Excerpt

## TABLE OF CONTENTS

Acknowledgements

Abstract

Abbreviations

List of Figures

List of Tables

Chapter 1: Introduction

1.1 Problem Statement

1.2 Aim and Objective

1.3 Scope of Work

1.4 Organisation of Thesis

Chapter 2: Literature Review

2.1 Introduction

2.2 Literature Review

Chapter 3: Modular Multilevel Converter

3.1 Introduction

3.2 Modular Multilevel Converter (MMC)

3.2.1 Features of Modular Multilevel Converter (MMC)

3.2.2 Advantages of Modular Multilevel Converter (MMC)

3.2.3 Working of Modular Multilevel Converter (MMC)

3.3 Sub Module Topologies

3.3.1 Half Bridge Sub Module

3.3.2 Full Bridge Sub Module

3.3.3 Clamp Double Sub Module

Chapter 4: Average and RMS Values Calculation

4.1 Introduction

4.2 Circuit Ananlysis

4.3 Conversion Losses

4.3.1 Conduction Losses

4.3.2 Switching Losses

4.4 Half Bridge Sub Module Calculations

4.4.1 Average Value of the Current

4.4.2 RMS Value of the Current

4.5 Full Bridge Sub Module Calculations

4.5.1 Average Value of the Current

4.5.2 RMS Value of the Current

4.6 Clamp Double Sub Module Calculations

4.6.1 Average Value of the Current

4.6.2 RMS Value of the Current

Chapter 5: Power Losses Estimation

5.1 Introduction

5.2 Half Bridge Sub Module

5.2.1 Conduction Losses

5.2.2 Switching Losses

5.3 Full Bridg Sub Module

5.3.1 Conduction Losses

5.3.2 Switching Losses

5.4 Clamp Double Sub Module

5.4.1 Conduction Losses

5.4.2 Switching Losses

Chapter 6: Solution based on MATLAB Simulation: Case Study

6.1 Introduction

6.2 Half Bridge Sub Module

6.3 Full Bridge Sub Module

6.4 Clamp Double Sub Module

6.5 Summary

Chapter 7: Conclusion

7.1 Conclusion of Research Work

7.2 Future Directions

References

## ACKNOWLEDGEMENTS

First of all, I am thankful to ALMIGHTY ALLAH who gave me the strength and courage to complete this project and thesis. I am highly indebted to my supervisor Dr.

M. Naeem Arbab, Professor Department of Electrical Engineering University of Engineering and Technology Peshawar, who has always remained an invaluable source of knowledge, inspiration and who helped and guide me throughout my project.

Finally I am thankful to my mother, wife and children whose prayers and support provided me the constancy to accomplish my thesis.

## ABSTRACT

Modular Multilevel Converter (MMC) has become the most concerned converter topology in the High Voltage Direct Current (HVDC) transmission system, in recent times. The low switching frequency, low converter losses and flexible control made it most attractive topology. It is important to make a research on the loss calculation method of MMC and state formulae for the losses as it is a vital step during the design stage of the MMC based HVDC system.

In this research work, the structure of MMC based HVDC system is discussed. Three sub module topologies’; half bridge, full bridge and clamp double sub module, are discussed. A method based on the average and root mean square (RMS) values of the current passing through the sub module is discussed. The conversion losses in the switching devices of the sub modules are calculated using the method. A cases study is taken into consideration then with certain parameters. Using these parameters a MATLAB program is developed. With the help of the program the losses and efficiency curves for each switching device by taking each sub module separately are obtained respectively. A comparison of the losses and efficiency of each sub module is also discussed. At the end those factors which effect the losses and efficiency of the sub module are discussed along with the certain aspects for the directions of future work.

## ABBREVIATIONS

illustration not visible in this excerpt

## LIST OF FIGURES

Figure 1-1 HVDC Transmission System

Figure 3-1 MMC based HVDC System

Figure 3-2 Half Bridge Sub Module

Figure 3-3 Full Bridge Sub Module

Figure 3-4 Clamp Double Sub Module

Figure 6-1 Diode D1 Average Current and Conduction Losses

Figure 6-2 Diode D2 Average Current and Conduction Losses

Figure 6-3 Transistor T1 Average Current, Conduction and Switching Losses

Figure 6-4 Transistor T2 Average Current, Conduction and Switching Losses

Figure 6-5 Total Power Losses of Half Bridge Sub Module

Figure 6-6 Efficiency of Half Bridge Sub Module

Figure 6-7 Diode D1 Average Current and Conduction Losses

Figure 6-8 Diode D4 Average Current and Conduction Losses

Figure 6-9 Transistor T1 Average Current,Conduction and Switching Losses

Figure 6-10 Transistor T4 Average Current,Conduction and Switching Losses

Figure 6-11 Diode D1 Average Current and Conduction Losses

Figure 6-12 Diode D3 Average Current and Conduction Losses

Figure 6-13 Transistor T1 Average Current,Conduction and Switching Losses

Figure 6-14 Transistor T3 Average Current, Conduction and Switching Losses

Figure 6-15 Diode D2 Average Current and Conduction Losses

Figure 6-16 Diode D4 Average Current and Condcution Losses

Figure 6-17 Transistor T2 Average Current,Conduction and Switching Losses

Figure 6-18 Transistor T4 Average Current,Conduction and Switching Losses

Figure 6-19 Diode D2 Average Current and Conduction Losses

Figure 6-20 Diode D3 Average Current and Conduction Losses

Figure 6-21 Transistor T2 Average Current,Conduction and Switching Losses

Figure 6-22 Transistor T3 Average Current,Conduction and Switching Losses

Figure 6-23 Toral Power Losses of Full Bridge Sub Module

Figure 6-24 Efficiency of Full Bridge Sub Module

## LIST OF TABLES

Table 3-1 Switching States of Half Bridge Sub Moduel

Table 3-2 Switching States of Full Bridge Sub Module

Table 3-3 Switching States of Clamp Double Sub Module

Table 6-1 Parameters Used for Analysis

Table 6-2 Comparison of Total Power Losses (W) at different phase angles

Table 6-3 Comparison of Efficiencies (%) at different phase angles

## Chapter 1: Introduction

### 1.1 Problem Statement

In early days, as the demand of electricity was low, small or low rated power stations were built nearer to the loads. As the demand of electricity started to increase, the power stations size also increased, which resulted in building the power stations at a place where the fuel or source was available but quite away from the loads. To supply electrical energy to the load from a distanced source, a supply system is needed. The supply system can be broadly classified into two parts;

1. AC transmission system

2. DC transmission system

Nowadays, as the generating stations are located at remote sites, the transmission of electric power at high voltages is needed due to its many benefits such as reduction in losses in resistance of wire and delivering the same power at half of the current. The supply systems used are;

1. High Voltage using Alternating Current (AC)

2. High Voltage using Direct Current(DC) or HVDC

High Voltage using AC is in use for many years, but due to its limitations and with the increasing infiltration of renewable sources, HVDC has been receiving active consideration of engineers and researchers. An HVDC transmission system is shown in Figure 1-1. It consists of following main components;

1. AC source which is delivering power P1 to the load; receiving power P2.

2. Transformers to step up and step down the voltages at the required level.

3. Converters C1 and C2 used to converter the voltage from AC to DC and vice versa at the required point.

4. DC link, cables and overhead lines used to carry the DC power.

The converters; rectifiers, AC to DC and inverters, DC to AC, have been a topic of interest for the researchers for many years. With the development in the field of power electronics the problems faced into the design of converters for HVDC applications have been resolved up to certain extent. The converters used in early days are classified into two groups:

1. Line - commutated converters (LCC)

2. Voltage - sourced converters (VSC)

illustration not visible in this excerpt

Figure 1-1 HVDC Transmission System

LCC are made of electronic switches which can be turned on only, such as mercury arc valves and thyristors. While VSC consists of electronic switches which can be both turned on and off, such as Insulated Gate Bipolar Transistors (IGBTs) and Integrated Gate-Commutated Thyristors (IGCTs). VSC has advantages which makes it better than LCC such as;

1. Smaller size

2. Small size of filters

3. Fast active power reversal.

Traditionally VSC uses topologies such as two level, three level and neutral point diode - clamped converter. But in recent years, Modular Multilevel Converter (MMC) has emerged as a replacement for the traditional converter topologies. In 2003, Lesnicar and Marquardt ^{1} first proposed this topology. Since, then it has been in used by commercial industries such as Siemens’ HVDC Plus and ABB’s HVDC Light. This converter topology has advantages of easy assembly and flexibility in converter design.

Losses estimation in converters is an important stage during the design process of the converters. This helps the designer to optimize the overall system performance through a compromise of several design indices ^{19}. There are number of literatures on losses estimation in MMC, but research in this field is still very challenging because of the controlling and switching strategies employed to the transistors. A number of methods have been applied to calculate the losses in the converter, but there is still a lot of work to be done. In this research, three sub module topologies; half bridge, full bridge and clamp double sub module, employed in MMC are considered and conversion losses(conduction and switching losses) equations of each switching device in each sub module is derived. Average value and root mean square value of current through the semiconductor devices are calculated and are used to estimate the conversion losses and efficiency of the sub modules. Based on the derived equations a MATLAB program is developed. Then, using certain numerical values the results; losses and efficiencies graphs are obtained.

### 1.2 Aim and Objective

The research is done aiming to calculate the conversion losses and efficiency of different sub module topologies used in MMC based HVDC system with following objectives:

- Better understanding of working and operating principles of Modular Multilevel Converter (MMC).

- Study of different conversion losses in sub modules.

- Derivation of equations of conversion losses of each switching device of each sub module topology.

- Derivation of efficiency of each sub module topology.

- Effect of different parameters on conversion losses and efficiency of each sub module topology.

### 1.3 Scope of Work

Numbers of mathematical techniques are proposed to calculate the conversion losses and efficiency of Modular Multilevel Converter (MMC). In this research work average and root mean square values are used to calculate the conversion losses by considering different sub module topologies used in modular multilevel converter based HVDC system. Based on these calculations, efficiency of each sub module topology is calculated. Focusing on this, the research is divided into the following tasks:

- Derivation of average values and RMS values of current flowing through each switching device for each sub module topology.

- Derivation of conversion losses equations.

- Developing MATLAB based program on the basis of the derived equations.

- Analyzing the results by studying the effect of different parameters on the conversion losses and efficiency.

Following contributions are made by doing this research:

- The loss calculation method helps to better understand the working of considered sub module topologies.

- Method can be used by different industries to calculate and understand the losses and efficiencies of MMC.

- Method can also be helpful to understand the effect of different parameters on the conversion losses and efficiency of the sub modules.

### 1.4 Organization of the Thesis

The key task of this research is to help the researchers in calculating the losses of the switching devices in MMC. In this thesis each and every chapter is organized to give the reader a brief knowledge about the concerned topic in an easy and understandable manner.

Chapter 1, Introduction, is about a brief introduction about HVDC system, stating the problems faced in the analysis of MMC, scope of the research work and the organization of the thesis.

Chapter 2, Literature Review, is about review of work done in the field MMC, more concerned to the losses estimation. For this work, study has been done of the literature since 2003. All the important techniques and methods used for the losses calculation and efficiency have been discussed in this chapter. The results obtained in these literatures are also discussed.

Chapter 3, Modular Multilevel Converter, discusses different converter topologies used before modular multilevel converter. It discusses the construction and working of MMC. The advantages of MMC on other converter topologies and different sub module topologies used are also discussed.

Chapter 4, Average and RMS Values Calculations, explains the proposed method to calculate the losses and efficiency of the sub modules. Three different sub module topologies; half bridge, full bridge and clamp double sub module are discussed and the equations regarding average value and RMS values are derived. In Chapter 5, Power Losses Estimation, the conversion losses of each sub module topology are calculated by using the derived relationships of average and RMS values of the current in chapter 4.

Chapter 6, Solution Based on MATLAB Simulation: Case Study, is about the analysis of the derived equations using the MATLAB. In this chapter the results and graph obtained for each sub module topology are discussed. The parameters which effect the losses and efficiency are also discussed.

Chapter 7, Conclusion, the final chapter, summarizes the work done and conclusion is given of the overall research work and also discusses the future work that can be done in the field.

## Chapter 2: Literature Review

### 2.1 Introduction

Losses estimation in converters is a vital step during the design and planning stage of HVDC system. Accurate losses estimation helps the designers to make the system operation economical. Also, it helps the designer to optimize the overall system performance and to select the heat sinking equipment and cooling systems for the system.

In recent times, MMC has been widely accepted as the most concerned converter topology in the HVDC transmission system. It has features of low switching frequency, low converter losses and flexible control. Due to increase in the usage of MMC topology in HVDC system, the losses and efficiency estimation importance has increased, as it will help to understand the losses and parameters effecting the losses resulting in more efficient MMC based HVDC system.

There are number of research literatures on the losses estimation of MMC since its evaluation in 2003, but it is still a challenge for the researchers to estimate the losses because of its very complex topology structure; large number of sub modules and the switching technique employed. Due to these reasons, stating the formulae for the losses of each switching device is difficult.

In this research work, the topology of MMC is discussed, considering different sub module topologies used. Then, an analytical method based on average current and root mean square (RMS) of the current passing through the sub module is presented to calculate the conversion losses in the switching devices of MMC. Then, considering a case, a MATLAB program is developed and the losses and efficiency curves are obtained for the sub module topologies. Also, those factors which effect the losses and efficiency of the sub module are discussed. The MMC structure and the proposed approach is discussed in chapters 3 and 4 respectively.

### 2.2 Literature Review

In the following text recent work by the researchers regarding losses estimation is presented. A brief overview about the methods used by the researchers to calculate the losses in MMC is stated. Different methods used to calculate the losses and efficiency of MMC are studied along with the parameters and conditions considered in the calculations. Parameters such as input power, modulation index and power angle are taken under consideration for the calculations.

MMC was suggested by Marquardt and Lesnicar in 2003 ^{1} for very high voltage applications. The design later opened gates for the researchers to do investigations. Fundamental concepts and control scheme were introduced in this literature. Here, the basic component of the converter was named as sub module.

With respect to industrial applications Marquardt and Lesnicar ^{2} in 2004 discussed MMC with the construction and working principle. Interference problems associated with electromagnetic currents, surge currents and stray inductances are also discussed. In 2010, Marquardt ^{3} explained and investigated MMC for demanding future applications in power transmission such as grid connection of large off-shore wind parks and solar thermal power generation. Failure management in multi terminal HVDC networks is also discussed.

Different sub modules topologies such as half bridge, full bridge and clamp double sub modules are discussed by Noman Ahmed and Arif Haider ^{4}. Short circuit current behavior of MMC is also discussed in this literature.

In ^{5} power losses of MMC are discussed based on the two capacitor voltage balancing algorithm. This algorithm is based on ranking of capacitor voltage values and pulse width modulation. The impact of control methods on MMC is also discussed.

A new pulse width modulation technique for an arbitrary number of voltage levels for MMC is studied in ^{6}. The semiconductor losses are then estimated using the proposed modulation scheme. Losses distribution and semiconductor current ratings are also evaluated. Total harmonic distortion (THD), weighted total harmonic distortion (WTHD) and efficiency of the sub module are evaluated in the paper.

An improved modulation method based on waveform is presented by Ke Li, and Chengyong Zhao ^{7} for MMC. The technique produces lower switching frequency (50 Hz), hence resulting in lower swithching losses as compared to 2 and 3 level VSC. The methodology is validated by simulations in PSCAD/EMTDC. The characteristic waveforms of the converter is studied by Steffen Rohner, Steffen Bernet, Marc Hiller and Rainer Sommer in ^{8}. A 7.2 kV and 7.5 MVA converter is taken as an example for the study of the characteristics. The circulating current cause is also discussed.

A method using digital calculation way is proposed in ^{9}. Simulation in digital calculation is done for the switching waveforms. The voltage and current waveforms of a line frequency period are then obtained. For various operating conditions digital calculation program is developed and then considering certain parameters; power angle and different values of system power, the conversion losses are studied. The impact of switching frequency and circulating current on the losses are also studied. In ^{10} two approaches are presented to calculate the losses in MMC. The first one is analytical method based on the average model of the MMC. While the second one is based on the Simulink/ MATLAB detailed model, which is used as a reference for the analytical one. The former is proved as sufficient and faster method for the evaluation of power losses.

A phase shift carrier based pulse width modulation (PSC-PWM) method for MMC is described by Qingrui Tu, Zheng Xu, and Lie Xu ^{11}. To reduce the switching losses of the switching devices, reduced switching frequency voltage (RSV) balancing algorithm is presented in the paper. The algorithm reduces the switching frequency of the devices, hence reducing the concerned losses. PSCAD/EMTDC model is used to show the effectiveness of PSC-PWM method and RSV algorithm. The proposed algorithms show less converter power losses.

Conduction losses of MMC based on Silicon Carbide (SiC) switches is studied in ^{12}.

A comparison is carried out between losses of MMC based on Insulated Gate Bipolar Transistors (IGBTs) and MMC based on SiC. The results show that the MMC based on SiC are more efficient than the MMC based on IGBT.

An analytical method for power losses calculation of MMC is presented in ^{13}. The principle for the method is the forthcoming IEC 62751. Half bridge and full bridge sub modules are taken as case study. A comparison is made between the losses of the two modules, showing that the full bridge sub module has more losses as compared to the half bridge sub module.

The junction temperature has a certain effect on the losses of the semiconductor devices. The impact of this junction temperature on the losses of MMC has been studied in ^{14} using the junction temperature coefficient. To calculate the switching losses, a curve fitting method is proposed. A method is also presented to calculate the on-state losses according to the different on state current of the upper and lower leg. With the help of MATLAB, the losses are calculated for a case study. Also the parameters which effect the losses are also discussed.

In ^{15}, a loss comparison is presented among the three different sub module topologies; half bridge, full bridge and clamp double sub modules by implementing IGBTs and Integrated Gate-Commutated Thyristors (IGCTs).

The above mentioned literatures discussed are mostly relevant to the half bridge sub module topology. In ^{16}, the control scheme with capacitor voltage balancing method for full bridge sub module is discussed. The paper also discusses the construction and working of full bridge sub module topology. The model and the controller are investigated for different operating conditions ^{16}.

In ^{17}, clamp double sub module’s (CDSM) control is discussed. The construction and operation principle are also described. The modes of operation; normal and blocking mode are discussed. The charging of the capacitor is proposed by the method of simple grouping sequentially controlled charge is proposed.

CDSM is discussed by Rainer Marquardt in ^{18}. The power losses, DC bus short circuit limitation and controllability of the MMC based on CDSM are discussed. The power losses and efficiency of the sub module is calculated. The extra diode and transistor effect on the losses are also discussed.

An analytical method to calculate the efficiency of Voltage Source Converter (VSC) based HVDC transmission system using average value and root mean square (RMS) values of the current through the semiconductor devices is presented in ^{19}. The values are then used to estimate the conduction and switching losses. Two and three level converters are taken as case studies. The results are then confirmed with the help of digital simulations.

From the above review of literatures it is concluded that losses and efficiency estimation of MMC is a challenging task. Determination of accurate values of losses is challenging due to the control and switching methods used in MMC. So, losses and efficiency estimation method is proposed in this research by using the average value and the RMS value of the current through the semiconductor devices. These values are then utilize to derive the conduction, switching losses and efficiency of three sub module topologies; half bridge, full bridge and clamp double sub modules. The data used for the simulation is taken from the table given in the literatures reviewed.

## Chapter 3: Modular Multilevel Converter

### 3.1 Introduction

The manifestly changing current energy scenario has led to the implementation of renewable energy sources. HVDC has become more relevant than ever as it is more suitable for the renewable energy sources. More stochastic energy production calls for solutions that can transport power from areas with high generation to areas with lower generation ^{20}. Connecting distanced generation sources to the loads require HVDC transmission with solid and steadfast converter technology having high power capability. Due to this, innovative operational equipment based on power electronics makes it appearance. HVDC converters have gone through technological advancements for the last 50 years. Converters can be broadly classified into two main types:

i. Line Commutated Converters (LCC)

ii. Voltage Source Converters (VSC)

Line commutated converters (LCC) are thyristors based, which were introduced in 1970s. LCC is the pioneer converter technology. It is the only converter which has the capability of high power rating and is used for the bulk power transmission. Low losses in the converter are another advantage of LCC. The biggest disadvantage of LCC is that it absorbs a changing amount of reactive power; as a result adjustable reactive compensation is required.

Voltage source converters (VSC) use Insulated Gate Bipolar Transistor (IGBT) technology. The classical version of VSC is based on two and three level converters. The voltage level for two level converter is ±V and the three level converter has voltage level of ±V and zero. Pulse width modulation method is used to produce the desired voltage waveform. VSC based HVDC system does not require reactive power compensation, as both the active and reactive power flow can be controlled. The disadvantage of VSC is that the losses are more than the LCC.

Converters based on multilevel, more than three voltage levels, classification can be done as:

i. Neutral Point Clamped (NPC)

ii. Flying Capacitor (FC)

iii. Cascaded H-Bridge Converter (CHB) iv. Modular Multilevel Converter (MMC)

### 3.2 Modular Multilevel Converter (MMC)

A three phase Modular Multilevel Converter (MMC) is shown in Figure 3-1. Each phase leg or limb consists of two arms, upper arm and lower arm, connected in series ^{21}. Each arm consists of an arm inductance and series connected “N” sub modules or cells, a basic building block of MMC. The functions of arm inductance are:

1. Limits the current during the fault.

2. Acts as filter for ac current.

3. Limits the circulating current among the legs which is of high harmonics.

illustration not visible in this excerpt

Figure 3-1 MMC based HVDC System

All the sub modules are identical but individually controlled. A sub module is a two terminal bidirectional device. Each sub module consists of IGBTs, freewheeling diodes and capacitor. Each sub module produces voltage of either V or -V or zero. A number of sub module topologies; half bridge, full bridge and clamp double sub module that are being used are discussed later in the chapter.

#### 3.2.1 Features of Modular Multilevel Converter (MMC)

Compared to conventional VSCs, MMC offers certain features, which seems certain “strange” ^{22}.

1. The internal arm currents are flowing continuously ^{22}.

2. Half of the AC current is flowing through each arm ^{22}.

3. Arm inductance does not generate any voltage for the semiconductors ^{22}.

4. The sub module are two terminal devices, they do not require DC side storage capacitance ^{22}.

5. Voltage balancing of the sub module is not necessary ^{22}.

6. The DC link voltage can be controlled actively by the converter ^{22}.

#### 3.2.2 Advantages of Modular Multilevel Converter (MMC)

The MMC offers certain advantages over traditional VSC.

1. The power and voltage ratings of the converter can be easily scaled.

2. Since the number of sub modules can be increased, as a result levels of voltage and power are increased, producing nearly sinusoidal output voltage.

3. In case of sub module failure, redundant sub module is used; this increases the reliability of the converter.

4. Efficiency of the sub module is high, as low switching frequency is used, which results in low switching losses.

Due to these advantages, MMC is the most appropriate converter topology for HVDC applications.

#### 3.2.3 Working of Modular Multilevel Converter (MMC)

The power transfer between the AC and DC side of the converter is due to the circulating current. This circulating current is produced due the voltage difference between the DC side voltage and the DC component of the voltage of the sub module. With the help of proper control the differential voltage is achieved. To maintain the average sub module capacitor voltage, the average power transfer through the sub module must be equal to zero, this is the basic operating principle of the MMC. The instantaneous power at the sub module has oscillations, having frequency multiple of the fundamental frequency. These oscillations results in the variation of capacitor voltage. Considering a three phase; U, V and W, MMC based HVDC system.

**[...]**

- Quote paper
- Ehtasham Mustafa (Author), 2014, Analytical Efficiency Evaluation of Modular Multilevel Converter (MMC) for High Voltage Direct Current System (HVDC), Munich, GRIN Verlag, https://www.grin.com/document/316343

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