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

A Bidirectional Soft-Switched DAB-Based Single-Stage Three-Phase AC–DC Converter for V2G Application

In vehicle-to-grid applications, the battery charger of the electric vehicle (EV) needs to have a bidirectional power flow capability. Galvanic isolation is necessary for safety. An ac-dc bidirectional power converter with high-frequency isolation results in high power density, a key requirement for an on-board charger of an EV. Dual-active-bridge (DAB) converters are preferred in medium power and high voltage isolated dc-dc converters due to high power density and better efficiency. This paper presents a DAB-based three-phase ac-dc isolated converter with a novel modulation strategy that results in: 1) single-stage power conversion with no electrolytic capacitor, improving the reliability and power density; 2) open-loop power factor correction; 3) soft-switching of all semiconductor devices; and 4) a simple linear relationship between the control variable and the transferred active power. This paper presents a detailed analysis of the proposed operation, along with simulation results and experimental verification

Modeling and Control of a 7-Level Switched Capacitor Rectifier for Wireless Power Transfer Systems

Wireless power continues to increase in popularity for consumer device charging. Rectifier characteristics like efficiency, compactness, impedance tunability, and harmonic content make the multi-level switched capacitor rectifier (MSC) an exceptional candidate for modern WPT systems. The MSC shares the voltage conversion characteristics of a post-rectification buck-boost topology, reduces waveform distortion via its multi-level modulation scheme, demonstrates tank tunability via the phase control inherent to actively switched rectifiers, and accomplishes all this without a bulky filter inductor. In this work, the MSC WPT system operation is explained, and a loss model is constructed. A prototype system is used to validate the models, showing exceptional agreement with the predicted efficiencies. The modeled MSC efficiencies are between 96.1% and 98.0% over the experimental power range up to 20.0 W. Two significant control loops are required for the MSC to be implemented in a real system. First, the output power is regulated using the modulation of the rectifier\u27s input voltage. Second, the switching frequency of the rectifier must exactly match the WPT carrier frequency set by the inverter on the primary side. Here, a small signal discrete time model is used to construct four transfer functions relating to the output voltage. Then, four novel time-to-time transfer functions are built on top of the discrete time model to inform the frequency synchronization feedback loop. Both loops are tested and validated in isolation. Finally, the dual-loop control problem is defined, closed form equations that include loop interactions are derived, and stable wide-range dual-loop operation is demonstrated experimentally

A hybrid control method to suppress the three time fundamental frequency neutral-point voltage fluctuation in a VIENNA rectifier

This paper presents a solution to the control of the three times fundamental frequency fluctuation of the neutral-point in a VIENNA rectifier. A hybrid method combining a dynamic adjustment factor with a voltage deviation control of the split DC-link is proposed. The fluctuation of the neutral-point has been analyzed and the reason for the three times fundamental frequency fluctuation has been described using a mathematic model. As well as minimizing the three times fundamental frequency component in the neutral-point voltage the proposed control method also provides immunity to the influence of changes in the capacitor voltage. Furthermore, significant fluctuation in the neutral-point voltage caused by asymmetric capacitor parameters or unbalanced load can be effectively reduced by using a hybrid control method combining additional adjustment coefficients. The feasibility and effectiveness of the proposed strategy has been verified through the presented simulation and experimental results

Modeling and Control of a Doubly-Fed Induction Generator for Wind Turbine-Generator Systems

Wind energy plays an increasingly important role in the world because it is friendly to the environment. During the last decades, the concept of a variable-speed wind turbine (WT) has been receiving increasing attention due to the fact that it is more controllable and efficient, and has good power quality. As the demand of controllability of variable speed WTs increases, it is therefore important and necessary to investigate the modeling for wind turbine-generator systems (WTGS) that are capable of accurately simulating the behavior of each component in the WTGS. Therefore, this thesis will provide detailed models of a grid-connected wind turbine system equipped with a doubly-fed induction generator (DFIG), which includes the aerodynamic models of the wind turbine, the models of the mechanical transmission system, the DFIG models and the three-phase two-level PWM voltage source converter models. In order to obtain satisfying output power from the WTGS, control strategies are also necessary to be developed based on the previously obtained WTGS models. These control schemes include the grid-side converter control, the generator-side converter control, the maximum power point tracking control and the pitch angle control. The grid-side converter controller is used to keep the DC-link voltage constant and yield a unity power factor looking into the WTGS from the grid-side. The generator-side converter controller has the ability of regulating the torque, active power and reactive power. The maximum power point tracking control is used to provide the reference values for the active power at the stator terminals. The pitch angle control scheme is used to regulate the pitch angle and thus keep the output power at rated value even when the wind speed experiences gusts. Various studies in the literature have reported that two-level converters have several disadvantages compared with three-level converters. Among the disadvantages are high switching losses, high dv/dt, and high total harmonic distortion (THD). Hence, the models and field oriented control schemes for three-level neutral-point-clamped (NPC) converters are also investigated and applied to a WTGS. Besides, an advanced modulation technology, namely, space vector PWM (SVPWM), is also investigated and compared to traditional sinusoidal PWM in a WTGS

A new three-level indirect matrix converter with reduced number of switches

Matrix converters are sine wave in/out forced commutated single-stage AC/AC direct frequency changers using a single bi-directional switch to connect any input to any output. Indirect two-stage matrix converters provide similar input and output performance with no passive component in the dc-link. The difference is that some of these topologies require less switching devices or are able to achieve multilevel output voltage capability. This paper proposes a new indirect matrix converter topology with a three-level phase to neutral output voltage capability and reduced number of devices. A new modulation scheme, simpler that a standard 3-level scheme, is proposed. The performance of the converter in terms of input and output waveform quality and power losses is evaluated through simulations and on an experimental prototype

High Current Density Low Voltage Isolated Dc-dc Converterswith Fast Transient Response

With the rapid development of microprocessor and semiconductor technology, industry continues to update the requirements for power supplies. For telecommunication and computing system applications, power supplies require increasing current level while the supply voltage keeps decreasing. For example, the Intel\u27s CPU core voltage decreased from 2 volt in 1999 to 1 volt in 2005 while the supply current increased from 20A in 1999 to up to 100A in 2005. As a result, low-voltage high-current high efficiency dc-dc converters with high power-density are demanded for state-of-the-art applications and also the future applications. Half-bridge dc-dc converter with current-doubler rectification is regarded as a good topology that is suitable for high-current low-voltage applications. There are three control schemes for half-bridge dc-dc converters and in order to provide a valid unified analog model for optimal compensator design, the analog state-space modeling and small signal modeling are studied in the dissertation and unified state-space and analog small signal model are derived. In addition, the digital control gains a lot of attentions due to its flexibility and re-programmability. In this dissertation, a unified digital small signal model for half-bridge dc-dc converter with current doubler rectifier is also developed and the digital compensator based on the derived model is implemented and verified by the experiments with the TI DSP chip. In addition, although current doubler rectifier is widely used in industry, the key issue is the current sharing between two inductors. The current imbalance is well studied and solved in non-isolated multi-phase buck converters, yet few discusse this issue in the current doubler rectification topology within academia and industry. This dissertation analyze the current sharing issue in comparison with multi-phase buck and one modified current doubler rectifier topology is proposed to achieve passive current sharing. The performance is evaluated with half bridge dc-dc converter; good current sharing is achieved without additional circuitry. Due to increasing demands for high-efficiency high-power-density low-voltage high current topologies for future applications, the thermal management is challenging. Since the secondary-side conduction loss dominates the overall power loss in low-voltage high-current isolated dc-dc converters, a novel current tripler rectification topology is proposed. Theoretical analysis, comparison and experimental results verify that the proposed rectification technique has good thermal management and well-distributed power dissipation, simplified magnetic design and low copper loss for inductors and transformer. That is due to the fact that the load current is better distributed in three inductors and the rms current in transformer windings is reduced. Another challenge in telecommunication and computing applications is fast transient response of the converter to the increasing slew-rate of load current change. For instance, from Intel\u27s roadmap, it can be observed that the current slew rate of the age regulator has dramatically increased from 25A/uS in 1999 to 400A/us in 2005. One of the solutions to achieve fast transient response is secondary-side control technique to eliminate the delay of optocoupler to increase the system bandwidth. Active-clamp half bridge dc-dc converter with secondary-side control is presented and one industry standard 16th prototype is built and tested; good efficiency and transient response are shown in the experimental section. However, one key issue for implementation of secondary-side control is start-up. A new zero-voltage-switching buck-flyback isolated dc-dc converter with synchronous rectification is proposed, and it is only suitable for start-up circuit for secondary-side controlled converter, but also for house-keeping power supplies and standalone power supplies requiring multi-outputs

Assessment of novel power electronic converters for drives applications

Phd ThesisIn the last twenty years, industrial and academic research has produced over one hundred new converter topologies for drives applications. Regrettably, most of the published work has been directed towards a single topology, giving an overall impression of a large number of unconnected, competing techniques. To provide insight into this wide ranging subject area, an overview of converter topologies is presented. Each topology is classified according to its mode of operation and a family tree is derived encompassing all converter types. Selected converters in each class are analysed, simulated and key operational characteristics identified. Issues associated with the practical implementation of analysed topologies are discussed in detail. Of all AC-AC conversion techniques, it is concluded that softswitching converter topologies offer the most attractive alternative to the standard hard switched converter in the power range up to 100kW because of their high performance to cost ratio. Of the softswitching converters, resonant dc-link topologies are shown to produce the poorest output performance although they offer the cheapest solution. Auxiliary pole commutated inverters, on the other hand, can achieve levels of performance approaching those of the hard switched topology while retaining the benefits of softswitching. It is concluded that the auxiliary commutated resonant pole inverter (ACPI) topology offers the greatest potential for exploitation in spite of its relatively high capital cost. Experimental results are presented for a 20kW hard switched inverter and an equivalent 20kW ACPI. In each case the converter controller is implanted using a digital signal processor. For the ACPI, a new control scheme, which eliminates the need for switch current and voltage sensors, is implemented. Results show that the ACPI produces lower overall losses when compared to its hardswitching counterpart. In addition, device voltage stress, output dv/dt and levels of high frequency output harmonics are all reduced. Finally, it is concluded that modularisation of the active devices, optimisation of semiconductor design and a reduction in the number of additional sensors through the use of novel control methods, such as those presented, will all play a part in the realisation of an economically viable system.Research Committee of the University of Newcastle upon Tyn

Modelling, simulation and real time implementation of a three phase AC to AC matrix converter

Matrix converters (MCs) are essentially forced commutated cycloconverters with inherent four quadrant operation consisting of a matrix of bidirectional switches such that there is a switch for each possible connection between the input and output lines. Matrix converter directly converts the AC input voltage at any given frequency to AC output voltage with arbitrary amplitude at any unrestricted frequency without the need for a dc link capacitor storage element at the input side.The introduction of bidirectional switches using power transistors and IGBTs made easy realization of the matrix converter. The real development of the matrix converter starts with the work of Venturini and Alesina who proposed a mathematical analysis and introduced the Low-Frequency Modulation Matrix concept to describe the low frequency behavior of the matrix converter [1-3]. In this, the output voltages are obtained by multiplication of the modulation matrix or transfer matrix with the input voltages. One of the essential requirements for switching three phase AC to three phase AC MC is that two or more bidirectional switches connected to any one output phase should NOT be closed simultaneously, as this will cause dangerously high short circuit current. Similarly any one bidirectional switch connected to each output phase should remain closed to provide a current path with inductive load.This thesis mainly provides an account of the three phase AC to three phase AC MC modelling concept with SIMULINK software using fundamental Venturini and Optimum Venturini modulation algorithm [1-8], advanced modulation algorithm such as that proposed by Sunter-Clare [11-12] and by Ned Mohan [13-14, 16-17], application of these algorithms for the Vector control of three phase Induction Motor (IM) drive [15], real time hardware in the loop simulation [51,54-55] for a three phase AC to single phase AC MC, three phase AC to three phase AC Multilevel MC (MMC) with three [18-19] and six flying capacitors per output phase using PSCAD software (as SIMULINK started shooting trouble), Indirect (ISVM) [25-30] and Direct (DASVM, DSSVM, CZASVM) [31-38] Space Vector Modulation, newly discovered dual programmable AC to DC rectifier concept using three phase AC to three phase AC MC [43-45], Delta-Sigma Modulated MC [46-49] and single phase AC to three phase AC MC [50].In addition a novel concept of a single phase / three phase AC to single phase / three phase AC converter using a DC link, complementary N and P MOSFETs and IGBTs is presented. A chapter on model verification is also presented where selected SIMULINK models from various chapters have been verified by using either PSCAD or PSIM software. An appendix on PIC microcontroller PIC16F84A application to saw-tooth carrier waveform generation and switching three phase AC to three phase AC converters using a DC link is added. Another appendix on speed control and brake by plugging of three phase induction motor fed by matrix converter is presented. List of publications from this thesis is presented on third appendix

Z-source matrix rectifier

This paper presents a novel Z-source matrix rectifier(ZSMR). To overcome the inherent disadvantage that the voltage transfer ratio for traditional matrix rectifier cannot be more than 0.866, a Z-source network has been combined with the matrix rectifier. The proposed rectifier realizes a voltage-boost function and the Z-source network also serves as power storage and guarantees double filtration grade at the output of the rectifier. The open-circuit zero state is required to obtain the voltage-boost function and ensure the output angle of the current vector to be invariant to obtain the expected power factor. In addition, to widely extend the voltage transfer ratio of the proposed rectifier, this paper presents the switched-inductor matrix rectifier(SL-ZSMR) and tapped-inductor matrix rectifier(TL-ZSMR). The corresponding circuit topologies, control strategies and operating principles are introduced. Both simulation and experiment results are shown to verify the theoretical analysis