Multilevel Converters: An Enabling Technology for High-Power Applications

| Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.Ministerio de Ciencia y Tecnología DPI2001-3089Ministerio de Eduación y Ciencia d TEC2006-0386

The Essential Role and the Continuous Evolution of Modulation Techniques for Voltage-Source Inverters in the Past, Present, and Future Power Electronics

The cost reduction of power-electronic devices, the increase in their reliability, efficiency, and power capability, and lower development times, together with more demanding application requirements, has driven the development of several new inverter topologies recently introduced in the industry, particularly medium-voltage converters. New more complex inverter topologies and new application fields come along with additional control challenges, such as voltage imbalances, power-quality issues, higher efficiency needs, and fault-tolerant operation, which necessarily requires the parallel development of modulation schemes. Therefore, recently, there have been significant advances in the field of modulation of dc/ac converters, which conceptually has been dominated during the last several decades almost exclusively by classic pulse-width modulation (PWM) methods. This paper aims to concentrate and discuss the latest developments on this exciting technology, to provide insight on where the state-of-the-art stands today, and analyze the trends and challenges driving its future

Voltage Balancing Control Strategy in Converter System for Three-Level Inverters

Outcome of DC-link capacitor voltage variation on inverter switching states is accessible and designed for three-level inverter. In this paper for back-to-back system by including five-level diode clamped topologies we are proposing a novel DC link balancing method. The algorithm which we proposed here is the improvement of variable switching frequency control policy which was previously introduced by means of three-level back-to-back system which depends on calculations of adjacent capacitor voltages which focuses on three-level DC link network to identify the information about potential variation in consecutive nodes. As per the above proposal, all four capacitors in DC link network are effectively balancing the voltage. Due to optimization of switching losses the proposed method has advantages over the variable switching frequency.DOI:http://dx.doi.org/10.11591/ijece.v3i1.1471

Addressing control and capacitor voltage regulation challenges in multilevel power electronic converters

Multilevel power electronic converters are the current industry solutions for applications that demand medium voltage, reasonable efficiency, and high power quality. The proper operation of these types of power converters requires special control, modulation methods, and capacitor voltage regulation techniques. Both developing capacitor voltage regulation methods and addressing their associated issues with such fall within the primary focus of this dissertation. In this dissertation an investigation was conducted on the capacitor voltage regulation constraints in cascaded H-bridge multilevel converters with a staircase output voltage waveform. In the proposed method, the harmonic elimination technique is used to determine the switching angles. A constraint was then derived to identify modulation those indices that lead to voltage regulation of the capacitor. This constraint can be used in optimization problems for harmonic minimization to guarantee capacitor voltage regulation in these types of converters. Furthermore, a capacitor voltage regulation method was developed using redundant state selection for a flying capacitor active rectifier. This method reduces the number of switching instances by using both online and offline state selection procedure. Additionally, a start-up procedure is proposed that pre-charges the all of capacitors in the rectifier to both avoid overstressing the switches and obtain a smoother start-up. Finally, a flexible capacitor voltage regulation method is proposed that provides the ability to control the voltage of the capacitors in both cascaded H-bridge and hybrid multilevel converters. In this method, the capacitor voltage in each individual H-bridge cell is independently regulated by controlling the active power of each cell

Advanced control of grid-connected multilevel power electronic rectifiers

Multilevel power electronic converters have been gaining attention due to their ability to supply high amounts of power and to handle high voltage levels. In this dissertation, grid connected AC-DC rectifier application is investigated with different topologies and control scheme. At first, neutral point clamped (NPC) rectifier is employed to transfer power from the grid to the load. The NPC rectifier has two capacitors in order to build multilevel output voltage. However, it causes voltage unbalancing problem. Therefore, the new method has been proposed to regulate each capacitor voltage at the same voltage level. Experimental results show that it is effective to balance capacitor voltages of the NPC and it can improve total harmonic distortion (THD) of the grid current as a result. Furthermore, 7 voltage levels can be achieved by using hybrid multilevel rectifier which consists of an NPC and cascaded H-bridges (CHB). Because the hybrid multilevel rectifier has total 8 capacitors which are completely discharged at first, large inrush currents from the grid might cause hazards. Therefore, the paper develops a pre-charge routine for building it up to steady state operation in which unity power factor control (PFC) and load voltage control are achieved. Finally, multiple reference frame theory (MRF) is used to improve THD of the grid currents when the hybrid multilevel rectifier is connected with distorted grid voltage source. After calculating 5th harmonic of the grid current in real time, the voltage reference for the hybrid multilevel rectifier will be compensated in a feedback loop. Experimental results show validity in improving THD of the grid currents. --Abstract, page iv

Development of a multilevel converter topology for transformer-less connection of renewable energy systems

The global need to reduce dependence on fossil fuels for electricity production has become an ongoing research theme in the last decade. Clean energy sources (such as wind energy and solar energy) have considerable potential to reduce reliance on fossil fuels and mitigate climate change. However, wind energy is going to become more mainstream due to technological advancement and geographical availability. Therefore, various technologies exist to maximize the inherent advantages of using wind energy conversion systems (WECSs) to generate electrical power. One important technology is the power electronics interface that enables the transfer and effective control of electrical power from the renewable energy source to the grid through the filter and isolation transformer. However, the transformer is bulky, generates losses, and is also very costly. Therefore, the term "transformer-less connection" refers to eliminating a step-up transformer from the WECS, while the power conversion stage performs the conventional functions of a transformer. Existing power converter configurations for transformer-less connection of a WECS are either based on the generator-converter configuration or three-stage power converter configuration. These configurations consist of conventional multilevel converter topologies and two-stage power conversion between the generator-side converter topology and the high-order filter connected to the collection point of the wind power plant (WPP). Thus, the complexity and cost of these existing configurations are significant at higher voltage and power ratings. Therefore, a single-stage multilevel converter topology is proposed to simplify the power conversion stage of a transformer-less WECS. Furthermore, the primary design challenges – such as multiple clamping devices, multiple dc-link capacitors, and series-connected power semiconductor devices – have been mitigated by the proposed converter topology. The proposed converter topology, known as the "tapped inductor quasi-Z-source nested neutral-point-clamped (NNPC) converter," has been analyzed, and designed, and a prototype of the topology developed for experimental verification. A field-programmable gate array (FPGA)-based modulation technique and voltage balancing control technique for maintaining the clamping capacitor voltages was developed. Hence, the proposed converter topology presents a single-stage power conversion configuration. Efficiency analysis of the proposed converter topology has been studied and compared to the intermediate and grid-side converter topology of a three-stage power converter configuration. A direct current (DC) component minimization technique to minimize the dc component generated by the proposed converter topology was investigated, developed, and verified experimentally. The proposed dc component minimization technique consists of a sensing and measurement circuitry with a digital notch filter. This thesis presents a detailed and comprehensive overview of the existing power converter configurations developed for transformer-less WECS applications. Based on the developed 2 comparative benchmark factor (CBF), the merits and demerits of each power converter configuration in terms of the component counts and grid compliance have been presented. In terms of cost comparison, the three-stage power converter configuration is more cost-effective than the generatorconverter configuration. Furthermore, the cost-benefit analysis of deploying a transformer-less WECSs in a WPP is evaluated and compared with conventional WECS in a WPP based on power converter configurations and collection system. Overall, the total cost of the collection system of WPP with transformer-less WECSs is about 23% less than the total cost of WPP with conventional WECs. The derivation and theoretical analysis of the proposed five-level tapped inductor quasi-Z-source NNPC converter topology have been presented, emphasizing its operating principles, steady-state analysis, and deriving equations to calculate its inductance and capacitance values. Furthermore, the FPGA implementation of the proposed converter topology was verified experimentally with a developed prototype of the topology. The efficiency of the proposed converter topology has been evaluated by varying the switching frequency and loads. Furthermore, the proposed converter topology is more efficient than the five-level DC-DC converter with a five-level diode-clamped converter (DCC) topology under the three-stage power converter configuration. Also, the cost analysis of the proposed converter topology and the conventional converter topology shows that it is more economical to deploy the proposed converter topology at the grid side of a transformer-less WECS