Category Analysis, Design and Implementation of a High Efficiency Multilevel Converter for Renewable Energy Systems

Biography

Sergio Daher was born in Fortaleza-Brazil on the 3rd of August 1971; son of Elias Daher and Josephina Sophia Lira Daher. Since young, he showed interest in sciences and latter he gave special attention to the electrical and electronic engineering areas, what remains until today. His main areas of interest are: Power Electronics, Renewable Energy Systems, Electronic Instrumentation and Control Systems.

He believes that is possible to make the world better: all together, starting now, within our life context, doing small daily contributions.

Academic Studies

Подпись:Technical in Electricity (15th January 1990)

Federal Technical School of Ceara – ETFCe – Fortaleza – Brazil

Graduation in Electrical/Electronic Engineer (28th January 1995) Universidade Federal da Paraiba – UFPb – Campina Grande – Brazil

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Transformer optimization calculation sheet

( MATLAB 5.3.0.620a (R11) – Student Edition )

Подпись: 0 1 2 1 2 2
Transformer optimization calculation sheet Подпись: %Base model: 18mOhm

% Experimental data used (Efficiency x power) % File: n48v. mat

%|-

– Experimental………

%

Pout

Efficiency[%]

%

60

76.27

%

120

86.36

%

238

92.3

%

297

93.52

%

355

94.31

%

449

95.08

%

561

95.59

%

673

95.84

%

783

95.95

%

945

96

%

1057

95.93

%

1167

96.01

%

1275

95.86

%

1359

95.72

%

1463

95.53

%

1565

95.33

%

1666

95.12

%

1917

94.7

%

2015

94.48

%

2112

94.24

%

2208

94

%

2283

94.01

%

2377

93.76

%

2469

93.49

%

2560

93.23

%

2631

93.02

%

2808

92.51

%

2963

92.07

%

3318

91.24

%

3475

90.73

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Over-current protection circuits

The implemented prototype is protected against over-current conditions at its input and output by using the circuits shown in figures (a) and (b), respectively. The input protection circuit makes use of the battery current sensor. It simply compares the sensor output with an adjustable voltage reference and generates a pulse that is applied to both main and auxiliary microcontrollers. It should be noted that this circuit only detects an over-current in one direction (in this case, from the battery to the converter).

Over-current protection circuits

The output protection circuit is connected in parallel with the output filter inductor and it is less accurate then the input protection circuit. In fact, it will act only if a fast and large change in the load occurs, such as the case of short-circuit...

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Schematic circuits

Schematic of main controller

Ib_ad

ЛЛЛ/—° (Battery Current)

22R

4v7 Vb ad

WV—о ( Battery Voltage)

22R

4v7 Vo ad

ЛЛЛ,—0 (Output Voltage)

22R

 

і Vc+o

 

From

measurement

circuits

 

■WV—oTemp_ad
22R (Temperature Sensor)

 

лл/v—° Ip_ad 22R (Trans. Current)

4v7

 

Schematic circuits

to

auxiliar

microcontroller

 

From

protections

circuits

О ISc

 

О OSc

 

power supply

 

13

^ULN2003

 

ULN2003

 

Schematic circuits

Schematic circuits
Schematic of auxiliary controller

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Multilevel waveform design tool – user guide

1. General information

The Multilevel waveform design tool was developed in C language, using Builder 1.0 (for Windows) visual programming environment. All sources are available from the author ( sdaher@secrel. com. br / daher@re. e-technik. uni-kassel. de ). Main features are:

– Construction of multilevel waveforms up to 35 levels per quarter cycle, with the possibility to include user-defined hold-on-at-zero time;

– Determination of waveforms with reduced THD contents through fine adjustment of the switching angels ("improved" waveforms);

– Inclusion of configurable smoothness in waveforms with non-zero hold-on-at-zero time;

– Calculus of THD and Modulation index for all generated waveforms;

– Generation of look-up tables in C-code format for direct implementation in a microcontroller;

–...

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Table of normalized currents (N = 5)

(Estimated normalized currents at the transformer coils and output stage switches)

Output current: Ios = 1.0 Input current: Ip

Peak transformer ratio: ^pk = 1.0

In : current thought the nth output stage switch.

Note that transformer secondary currents are equal to the correspondent series-switch current (odd switch).

p__

Ip

I1

I2

I3

I4

I5

I6

I7

I8

I9

I10

7

0.877

0.806

0.592

0.891

0.455

0.972

0.236

0.000

1.000

0.000

1.000

8

0.875

0.602

0.798

0.653

0.757

0.719

0.695

0.668

0.744

0.000

1.000

9

0.874

0.792

0.610

0.493

0.870

0.550

0.835

0.820

0.573

0.000

1.000

10

0.873

0.616

0.788

0.749

0.662

0.450

0.893

0.883

0.470

0.000

1.000

11

0.872

0.784

0.621

0.856

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Future work

There are several interesting topics for further research:

■ The implemented controller does not take into account that the inverter output voltage can present a DC-level if the inverter feeds unsymmetrical loads, such as half-wave loads. Future implementations must include a mechanism to monitor and eliminate any possible output voltage DC-level.

■ The implemented controller adjusts the output voltage amplitude by only changing the number of output levels. Future implementations might include fine adjustment of the output voltage amplitude by also altering the output voltage waveform shape.

■ In this work, the correction of transformer-unbalancing was investigated only at steady-state level. Future work might study the transformer dynamics and should consider it on the controller design.

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Conclusions 8.1 Conclusions

This dissertation had investigated a high efficiency multilevel inverter topology which has great potential for application in stand-alone renewable energy systems. The presented study was focused on small systems (< 10 kWp) and it was showed that most typical system configurations store energy in battery banks in order to overcome the intermittence and peak power limitation of renewable energy sources. In this context, it was also identified that these systems require inverters with improved characteristics of reliability, capability to start heavy loads, efficiency and robustness. In fact, all these benefits can be achieved through the use of multilevel topologies.

Although several multilevel topologies have been successfully employed in industrial and power systems applications, it has ...

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