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

Voltage balancing in three-level neutral-point-clamped converters via Luenberger observer

This paper addresses the problems associated with the dc-link capacitor voltages of the three-level neutral-point-clamped power converter: the imbalance of the capacitor voltages as well as the presence of an ac-voltage low-frequency oscillation in the dc link of the converter. In order to cope with them, a mathematical analysis of the capacitor voltage difference dynamics, based on a direct average continuous model, is carried out, considering a singular perturbation approach. The analysis leads to a final expression where a sinusoidal disturbance appears explicitly. Consequently, the two problems can be handled together using the ordinary formulation of a problem of regulating the output of a system subject to sinusoidal disturbances, applying classical control theory to design the controller. In this way, the controller is designed including the disturbance estimate provided by a Luenberger observer to asymptotically cancel the disturbance, while keeping also balanced the capacitor voltages. Experiments for a synchronous three-level neutral-point-clamped converter prototype are carried out to evaluate the performance and usefulness of the converter working as a grid-connected inverter under the proposed control law.MICINN-FEDER DPI2009-09661Junta de Andalucía P07-TIC-0299

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

Sliding Mode Control Strategy for Three-Phase Three-Level T-Type Shunt Active Power Filters

In this paper, a sliding mode control (SMC) strategy is proposed for three-phase three-level T-type shunt active power filters (SAPF). The proposed control strategy has the ability to balance the capacitor voltages with respect to the neutral-point. The proposed SMC strategy is formulated in the natural frame which eliminates abc/dq transformation and two PI controllers compared to the design in the dq frame. In natural frame, only one PI controller is needed to generate the amplitude of grid current reference. The output of the PI controller is multiplied by the unity sinusoidal waveforms, obtained from the grid voltages, so as to obtain the grid current references. The filter current references are obtained by subtracting the measured load currents from grid current references. The performance of the proposed control method is investigated by simulation study during steady-state and transients caused by load change. It is shown that the grid currents are almost sinusoidal with small THD, grid currents and dc-link voltage track their references and capacitor voltages are balanced with respect to the neutral-point

Evaluation of a Trapezoidal Predictive Controller for a Four-Wire Active Power Filter for Utility Equipment of Metro Railway, Power-Land Substations

The realization of an improved predictive current controller based on a trapezoidal model is described, and the impact of this technique is assessed on the performance of a 2 kW, 21.6 kHz, four-wire, Active Power Filter for utility equipment of Metro Railway, Power-Land Substations. The operation of the trapezoidal predictive current controller is contrasted with that of a typical predictive control technique, based on a single Euler approximation, which has demonstrated generation of high-quality line currents, each using a 400 V DC link to improve the power quality of an unbalanced nonlinear load of Metro Railway. The results show that the supply current waveforms become virtually sinusoidal waves, reducing the current ripple by 50% and improving its power factor from 0.8 to 0.989 when the active filter is operated with a 1.6 kW load. The principle of operation of the trapezoidal predictive controller is analysed together with a description of its practical development, showing experimental results obtained with a 2 kW prototype

A simplified space-vector modulation algorithm for four-leg NPC converters

To interface generation sources and loads to four-wire distribution networks is important to use power converters and modulation methods which provide high performance, flexi¬bility and reliability. To achieve these goals, this paper proposes a simple and efficient Space Vector Modulation (SVM) algorithm in α3-y coordinates for Neutral Point Clamped (NPC) converters. The proposed SVM method reduces a three-dimensional (α3-y) search of the modulating vectors into a simple two-dimensional (α3) problem. Moreover, the algorithm provides full utilisation of the dc-link voltage, full utilisation of the redundant vectors and it can be applied to any other four-leg converter topology. The proposed SVM has been successfully validated using a 6kW three-level four-leg NPC converter, achieving control over the voltages of the dc-link capacitors and simple definition of switching pattern for shaping frequency spectrum

High-Efficiency NPC Multilevel Converter Using Super-Junction MOSFETs

Super-junction MOSFETs exhibit low on-state resistances and low switching losses. However, the reverse recovery behavior of their intrinsic diodes and their output capacitance characteristics make their deployment in freewheeling locations challenging. In this paper, a new snubber circuit arrangement has been proposed for a three-level converter to minimize the effect of the output capacitance. This is used in conjunction with diode deactivation circuitry to address the diode recovery behavior. Results are given for a three-phase three-level neutral point clamped converter running from an input voltage of 720 V and supplying a 3-kVA load. The converter operates with no forced cooling and efficiency is estimated at 99.3%. Apart from lower energy consumption, an advantage of high efficiency is a reduced converter mass due to reduced cooling requirements

A hybrid modulation technique for the dc-bus voltage balancing in a three-phase npc converter

In this paper a new pulse width modulation technique for three-phase neutral point clamped (NPC) converter is presented, with the aim to actively control the dc-bus capacitors' voltages. To meet this requirement, usually NPC modulation techniques are either based on the sole common mode voltage injection (CM1) or on the sole multi-step (MS) switching mode of operation. Contrarily, the presented approach combines these two strategies, taking advantages of all their main benefits while keeping the switching transitions to the minimum required number. The approach has been numerically tested and compared with some of the other strategies, showing an overall better behaviour, especially for high modulation indices

High Power, Medium Frequency, and Medium Voltage Transformer Design and Implementation

Many industrial applications that require high-power and high-voltage DC-DC conversion are emerging. Space-borne and off-shore wind farms, fleet fast electric vehicle charging stations, large data centers, and smart distribution systems are among the applications. Solid State Transformer (SST) is a promising concept for addressing these emerging applications. It replaces the traditional Low Frequency Transformer (LFT) while offering many advanced features such as VAR compensation, voltage regulation, fault isolation, and DC connectivity. Many technical challenges related to high voltage stress, efficiency, reliability, protection, and insulation must be addressed before the technology is ready for commercial deployment. Among the major challenges in the construction of SSTs are the strategies for connecting to Medium Voltage (MV) level. This issue has primarily been addressed by synthesizing multicellular SST concepts based on modules rated for a fraction of the total MV side voltage and connecting these modules in series at the input side. Silicon Carbide (SiC) semiconductor development enables the fabrication of power semiconductor devices with high blocking voltage capabilities while achieving superior switching and conduction performances. When compared to modular lower voltage converters, these higher voltage semiconductors enable the construction of single-cell SSTs by avoiding the series connection of several modules, resulting in simple, reliable, lighter mass, more power dense, higher efficiency, and cost effective converter structures. This dissertation proposes a solution to this major issue. The proposed work focuses on the development of a dual active bridge with high power, medium voltage, and medium frequency control. This architecture addresses the shortcomings of existing modular systems by providing a more power dense, cost-effective, and efficient solution. For the first time, this topology is investigated on a 700kW system connected to a 13kVdc input to generate 7.2kVdc at the output. The use of 10kV SiC modules and gate drivers in an active neutral point clamped to two level dual active bridge converter is investigated. A special emphasis will be placed on a comprehensive transformer design that employs a multi-physics approach that addresses all magnetic, electrical, insulation, and thermal aspects. The transformer is designed and tested to ensure the system’s viability