.1V and 49.1V which is as shown in Fig. 3.9 and 3.10.

Fig. 3.7 output voltage of PV Module-1

Fig. 3.8 output voltage of PV Module-2

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

Fig. 3.9 Boosted Voltage of PV Module-1 fed as input (V1) to the 7-level PUC Inverter

Fig. 3.10 Boosted Voltage of PV Module-2 fed as input (V2) to the 7-level PUC Inverter

The inverter load voltage and current at 1000W/m2 is taken and should be shown in Fig. 3.11 & 3.12 by observing those waveforms it is clear that the outputs are almost nearer to sinusoidal hence there is no need of filter requirements. Load current harmonic distortion is around 6.08% as presented in Fig. 3.1.

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

Fig. 3.11 Load Voltage Waveform

Fig. 312 Load Current Waveform

Fig. 3.13 FFT analysis of load current Dept. of EEE, Dr. AIT

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

CONCLUSION This topic shows the implementation of transformer-less advanced multilevel topology in field of PV applications. The main aim of this proposed concept is reduction in the device count and to improve the overall performance with respective to cost reduction compared to existing multilevel topologies. Moreover the figure of merit of this concept is there is no need of transformers hence it avoids the bulky installations. The P&O MPPT technique is also used to continuously track the maximum power point. This whole concept was verified in Matlab/Simulink

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Advanced seven level transformer-less multilevel inverter topology 2018-2019

REFERENCES [1] A. Nabae, I. Takahashi and H. Akagi, "A New Neutral-Point-Clamped PWM Inverter," in IEEE Transactions on Industry Applications, vol. IA-17, no. 5, pp. 518-523, Sept. 1981.

[2] T. A. Meynard and H. Foch, "Multi-level conversion: high voltage choppers and voltagesource inverters," Power Electronics Specialists Conference, 1992. PESC '92 Record., 23rd Annual IEEE, Toledo, 1992, pp. 397-403, vol.1.

[3] Fang Zheng Peng, Jih-Sheng Lai, J. W. McKeever and J. VanCoevering, "A multilevel voltage-source inverter with separate DC sources for static VAr generation," in IEEE Transactions on Industry Applications, vol. 32, no. 5, pp. 1130-1138, Sep/Oct 1996.

[4]

A. Ajami, M. R. J. Oskuee, A. Mokhberdoran and A. Van den Bossche, "Developed cascaded multilevel inverter topology to minimise the number of circuit devices and voltage stresses of switches," in IET Power Electronics, vol. 7, no. 2, pp. 459-466, February 2014.

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