Advanced Multilevel Converters and Applications in Grid Integration

A comprehensive survey of advanced multilevel converter design, control, operation and grid-connected applications

Advanced Multilevel Converters and Applications in Grid Integration presents a comprehensive review of the core principles of advanced multilevel converters, which require fewer components and provide higher power conversion efficiency and output power quality.  The authors – noted experts in the field – explain in detail the operation principles and control strategies and present the mathematical expressions and design procedures of their components.

The text examines the advantages and disadvantages compared to the classical multilevel and two level power converters. The authors also include examples of the industrial applications of the advanced multilevel converters and offer thoughtful explanations on their control strategies. Advanced Multilevel Converters and Applications in Grid Integration provides a clear understanding of the gap difference between research conducted and the current industrial needs. This important guide:

• Puts the focus on the new challenges and topics in related areas such as modulation methods, harmonic analysis, voltage balancing and balanced current injection
• Makes a strong link between the fundamental concepts of power converters and advances multilevel converter topologies and examines their control strategies, together with practical engineering considerations
• Provides a valid reference for further developments in the multilevel converters design issue
• Contains simulations files for further study

Written for university students in electrical engineering, researchers in areas of multilevel converters, high-power converters and engineers and operators in power industry, Advanced Multilevel Converters and Applications in Grid Integration offers a comprehensive review of the core principles of advanced multilevel converters, with contributions from noted experts in the field.

• Language: English
• Publisher: Wiley
• Release date: Oct 15, 2018
• ISBN: 9781119475897

Book preview

Dedication

This book is dedicated to our families and parents.

List of Contributors

Ali I. Maswood

Nanyang Technological University

Singapore

Gabriel Heo Peng Ooi

Nanyang Technological University

Singapore

Georgios Konstantinou

University of New South Wales

Australia

Harikrishna Raj Pinkymol

Nanyang Technological University

Singapore

Hossein Dehghani Tafti

Nanyang Technological University

Singapore

Josep Pou

Nanyang Technological University

Singapore

Md Shafquat Ullah Khan

Nanyang Technological University

Singapore

Muhammad M. Roomi

Nanyang Technological University

Singapore

Ziyou Lim

Nanyang Technological University

Singapore

Preface

The ever‐increasing demand for energy and the evidence of global warming have forced many nations to divert renewable and clean energy sources into the mainstream for power generation. Statistical results have proven that the total amount of power capacity installed yearly is increasing tremendously in order to meet supply demands. In many countries, government bodies and authorities are also planning future‐sustainable cities. Solar power systems and wind power systems are getting more attention in various countries. Clean energy sources, such as fuel cell power systems, are still undergoing testing to serve as the primary energy sources for many housing blocks. Furthermore, most of the available high‐power rotating machines in industry require variable speeds and special control algorithms. A power conversion stage, using semiconductor switches, is required in these renewable energy systems and industrial applications. Therefore, a deep understanding of the design of high‐power converters is required for researchers and industrial engineers.

Two‐level power converters are applied in various industrial applications; however, they have limitations for high‐power conversion systems. They also have a low quality of output power, which necessitates the application of large filters with higher losses and lower power conversion efficiencies. On the other hand, semiconductor switches have limitations in terms of high current and high voltages. Therefore, implementing multilevel converters in high‐power and high‐voltage applications has become a trend recently. With the multilevel approach, better power quality can be achieved by synthesizing a higher number of output voltage levels. Many research works on multilevel topologies have been proposed, but the technology is still not available commercially. However, understanding the principles of advanced multilevel converters with enhanced efficiency, better output voltage quality, and reduced number of switches is necessary for power engineering researchers and industrial engineers, since a comprehensive presentation of these is not available in the books/reports in the market presently.

This book, Advanced Multilevel Converters and Applications in Grid Integration, presents the principles of advanced multilevel converters, which require lesser number of components and provide higher power conversion efficiency and output power quality. Their operational principles and control strategies are explained in detail. Furthermore, the mathematical expressions and design procedures of their components are also presented. Their advantages and disadvantages as compared to the classical multilevel and two‐level power converters are also provided. Some of the industrial applications of advanced multilevel converters are included in the proposed book, along with a deep explanation of their control strategies. Thereby, this book is able to provide a better understanding of the gap differences between the research conducted and the current industrial needs.

The book is divided into four parts:

Part I includes several modulation algorithms for classical multilevel converters and control strategies for the voltage balancing of capacitors. The reader will be exposed to the basics of multilevel converters in this part, which will help in gaining a better understanding of the principles of advanced multilevel converters.

Part II presents several multilevel rectifiers and their operational principles. Unidirectional and bidirectional multilevel rectifiers are initially discussed in this part, followed by discussions on multilevel diode‐clamped rectifiers. Finally, flying capacitor–based multilevel converters are explored. The operational principles of each configuration, along with the relevant mathematical presentations of their operations, are demonstrated in each chapter. A procedure for designing the components of the converter and the related voltage/current stress of the components are also illustrated. The performance of various control and modulation strategies on multilevel rectifiers are evaluated in each chapter using simulation and experimental results.

Part III demonstrates the various topologies of advanced multilevel inverters, including transformerless diode‐clamped and flying capacitor–based multilevel inverters. Furthermore, multilevel Z‐source inverters are also described in this part. Similar to Part II, each chapter provides the operational principles, mathematical formulations, and design procedures for these converters, as well as the various control strategies and their evaluation results.

Part IV investigates the various industrial applications of the advanced multilevel converters presented in this book. Photovoltaic power plants, wind power plants, fuel cell power generation, and flexible alternating current transmission systems (FACTS) are covered in this chapter. Several control strategies in these systems are presented, followed by their evaluation results during various operation conditions.

Part I

A review on Classical Multilevel Converters

1

Classical Multilevel Converters

Gabriel H. P. Ooi Ziyou Lim and Hossein Dehghani Tafti

1.1 Introduction

Power electronic converters are classified mainly based on the current (CS) or voltage source (VS). In the early days around the 1980s, current source inverters (CSI) were popular when thyristor‐based semiconductors were first developed [1]. The CSIs are also known as load‐commutated inverters (LCI) in the industry, which mainly comprise the gate turn‐off thyristor (GTO) or the integrated gate‐commutated thyristor (IGCT) in the circuit. Usually, CSIs are operated under short‐circuit conditions using current‐controlled switching devices; hence, a low switching frequency is required. Besides, the gate driver circuitry design can become too complicated.

In 1964, Ray E. Morgan had proved that the performance of an inverter will be more efficient for a fast switching operation where switching losses are greatly reduced through the experimented bridge chopper inverter. Despite the IGCT allowing a higher switching frequency (up to a few kilohertz) than the GTO, the switching losses are considerably high. Hence, the switching frequency for the IGCT is typically limited to a few hundred hertz (around 500 Hz).

In the mid‐1990s, insulated‐gate bipolar transistors (IGBTs) designed to be open‐circuit voltage‐controlled semiconductor devices for fast switching purposes had become available in the market. As a result, IGBTs would have been more favorable in the voltage source inverter (VSI) where a lower switching loss, higher efficiency, and a higher reliability are achieved. Moreover, the VSI has a low current harmonic distortion due to the high switching frequency operation. The VSI is used in industrial applications due to the numerous advantages, and therefore, multilevel VSI is the prime focus of research and will be further discussed in this chapter.

1.2 Classical Two‐Level Converters

The configuration of the conventional three‐phase two‐level VSI as shown in Fig. 1.1 has been commonly used in the early 1960s for motor drives. The 2L‐VSIs were constructed using thyristor‐based switching devices during those times until the 1990s when IGBTs were used to replace the thyristors for faster switching purposes.

Diagram of a traditional three-phase two-level voltage source inverter, consisting only of two switching devices in each phase leg.

Figure 1.1 Traditional three‐phase two‐level voltage source inverter (2L‐VSI).

The topology of the three‐phase 2L‐VSI is simple in nature, consisting only of two switching devices in each phase leg, thus allowing the cost of implementation to be relatively low. Although a fast switching operation of up to 20 kHz can be performed with the IGBTs to reduce the switching losses in the 2L‐VSI, there is an implication of high stress in motor winding especially when a long cable is connected between the inverter and the motor [2]. The increase in switching frequency operation causes high dV/dt (large voltage spikes) due to the characteristic impedances of the cable; hence, it has a detrimental impact on the motor.

However, electromagnetic interference (EMI) becomes a major issue when the power electronic appliances are operating at a switching frequency range between 10 kHz and 30 MH. Since power electronic converters usually operate below the frequency of 10 MHz (typically around 10 kHz to 150 kHz), EMI is spread by the conduction of the wires. Hence, the conducted EMI is the key concern in the design of power converters instead of the radiated EMI. Thus, the EMI filter is required to suppress the conducted EMI for the 2L‐VSI with a 20 kHz switching frequency so that the filter size can also be minimized.

Apart from that, there is a limitation for the 2L‐VSI to drive high power (≥100 MW) applications due to the available voltage ratings of the semiconductor devices in the market. Based on the configuration of the 2L‐VSI, each of the semiconductor devices has to support the total amount of DC voltage supply. Therefore, the design of 2L‐VSI becomes complex when a number of semiconductor components are series‐connected together in order to withstand high DC voltage [3, 4]. As a result, the reliability of the 2L‐VSI will also be greatly affected due to the increase in the number of switching devices required in the circuit.

1.3 The Need for Multilevel Converters

Multilevel power converters, a new breed of power electronic converters, are being developed and evolved since the 1980s. The ability to overcome the limitations and the challenges faced in the traditional 2L‐VSI topology has been favorable in many industrial high‐power applications.

True to their name, multilevel converters synthesize the output voltage into a multiple stepped level (staircase‐like) waveform. By doing so, the voltage stress across each semiconductor device is greatly reduced. Additionally, by higher number of output voltage stepped levels, the voltage harmonic is decreased. Thus, the size of the output filter can be further reduced or even eliminated. In addition, good output power quality can be achieved even with a much lower switching frequency (less than 10 kHz). Hence, the problems of EMI and high switching losses can be avoided.

Multilevel converters have drawn great research interest and attention over the last decade because of their numerous advantages. Despite having been considered as proven mature technologies in many industries, multilevel converters still encounter many challenges, such as unbalanced capacitor voltages when a higher number of output voltage stepped levels is intended to be achieved. Therefore, an investigation of the existing multilevel topologies used in industries will be discussed in the following sections of this chapter. The advantages along with the drawbacks of each multilevel converter topology will be elaborated as well. These will serve in providing a clearer vision to remain focused on the research area and alternative solutions can be proposed to improve the performance of multilevel converters.

1.4 Classical Multilevel Converters

1.4.1 Multilevel Diode Clamped Converters

The three‐level neutral point clamped inverter (3L‐NPC), shown in Fig. 1.2, was created by Nabae and colleagues [5] as a multilevel power converter in 1981. The expansion of the 3L‐NPCI is discussed in [6] and the first 10 kVA four‐level inverter prototypes are investigated in [7].

Diagram of a three-phase multilevel diode clamped inverter, where both diodes are clamped to the middle of both the dc-link capacitors.

Figure 1.2 Three‐phase multilevel diode clamped inverter (MDCI).

Based on the fundamental 3L‐NPC (Fig. 1.2), the three‐level output voltage stepped waveform is synthesized through both the diodes clamped to the middle of both the dc‐link capacitors. The middle connection of the dc‐link capacitors is referred to as the neutral point and also served as the output phase voltage reference point. Since the output voltage waveform is synthesized from the dc‐link capacitor voltage, the number of diode elements required to be clamped will increase when a higher number of output voltage level is desired. Therefore, the topology of 3L‐NPC evolves into a multilevel diode clamped inverter (MDCI).

The number of switches i and clamping diodes D required for the m‐level (mL) output phase voltage of the MDCIs can be determined using equations and , respectively:

1.1 equation

1.2 equation

The number of dc‐link capacitors Q needed for mL‐MDCIs can be known based on the following equation :

1.3 equation

Since there are no passive elements required for the MDCIs (excluding the dc‐link capacitors), only conduction losses and switching losses are accounted for the clamped diodes and switches in the converter.

When a higher number of output voltage stepped levels is desired, a higher number of capacitors will be needed in the dc‐link. Theoretically, the dc VS should be distributed equally among the number of dc‐link capacitors. However, in practical cases, the parameters of both active and passive elements can never be identically the same, due to which, an unbalanced condition occurs in the dc‐link capacitor voltages. Besides, the amount of charging and discharging current flowing through the diodes clamped between each pair of dc‐link capacitors is unequal because of the switching strategy used [6].

The unbalanced condition of the dc‐link capacitor voltages makes the MDCI topology become disadvantageous and challenging to achieve a higher number of output phase voltage levels. Several methods have been proposed to overcome the dc‐link unbalanced condition based on an active control [8], an active balancing circuit [9–11], or a passive RLC technique [12, 13].

Even though many research studies have contributed to resolve the unbalanced condition for three‐ to five‐level MDCI, [8] makes the control a little more complex or more losses are incurred from the active balancing circuit [9–11] and the passive RLC technique [12, 13]. Additional costs are also incurred to implement these proposed methods for a higher number of output phase voltage levels.

These factors could be the reason that MDCI with more than a three‐level output phase voltage is still not available in any of the industrial markets yet. Currently, 3L‐NPC is the only MDCI topology popularly available in the market. This is due to its simplicity in design, low costs, and high reliability as compared with other multilevel topologies. The 3L‐NPC is widely used in industrial medium power motor drives as well as power inverters for wind and solar systems.

1.4.2 Multilevel Flying Capacitor Converter

A multilevel flying capacitor inverter (MFCI) as shown in Fig. 1.3 was named the versatile multilevel commutation cell originally in 1992 [14]. MFCI offers an alternative approach for multilevel conversion. Based on its original name, the topology of the MFCI is constructed with the multiple cells concept. By increasing the number of cells in the MFCI, a higher number of output phase voltage levels can be obtained as well. The design does not only allow the topology to be versatile but also achieves the objective of a multilevel approach with the property of voltage sharing among the switches.

Diagram of a three-phase n-cell multilevel flying capacitor inverter, where the synthesized output phase voltage levels are not dependent on the dc-link capacitors.

Figure 1.3 Three‐phase n‐cell multilevel flying capacitor inverter (MFCI).

The MFCI has a similar structure as the MDCI, where capacitors are clamped between the switches instead of the diode elements. Hence, each cell in the MFCI consists of two semiconductor devices with a capacitor clamped. The total number of switches i and capacitors p required for mL‐MFCI can be determined based on the number of cells (n cells ) implemented as shown in equations and , respectively:

1.4 equation

1.5 equation

Based on the topology of the MFCI as shown in Fig. 1.3, the synthesized output phase voltage levels are not dependent on the dc‐link capacitors. Hence, the MFCI, unlike the MDCI, will not face the same challenge of the unbalanced dc‐link capacitor voltages when a higher number of output phase voltage levels is desired.

However, more capacitors are needed for the mL‐MFCI with a higher voltage level. Capacitors usually lead to a large inrush of current during the start‐up period. Therefore, pre‐charge circuits are needed to avoid high current during the transient period which may damage the power rating of the semiconductor devices [15]. In order to minimize the current from overstressing the switches during the initial operation, the common practice in any converter is to place a resistor in between the DC source and the dc‐link capacitors. High losses may be experienced during this process, thus a switch is also connected in parallel to the pre‐charge resistor and will be turned on after pre‐charging. So excess losses can now be avoided during normal operations by shorting the pre‐charge resistor through the switch.

Besides that, capacitors are passive elements that also serve as energy storage devices. The amount of energy stored in a capacitor will affect the voltage quality across it. The analytical and experimental results obtained in [16] have proven that a higher carrier frequency can reduce or minimize the capacity of the flying capacitors, and low voltage ripples can be achieved. However, a higher carrier frequency will limit the MFCI's high power conversion and also lead to issues such as