1
Background

1.1 Power Electronics Converter Topologies and Applications in Modern Power Systems

1.1.1 Introduction

In modern society, electrical energy is the most convenient and widely available form of energy, making it the most crucial energy source. However, in recent years, with rapid economic development, global electricity consumption has surged, leading to prominent issues of energy scarcity and environmental pollution. On one hand, electrical energy cannot meet the demands of industrial production and people's daily lives. On the other hand, extensive reliance on traditional fossil fuels for electricity generation has caused severe environmental problems and inefficient utilization of electrical energy [1].

According to statistics from the International Energy Agency in 2014, from 1973 to 2012, the proportion of coal and oil in global terminal energy consumption decreased by 3.6% and 7.5%, respectively. In contrast, the share of electricity consumption increased from 9.4% to 18.1%, ranking second only to oil, as shown in Figure 1.1. It is projected that by 2030, electricity will constitute 25% of global terminal energy consumption, and by 2050, this share is expected to surpass 50%, as depicted in Figure 1.2 [2-5].

Power electronics technology, serving as the vital link for energy conversion and a necessary means to address environmental pollution in the context of new energy sources, has permeated various aspects of electrical applications. This includes applications in power systems, industry, transportation, aerospace, information technology, and telecommunications, as depicted in Figure 1.3 [6]. It has directly or indirectly generated significant economic and societal benefits. In the future, approximately 90% of electrical energy will need to be processed through power electronics technology to enhance energy efficiency and production efficiency, thereby maximizing the utilization of renewable energy sources [7].

AC variable frequency drive technology is a significant application of power electronics in energy-efficient and high-capacity AC transmission control systems. Within this technology, AC converters play a crucial role as integral components of AC speed control systems. Currently, AC converters are extensively employed in high-power AC motor drive systems and power systems [8]. The classification of converters can be seen in Figure 1.4 [9].

The frequency converter, known as a thyristor-based AC/AC converter circuit, directly converts AC power of a certain frequency into adjustable-frequency AC power. As it lacks a direct current (DC) stage, it falls into the category of direct-frequency conversion circuits. However, this type of converter has notable drawbacks, with its output upper-frequency limit not exceeding 1/3 to 1/2 of the grid frequency. For single-phase AC circuits, two sets of converters are needed, while three-phase circuits require six sets, resulting in numerous components and highly complex control systems.

Figure 1.1 (a) Comparison of energy consumption structure between 1973 and 2012; (b) Global terminal energy consumption structure from 2010 to 2050.

Figure 1.2 Global terminal energy consumption structure from 2010 to 2050.

AC/DC/AC converter is presently one of the most widely used AC/AC frequency conversion circuits. This converter first rectifies AC power into DC power and then inverts DC power back into AC power. Due to the presence of a DC stage, this circuit falls under the category of indirect-frequency conversion circuits. Depending on whether the intermediate DC stage is composed of capacitors or inductors, it can be classified into voltage-source indirect AC/DC/AC converters and current-source indirect AC/DC/AC converters [8]. Among them, the voltage-source AC/DC/AC converter can be further divided into non-controlled rectifier + inverter (Figure 1.5a), which lacks boosting capability and generates high-input current harmonics, resulting in severe grid pollution. The controlled rectifier + inverter (Figure 1.5b) utilizes a boosting rectifier at the input stage, requiring the addition of an inductor. To mitigate harmonic pollution to the grid, inductor-capacitor (LC) or inductor-capacitor-inductor (LCL) filters need to be designed at the input stage. The primary drawback of both types of converters lies in the intermediate energy storage components, which not only have large volume and high mass but are also challenging to maintain, leading to lower power density in power converters.

Figure 1.3 Application fields of power electronics [6].

Figure 1.4 AC frequency converter classification.

The current-source AC/DC/AC converter Figure 1.6 introduces challenges related to the need for large-capacity flat-wave reactors and issues like current distortion and oscillations caused by AC-side LC filter. In comparison to voltage-source converters, it is more costly and complex to control, thereby limiting its application and research. However, with the advancement of superconducting technology, the current-source converter has found successful applications in superconducting energy storage. Furthermore, it has garnered significant attention in medium-voltage high-power wind power generation and motor drive applications [10, 11].

Figure 1.5 AC/DC/AC voltage-source converter (a) uncontrolled rectifier with inverter, (b) controlled rectifier with inverter.

Figure 1.6 AC/DC/AC current-source converter.

To overcome the drawbacks associated with converters featuring intermediate energy storage components and to enhance the power density and reliability of AC/AC converters, researchers began to explore the possibility of AC/AC converters without the use of DC energy storage elements. It was at this juncture that matrix converter (MC) emerged. MC is an electrical conversion device based on bidirectional switches and utilizes pulse-width modulation to generate the desired output voltage. Among various novel AC power converters, MC has gained significant attention from researchers worldwide due to its simple structure and full silicon integration, among other excellent performance attributes [9]. Depending on their structural characteristics, MCs can be classified into two categories: direct matrix converters (DMCs) and indirect matrix converters (IMCs). IMCs not only inherit the advantageous features of DMC but also possess the advantage of zero-current switching at the rectifier stage, significantly reducing control complexity, making them one of the most promising types of AC power converters. IMCs have further led to the development of three-level MCs and generalized sparse IMCs.

1.1.2 Matrix Converter

MCs have been in development for over 40 years, and substantial progress has been made in key areas such as topology design, modulation strategies, control theory, and device development [12-14].

1.1.2.1 Direct Matrix Converter

The concept of DMCs and bidirectional switches was first proposed by Gugi and Pelly [15]. In 1980, Venturini and Alesina introduced the idea of using transistors to construct bidirectional switches for implementing MCs. They developed a prototype based on this concept and presented a series of attractive results. The topology of a DMC is shown in Figure 1.7. This topology employs nine bidirectional switches to interconnect each input phase with every output phase, allowing for the synthesis of the desired output and input currents through a single-stage transformation. Since each bidirectional switch consists of two antiparallel insulated gate bipolar transistors (IGBTs), a DMC requires a total of 18 IGBT power devices [16].

The advantages of a DMC include: (i) bidirectional energy flow, achieving four-quadrant operation; (ii) both input and output currents are sinusoidal; (iii) power factor at the input side can be unity for any load; and (iv) no need for a DC energy storage stage, resulting in a compact circuit structure and high integration level [17].

Despite over 40 years of development, MC technology still faces challenges preventing widespread industrial adoption [18]. These challenges include: (i) maximum boost ratio limited to 0.866; (ii) a relatively high number of power devices, leading to complex commutation control; (iii) difficulty in control under abnormal grid voltage conditions due to the absence of an intermediate DC stage, impacting system performance; (iv) interference on the load side directly affects input-side performance, leading to suboptimal electromagnetic compatibility with the grid; and (v) complex protection circuits, large physical footprint, and higher cost.

1.1.2.2 Indirect Matrix Converter

In pursuit of simplifying the structure of DMCs, reducing the count of power switching components, minimizing system energy losses, and alleviating control intricacies, scholars have introduced a category of IMCs, as depicted in Figure 1.8. In this topology, the input-side rectification employs bidirectional switches, while the inversion stage relies on unidirectional switches, necessitating a total of 18 IGBT power devices. The initial conceptualization of this topology was attributed to Wei at University of...