This paper gives an overview of medium-voltage (MV) multilevel converters with a focus on achieving minimum harmonic distortion and high efficiency at low switching frequency operation. Increasing the power rating by minimizing switching frequency while still maintaining reasonable power quality is an important requirement and a persistent challenge for the industry. Existing solutions are discussed and analyzed based on their topologies, limitations, and control techniques. As a preferred option for future research and application, an inverter configuration based on three-level building blocks to generate five-level voltage waveforms is suggested. This paper shows that such an inverter may be operated at a very low switching frequency to achieve minimum on-state and dynamic device losses for highly efficient MV drive applications while maintaining low harmonic distortion.

Medium-Voltage Multilevel Converters—State of the Art, Challenges, and Requirements in Industrial Applications

Power electronics is a discipline spawned by real-life applications in industrial, commercial, residential, and aerospace environments. Much of its development revolves around some immediate need for solving specific power conversion problems. This comprehensive book focuses on the typical bifurcation scenarios and nonlinear behavior observed in switching power converters, expounding on their most relevant aspects from a practical viewpoint. A balanced emphasis on theory, computer methods, and laboratory experiments bridges the gap between the theoretical research and practical engineering applications.

Complex Behavior of Switching Power Converters

Power electronics circuits are rich in nonlinear dynamics. Their operation is characterized by cyclic switching of circuit topologies, which gives rise to a variety of nonlinear behavior. This paper provides an overview of the chaotic dynamics and bifurcation scenarios observed in power converter circuits, emphasizing the salient features of the circuit operation and the modeling strategies. In particular this paper surveys the key publications in this field, reviews the main modeling approaches, and discusses the salient bifurcation behaviors of power converters with particular emphasis on the disruption of standard bifurcation patterns by border collisions.

/pdf/complex-behavior-in-switching-power-converters-1stelymp8h.pdf

Complex behavior in switching power converters

Control of chaos: Methods and applications in engineering

Recent investigations on the bifurcation behavior of power electronic DC-DC converters have revealed that most of the observed bifurcations do not belong to generic classes such as saddle-node, period doubling, or Hopf bifurcations. Since these systems yield piecewise smooth maps under stroboscopic sampling, a new class of bifurcations occur in such systems when a fixed point crosses the border between the smooth regions in the state space. In this paper we present a systematic analysis of such bifurcations through a normal form: the piecewise linear approximation in the neighborhood of the border. We show that there can be many qualitatively different types of border collision bifurcations, depending on the parameters of the normal form. We present a partitioning of the parameter space of the normal form showing the regions where different types of bifurcations occur. We then use this theoretical framework to explain the bifurcation behavior of the current programmed boost converter.

http://yorke.umd.edu/Yorke_papers_most_cited_and_post2000/2000_04_Banerjee_G-H-Yuan_IEEE_1Dim_Switching_Circuits.pdf

Bifurcations in one-dimensional piecewise smooth maps-theory and applications in switching circuits

The DC-DC buck power converter, a widely used chopper circuit, exhibits subharmonics and chaos if current feedback is used. This paper investigates the dependence of the system behavior on its parameters. The bifurcation phenomena and a mapping of the parameter space have been presented. This knowledge is vital for designing practical circuits.

Bifurcation behavior of the buck converter

For the application of variable speed drive, submodule capacitors of the modular multilevel converter (MMC) suffer from large voltage fluctuations at low frequencies. This paper presents a control technique that regulates this fluctuation to a constant value at all speeds with rated load torque. Transformerless configuration of this drive is as follows. An MMC is used as a front-end converter and is directly connected to the grid. It keeps the grid current sinusoidal and is in phase with the grid voltage. It generates regulated dc current at the output. Another MMC, connected in cascade with the earlier one, drives a three-phase induction motor. Therefore, the input to this rear-end MMC is constant dc current, the output is variable magnitude and frequency voltage source that drives the motor. This paper proposes a control strategy for this drive. The proposed control method and its performance have been analyzed and verified through experimental results in this paper.

Control of Medium-Voltage AC Motor Drive for Wide Speed Range Using Modular Multilevel Converter

Conventional modular multilevel converter (MMC) has severe limitation to operate at low frequency for the application of variable-speed induction motor drive. Fluctuation of submodule (SM) capacitor voltage increases rapidly as the motor speed decreases with rated torque. Finally, it becomes impractical to operate the motor at zero frequency. Recently, MMC-based medium voltage induction motor drive has been proposed to tackle this problem without additional circulating current. However, it does not solve the problem near zero frequency with nonzero motor torque. This paper proposes new method to solve this problem of high peak-to-peak voltage ripple of SM capacitor at all frequencies including zero frequency. Vector control strategy for induction motor is adopted here to have accurate control of torque near zero frequency. First, the problem of low-frequency operation is analyzed. Then, the solution is proposed analytically. The proposed control technique and overall performance of the drive have been verified through experimental results.

Medium-Voltage Vector Control Induction Motor Drive at Zero Frequency Using Modular Multilevel Converter

A variable-speed controller, for an existing 225 kW fixed-speed wind power generator, is presented in this paper. A sensorless direct torque control algorithm-based controller is proposed for the squirrel-cage induction generator. Generator torque reference is derived based on sensorless maximum power tracking mode algorithm, up to the base speed of the generator. A three-phase front-end converter is used here to deliver the generated power to the grid with unity-power-factor operation at all wind speeds. This algorithm is based on direct active and reactive power control. The experimental waveforms from the actual installation site are presented in this paper with a comparison of the existing fixed-speed system. An interpretation of the results is also presented here.

Sensorless variable-speed controller for existing fixed-speed wind power generator with unity-power-factor operation

A novel control technique for sensorless vector control operation of a double-inverter-fed wound-rotor induction motor is presented. Two current controllers control the stator-side currents based on a vector control algorithm. Another V/f-type flux and frequency controller controls the rotor-side frequency directly. A novel frequency command profile for the rotor-side controller is suggested to make this sensorless drive operation reliable and reduce dependence on motor parameters at any rotor speed. A complete inverter power flow analysis is presented to show that the drive can deliver full torque from 0- to 2-p.u. speed for either direction of rotation. Thus, double the rated power can be extracted from the induction motor without overloading it. The proposed algorithm allows the drive to start on-the-fly without any rotor transducer. Results from a prototype 50-hp drive are presented.

/pdf/sensorless-field-oriented-control-for-double-inverter-fed-1x3t98i35j.pdf

G. Poddar

Papers

Bifurcation behavior of the buck converter

Control of Medium-Voltage AC Motor Drive for Wide Speed Range Using Modular Multilevel Converter

Medium-Voltage Vector Control Induction Motor Drive at Zero Frequency Using Modular Multilevel Converter

Sensorless variable-speed controller for existing fixed-speed wind power generator with unity-power-factor operation

Sensorless field-oriented control for double-inverter-fed wound-rotor induction motor drive