Decoupled DC-Link capacitor voltage control of DC-AC multilevel multileg converters

© 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksThis paper studies the coupling between the capacitor voltage control loops of diode-clamped (or functionally equivalent) multilevel multileg (multiphase) dc-ac converters. From a complete model of the plant revealing the coupling, a simple approach consisting in multiplying the vector of control commands by a constant matrix is proposed to decouple the control problem and achieve a better controller performance. Simulation and experimental results are presented to prove the superior performance of the proposed decoupled control.Postprint (author's final draft

Comprehensive analysis of voltage balancing techniques for 5L NPC converters

The reduced capability of the 5L-NPC Inverter to balance the voltages of the four DC-bus capacitors makes this converter unattractive for real power applications. This is especially true if the load demands active power. The Back-to-Back (B2B) configuration of two 5L-NPC converters and the use of a Space Vector Modulation (SVM) that exploits the voltage balancing capabilities of the redundant switching vectors, extend the operating conditions range in which a proper voltage balance can be achieved. However, if practical modulation restrictions are considered (limitation of voltage steps, dead times, switching losses, etc.) the voltage balance cannot be achieved for all operation conditions. This paper introduces the main restrictions that should be considered for the 5L-NPC modulation strategy. The voltage balancing limits of the proposed SVM scheme are shown and additional considerations to improve the voltage balancing capability are proposed and evaluated

Metodo de control del balance de tensiones de los condensadores del bus de continua en un convertidor de potencia de cinco niveles

Método de control del balance de tensiones de los condensadores del bus de continua de los convertidores de potencia multinivel, basado en la conmutación por división en el tiempo de la componente gamma de la señal promediada de control de los interruptores del convertidor, y que se caracteriza por la división del periodo de muestreo de las diferencias de tensiones de los condensadores en dos subintervalos en los que la componente gamma se fija a un determinado valor constante en cada uno de los mismos, seleccionando los valores constantes de aplicación de una tabla de pares de valores predeterminados de control, según los signos de las diferencias de tensiones; y donde el criterio de elección de estos valores constantes viene determinado por el efecto que producen en las variaciones netas de las diferencias de tensiones de los condensadores a lo largo de un periodo de muestro, dando lugar a una serie de regiones en el espacio de valores de la señal de control.Españ

DC-link Voltage Balancing Strategy based on SVM and Reactive Power Exchange for a 5L-MPC Back-to-Back Converter for Medium Voltage Drives

The reduced capability of multilevel converters with more than one intermediate node to balance the dc-link capacitors voltage, as well as the lack of standard modulation methods to improve their balancing performance, makes these converter topologies unattractive for real power applications. This is especially true when the load demands active power. One of these topologies is the five-level multipoint clamped (5L-MPC) converter. The back-to-back (B2B) configuration of two 5L-MPC converters and the use of a space vector modulation (SVM) that exploits the voltage balancing capability of the redundant switching vectors extend the operation conditions range in which a proper voltage balance can be achieved. However, if practical modulation restrictions are considered (limitation of voltage steps, dead times, switching losses, etc.), the voltage balance cannot be achieved for all operation conditions. In this paper, an SVM which takes into account practical restrictions is proposed. In order to guarantee the voltage balance at any operation condition, the grid-side rectifier exchanges reactive power with the grid-side LCL filter. Thus, the voltage balance of the dc-link is guaranteed while a unity grid-side power factor is achieved. The proposed modulation scheme and the voltage balancing strategy are experimentally validated in a 6.6 kV 1.5 MW 5L-MPC B2B converter

Application of Modular Multilevel Converters (MMC) Using Phase-Shifted PWM and Selective Harmonic Elimination in Distribution Systems

Reducing the size and weight of a power electric system is a prodigious challenge to researchers as the development of the latest technologies emerge in the field of electrical engineering. A similar urge is there to develop a light-weight mobile power substation (MPS) to use in the electric power distribution systems during emergency conditions. This thesis proposes a power electronics based solution using the modular multilevel converter (MMC) topology to design the MPS system. The market-available power semiconductor devices are analyzed and suitable devices are selected to design the system. The phase-shifted pulse width modulation (PS-PWM) and selective harmonic elimination (SHE) switching algorithms are selected to modulate the MMC terminals. To validate the proposed techniques simulation files are built in MATLAB/SIMULINKTM. Simulation results are presented and analyzed to verify the theoretical claims. These simulation results prove the feasibility of designing the MPS system with the proposed techniques

A Transformerless PCB Based Medium-Voltage Multilevel Power Converter with A DC Capacitor Balancing Circuit and Algorithm

This dissertation presents a new method of constructing a transformerless, voltage-sourced, medium-voltage multilevel converter using existing discrete power semiconductor devices and printed circuit board technology. While the approach is general, it is particularly well-suited for medium-voltage converters and motor-drives in the 4.16 kV, 500 - 1000 kW range. A novel way of visualizing the power stage topology is developed which allows simplified mechanical layouts while managing the commutation paths. Using so many discrete devices typically drives cost and complexity of the gate-drive system including its control and isolation; a gate-drive circuit is presented to address this problem. As with most multilevel topologies, the dc-link voltages must be balanced during operation. This is accomplished using an auxiliary circuit made up of the same power stage and an associated control algorithm. Experimental results are presented for a 4.16 kV, 746 kW, five-level power converter prototype. This dissertation also analyzes a new capacitor voltage-balancing converter along with a novel capacitor voltage balancing control algorithm. Analysis of the inverter system provides a new description of capacitor voltage stability as a function of system operating conditions

Hybrid Multilevel Converters with Internal Cascaded/Paralleled Structures for MV Electric Aircraft Applications

Using on-board medium voltage (MV) dc distribution system has been a megatrend for next-generation electric aircraft systems due to its ability to enable a significant system mass reduction. In addition, it makes electric propulsion more feasible using MV power electronic converters. To develop high-performance high-density MV power converters, the emerging silicon carbide (SiC) devices are more attractive than their silicon (Si) counterparts, since the fast switch frequency brought by the SiC can effectively reduce the volume and weight of the filter components and thus increase the converter power density. From the converter topology perspective, with the MV dc distribution, the state-of-the-art two-level converters are no longer suitable for next-generation electric aircraft system due to the excessive dv/dt and high voltage stress across the power devices.To address these issues while still maintaining cost-effectiveness, this work demonstrates a megawatt-scale MV seven-level (7-L) Si/SiC hybrid converter prototype implemented by active-neutral-point-clamped (ANPC) converter and H-bridges which is called ANPC-H converter in this work, and a MV five-level (5-L) Si/SiC hybrid ANPC converter prototype, which are hybrid multilevel converters with internal cascaded and paralleled structures, respectively. Using multilevel circuit topology, the voltage stress across the devices and converter output voltage dv/dt are reduced. The tradeoff between the system cost and efficiency was addressed by the adoption of the Si/SiC hybrid configuration with optimized modulation strategies. Comprehensive design and evaluation of the full-scale prototypes are elaborated, including the low-inductance busbar designs, power converter architecture optimization and system integration. To control the 7-L Si/SiC hybrid ANPC-H converter prototype, a low computational burden space-vector-modulation (SVM) with common-mode voltage reduction feature is proposed to fully exploit the benefits of 7-L Si/SiC hybrid ANPC-H converter. To further reduce the converter losses and simplify control algorithm, an active hybrid modulation is proposed in this work by applying low frequency modulation in Si cells and high frequency modulation in SiC cells, thus the control framework is simplified from the 7-L SVM to a three-level SVM. To control the 5-L Si/SiC hybrid ANPC converter prototype to overall loss minimization, the low frequency modulation and high frequency modulation are also adopted for Si cells and SiC cells respectively in 5-L Si/SiC hybrid ANPC converter prototype. Compared to the SVM-based hybrid modulation in 7-L ANPC-H converter, the hybrid modulation for 5-L hybrid ANPC adopts a simpler carrier-phase-shifted pulse width modulation for its inner-paralleled high frequency SiC cells, which extensively suppresses harmonics caused by high frequency switching. With the proposed modulation strategies, extensive simulation and experimental results are provided to evaluate the performance of each power stage and the full converter assembly in both the steady-state operation and variable frequency operations of the demonstrated hybrid converters

Hybrid Multilevel Converters with Internal Cascaded/Paralleled Structures for MV Electric Aircraft Applications

Using on-board medium voltage (MV) dc distribution system has been a megatrend for next-generation electric aircraft systems due to its ability to enable a significant system mass reduction. In addition, it makes electric propulsion more feasible using MV power electronic converters. To develop high-performance high-density MV power converters, the emerging silicon carbide (SiC) devices are more attractive than their silicon (Si) counterparts, since the fast switch frequency brought by the SiC can effectively reduce the volume and weight of the filter components and thus increase the converter power density. From the converter topology perspective, with the MV dc distribution, the state-of-the-art two-level converters are no longer suitable for next-generation electric aircraft system due to the excessive dv/dt and high voltage stress across the power devices.To address these issues while still maintaining cost-effectiveness, this work demonstrates a megawatt-scale MV seven-level (7-L) Si/SiC hybrid converter prototype implemented by active-neutral-point-clamped (ANPC) converter and H-bridges which is called ANPC-H converter in this work, and a MV five-level (5-L) Si/SiC hybrid ANPC converter prototype, which are hybrid multilevel converters with internal cascaded and paralleled structures, respectively. Using multilevel circuit topology, the voltage stress across the devices and converter output voltage dv/dt are reduced. The tradeoff between the system cost and efficiency was addressed by the adoption of the Si/SiC hybrid configuration with optimized modulation strategies. Comprehensive design and evaluation of the full-scale prototypes are elaborated, including the low-inductance busbar designs, power converter architecture optimization and system integration. To control the 7-L Si/SiC hybrid ANPC-H converter prototype, a low computational burden space-vector-modulation (SVM) with common-mode voltage reduction feature is proposed to fully exploit the benefits of 7-L Si/SiC hybrid ANPC-H converter. To further reduce the converter losses and simplify control algorithm, an active hybrid modulation is proposed in this work by applying low frequency modulation in Si cells and high frequency modulation in SiC cells, thus the control framework is simplified from the 7-L SVM to a three-level SVM. To control the 5-L Si/SiC hybrid ANPC converter prototype to overall loss minimization, the low frequency modulation and high frequency modulation are also adopted for Si cells and SiC cells respectively in 5-L Si/SiC hybrid ANPC converter prototype. Compared to the SVM-based hybrid modulation in 7-L ANPC-H converter, the hybrid modulation for 5-L hybrid ANPC adopts a simpler carrier-phase-shifted pulse width modulation for its inner-paralleled high frequency SiC cells, which extensively suppresses harmonics caused by high frequency switching. With the proposed modulation strategies, extensive simulation and experimental results are provided to evaluate the performance of each power stage and the full converter assembly in both the steady-state operation and variable frequency operations of the demonstrated hybrid converters

Direct current control for grid connected multilevel inverters

Control schemes for inverters of different topologies and various numbers of voltage levels are of great interest for many standard as well as special applications. This thesis describes a novel, robust and high-dynamic direct current control scheme for multilevel voltage source inverters. lt is highly independent from load parameters and universally applicable. The new control method is examined and validated with real measurements . The aim of the thesis is to establish and prove a new concept of a direct current control algorithm for multilevel inverter topologies for grid connected systems. This application is characterized by unknown grid conditions including failure modes and other distortions, complex inverter topologies and a large variety and complexity of current control algorithms for multilevel inverters. Therefore the complexity of the system needs to be reduced. Additionally , the advantages of multilevel inverters and the dynamic performance and robustness of direct current control techniques shall be combined. Starting from a detailed literature study on inverter topologies and direct as well as indirect current control methods, the thesis includes three chapters containing relevant contributions to the achievement of the objectives. A method reducing the control-complexity of multilevel converters has been developed. The simplification method is based on a transformation that converts any three-phase voltage (or current) into a non-orthogonal coordinate system. This choice minimizes the complexity and effort to determine the location of those discrete voltage space vectors directly surrounding the required reference voltage vector. A further improvement is achieved by scaling all coordinates to integer values. This is advantageous for further calculations on microprocessors or FPGA based control systems. The main contribution of this thesis is a new direct current control method minimizing the disadvantages of existing direct methods. At the same time advantages of other control algorithms shall be applied. The new method is based on a simple mathematical equation, that is, the solution of a scalar product, to always select the one inverter output voltage vector best reducing the actual current error. This results in the designation "Scalar Hysteresis Control - SHC". An advanced seeking algorithm ensures robust current control capability even in case of unknown, unsymmetrical or changing loads, in case of rapid set-point changes or in cases of unknown phase voltages . The new method therefore shows excellent properties in terms of simplicity , robustness, dynamics and independence from the inverter level count and the hardware topology . The properties of the control method are verified by means of simulations and real measurements on two-, three- and five-level inverters over the complete voltage operating range. Finally, all contributions are collected together and assessed with regard to the objectives. From the proposed control method new opportunities for future work, further developments and extensions are evolving for continuing scientific researchEls sistemes de control d'inversors de diferents topologies i diferent varis nivells de tensió són de gran interès per moltes aplicacions estàndard i també per aplicacions especials. Aquesta tesi investiga sobre un mètode de control directe de corrent per convertidors multinivell en font de tensió que es mostra robust i presenta una elevada dinàmica en el control de corrent. El mètode és molt robust davant de canvies als paràmetres de la càrrega i aplicable a qualsevol tipus de convertidor. En aquesta tesi s'analitza el mètode i es valida mitjançant resultats experimentals. L'objectiu d'aquesta tesi és establir i demostrar un nou de mètode i algorisme de control directe de corrent aplicat especialment a inversors connectats a la xarxa. L'aplicació es caracteritza per la desconeixença dels paràmetres de la xarxa, incloent diferents modes de falla i distorsions en la seva tensió i una varietat de tipologies de convertidors multinivell. El mètode de control busca simplificar l'algorisme i que pugui ser aplicat en aquest entorn de forma robusta, de forma que es pugui estendre l'ús dels convertidors multinivell sense afegir més complexitat als algorismes de control i modulació. La tesi aborda el problema iniciant amb un anàlisi de la literatura existent en aquest tipus de mètodes de control directe i indirecte del corrent i els convertidors multinivell, per continuar amb l'anàlisi del mètode proposat i la seva demostració mitjançant resultats de simulacions i experimentals. El mètode de simplificació està basat en una transformació que transforma qualsevol sistema trifàsic a un sistema de coordenades no-ortogonal. Escollir aquest sistema de coordenades redueix la complexitat i l'esforç per determinar la ubicació d'aquells vectors espacials que directament envolten el vector de referencia. A més, totes les coordenades s'escalen a valors enters, que permet la programació de l'algorisme en sistemes de control basats en microprocessadors o FPGAs. La principal contribució d'aquesta tesi és un nou mètode de control de corrent que intenta minimitzar els desavantatges dels mètodes indirectes existents a l'actualitat, al mateix moment que s'intenta incorporar els avantatges dels mètodes indirectes. El mètode proposat es basa en una equació matemàtica simple, la solució d'un producte escalar, per trobar el vector de tensió espacial que minimitza l'error de corrent, en el que s'anomena "Scalar Hysteresis Control" o SHC. L'algorisme assegura un control robust del corrent sense la necessitat de conèixer la tensió de fase, o les càrregues, tant si són desequilibrades o canviants. També presenta una dinàmica molt elevada en cas de canvies en la referència. El nou mètode mostra unes propietats excel·lents en termes de simplicitat, robustesa, dinàmica i independència de la tipologia del convertidor i, en el cas de convertidors multinivell, del nombre de nivells. Les propietats del mètode de control són verificades mitjançant simulacions i resultats experimentals en convertidors de dos, tres i fins a cinc nivells de tensió en tot el rang d'operació, fins i tot en la zona de sobremodulació. A partir del mètode de control proposat, s'estan desenvolupant noves aplicacions i extensions, continuant també la contribució a la recerca científica.Postprint (published version