Multilevel Voltage-Source-Converter Topologies for Industrial Medium-Voltage Drives

This paper presents a technology review of voltage-source-converter topologies for industrial medium-voltage drives. In this highly active area, different converter topologies and circuits have found their application in the market. This paper covers the high-power voltage-source inverter and the most used multilevel-inverter topologies, including the neutral-point-clamped, cascaded H-bridge, and flying-capacitor converters. This paper presents the operating principle of each topology and a review of the most relevant modulation methods, focused mainly on those used by industry. In addition, the latest advances and future trends of the technology are discussed. It is concluded that the topology and modulation-method selection are closely related to each particular application, leaving a space on the market for all the different solutions, depending on their unique features and limitations like power or voltage level, dynamic performance, reliability, costs, and other technical specifications.     

Phase-controlled multilevel converters based on dual structureassociations

Multilevel converters, like neutral-point-clamped inverters or multilevel choppers, are particularly attractive in high-power applications. Nevertheless, in these structures, all switches are confronted to commutation stresses caused by their turn-on and turn-off control. Furthermore, the methods to balance the capacitor voltages or to control the neutral point voltage are complex enough. In this paper, the authors propose new multilevel converters based on series connection of zero-current-source (ZCS) inverter cells and parallel connection of zero-voltage-source (ZVS) inverters. These dual structure associations give soft-switching operation for all switches and allow the use of semiconductors, normally destined for medium-power applications, in high-power converters (up to 1 MW). The authors consider the structure design for several topologies to achieve DC-DC or DC-AC converters. The simulation results validate the simplicity of phase control techniques and give out the principal features of different topologies     

Advanced Control and Analysis of Cascaded Multilevel Converters Based on P-Q Compensation

This paper introduces new controls for the cascaded multilevel power converter. This converter is also sometimes referred to as a ldquohybrid converterrdquo since it splits high-voltage/low-frequency and low-voltage/pulsewidth-modulation (PWM)-frequency power production between ldquobulkrdquo and ldquoconditioningrdquo converters respectively. Cascaded multilevel converters achieve higher power quality with a given switch count when compared to traditional multilevel converters. This is a particularly favorable option for high power and high performance applications such as naval ship propulsion. This paper first presents a new control method for the topology using three-level bulk and conditioning inverters connected in series through a three-phase load. This control avoids PWM frequency switching in the bulk inverter. The conditioning inverter uses a capacitor source and its control is based on compensating the real and reactive (P-Q) power difference between the bulk inverter and the load. The new control explicitly commands power into the conditioning inverter so that its capacitor voltage remains constant. A unique space vector analysis of hybrid converter modulation is introduced to quantitatively determine operating limitations. The conclusion is then generalized for all types of controls of the hybrid multilevel converters (involving three-level converter cells). The proposed control methods and analytical conclusions are verified by simulation and laboratory measurements.     

A novel orthogonal-vectors-based topology of multilevel inverters

This paper presents a novel proposal of developing converter output voltage waveforms and novel converter topologies. The main idea is based on the assumption that the total converter output space vector is composed of two orthogonal space vectors. Two basic proposals are discussed. The first one is related to a converter built of two standard inverters: a main inverter (MI) and an auxiliary one. The converter output voltage space vector is composed of two orthogonal vectors generated by the respective inverters. The total power of the auxiliary inverter does not exceed 20% of the MI power. Thanks to the presented control method, the harmonic content of the output voltage is significantly reduced. The second proposal is related to a novel converter topology denoted as OVT-IHC. The converter is built of one two-level inverter and three isolated H-bridge circuit units. The structure and its performance are also discussed in the paper. The converter in question is able to generate 133 different output space vectors and permits achievement of a stepped adjustment of the RMS output voltage. Both topologies presented in the paper indicate some characteristics and advantages of multilevel inverters. The converters acting on the basis of the orthogonal vectors idea possess promising properties and are suitable to applications in medium-power converters. The paper presents main features and contribution to the theory.     

Design of a Flexible Control Platform for Soft-Switching Multilevel Inverters

In modern high-power medium voltage drives, multilevel converters are increasingly used. Employing slight topological modifications, soft-switching technology can be applied to multilevel converters to reduce the switching losses. As a result, the switching frequency can be increased, thereby reducing the output filter size. However, common converter controls have to be modified. In this paper, a flexible control platform is presented that allows rapid prototyping of soft-switching topologies. An analysis of different auxiliary resonant commutated pole (ARCP) topologies shows that all switching commands can be synthesized with synchronized signals of two-level ARCP converters. Therefore, a flexible state-machine for two-level converters was developed first, which can also be used to build controls for multilevel topologies. It supports drivers with built-in intelligence as well as the control of additional switches that are required in some ARCP neutral-point-clamped (NPC) topologies. The switching commands for the state machines can be generated by standard multilevel modulation methods. Illegal switching states are filtered and multiple simultaneous commutations per phase are prevented for ARCP NPC converters. To verify the functionality, the control scheme was realized in a field programmable gate array and a completely modular test converter was developed. This test converter can be used to quickly implement all common multilevel topologies and test different modulation strategies. Experimental results are presented in this paper.     

Multilevel Inverter Topologies for Stand-Alone PV Systems

This paper shows that versatile stand-alone photovoltaic (PV) systems still demand on at least one battery inverter with improved characteristics of robustness and efficiency, which can be achieved using multilevel topologies. A compilation of the most common topologies of multilevel converters is presented, and it shows which ones are best suitable to implement inverters for stand-alone applications in the range of a few kilowatts. As an example, a prototype of 3 kVA was implemented, and peak efficiency of 96.0% was achieved.     

Multilevel Multiphase Space Vector PWM Algorithm With Switching State Redundancy

Multilevel multiphase technology combines the benefits of multilevel converters and multiphase machines. Nevertheless, new modulation techniques must be developed to take advantage of multilevel multiphase converters. In this paper, a new space vector pulsewidth modulation algorithm for multilevel multiphase voltage source converters with switching state redundancy is introduced. As in three-phase converters, the switching state redundancy permits to achieve different goals like extending the modulation index and reducing the number of switchings. This new algorithm can be applied to the most usual multilevel topologies; it has low computational complexity, and it is suitable for hardware implementations. Finally, the algorithm was implemented in a field-programmable gate array, and it was tested by using a five-level five-phase inverter feeding a motor.      

多电平电压型变频器(上)

在大功率领域,中点钳位三电平和H桥级联中压变频器应用最广泛,本文在详细讨论它们优缺点的基础上给出其主要适用场合,并分别介绍了它们各自的改进方案。文中还介绍了三种较少应用的多电平变换器:有源中点钳位三电平逆变器;电容钳位四电平变频器和在相支路中接入耦合电抗的多电平变换器。     

多电平电压型变频器(下)

在大功率领域,中点钳位三电平和H桥级联中压变频器应用最广泛,本文在详细讨论它们优缺点的基础上给出其主要适用场合,并分别介绍了它们各自的改进方案。文中还介绍了三种较少应用的多电平变换器:有源中点钳位三电平逆变器;电容钳位四电平变频器和在相支路中接入耦合电抗的多电平变换器。     

大功率变频器的拓朴结构及其谐波抑制技术

本文对目前大功率变频器的主流拓扑结构及网侧输入电流和逆变器输出电流的谐波抑制技术进行了分析和介绍。     

A Novel Switching Sequence Design for Five-Level NPC/H-Bridge Inverters With Improved Output Voltage Spectrum and Minimized Device Switching Frequency

This paper presents a novel switching sequence design for the space-vector modulation of high-power multilevel converters. The switching sequences are optimized for the improvement of harmonic spectrum and the minimization of device switching frequency. Compared to other commonly used switching sequences, the output spectrum of the proposed design shows higher inverter equivalent switching frequency. Meanwhile, the device switching frequency is reduced by using a flexible switching pattern. The proposed switching sequence has been simulated and experimentally tested on a 5-level neutral point clamped H-bridge based inverter. The results from both simulations and experiments consistently verify the above-mentioned features.     

高压大功率变频器的谐波分析

当电动机容量较大时,大功率变频器的输入谐波对电网的影响以及输出谐波对电动机的影响成为了交流变频系统中突出的问题。为了减小大功率变频器的谐波,普遍采用多脉动整流、变压器耦合输出、多电平和单元级联技术,形成了以多脉动整流拓扑或多电平拓扑为输入级、以变压器耦合输出或多电平输出拓扑为输出级的大功率变频器主电路,以及多重化结构的大功率变频器主电路。本文对目前几种有代表性的高压变频器主电路拓扑及输入输出谐波进行了分析,并与IEEE-519标准进行比较,研究了变频器的谐波特性。     

Simple Unified Approach to Develop a Time-Domain Modulation Strategy for Single-Phase Multilevel Converters

Single-phase power converters are widely used in power applications as photovoltaics and fuel-cell power conditioners. In addition, multilevel converters are a well-known solution in order to achieve high-quality output waveforms in power systems. In this paper, a time-domain duty-cycle computation technique for single-phase multilevel converters named 1DM is presented. The proposed technique is based on geometrical calculations with outstanding simplicity and generality. The proposed modulation technique can be easily applied to any multilevel converter topology carrying out the necessary calculations. The most common multilevel converter topologies have been studied in this paper as examples to introduce the proposed modulation strategy. Any other multilevel converter topology could be studied, and the corresponding 1DM could be easily developed. In addition, the well-known optimized voltage balance strategy for voltage capacitor control using the redundant switching states of the system is applied working with the proposed 1DM method, showing that both techniques are compatible. Experimental and simulation results for several single-phase multilevel converters are shown to validate the proposed modulation technique.      

Cascaded H-bridge multilevel inverters with discrete variation of DC sources

This paper presents a new voltage regulation structure for multilevel inverters using discrete variation of DC sources. In the proposed method, discrete variation is achieved based on multi-tap transformers for applications where the DC sources are isolated and supplied by three-phase transformers, e.g., medium voltage (MV) motor drive applications. Moreover, by utilising the new solution, the number of required DC regulators is reduced significantly, for applications where the optimum modulation index with variable DC sources is used which suffer from large number of required converters. In addition, this paper proposes a new algorithm for optimising the switching angles, turn ratios of the multi-tap transformers and DC sources to maintain the voltage distortions at the standards levels for a wide range of output voltage regulation. To verify the feasibility of the new solution, a laboratory prototype is implemented based on a 2-cell cascaded H-bridge (CHB) multilevel inverter with unequal DC sources (with 9-level operation) while the proposed structure can be applied for other CHB multilevel inverters and switching strategies.     

A Space Vector Modulation Scheme to Reduce Common Mode Voltage for Cascaded Multilevel Inverters

Multilevel inverters can reduce the common mode voltage generated. Schemes have been reported for multilevel inverters that reduce the common mode voltage. However, most of the schemes result in reduced modulation depth, high switching losses, and high harmonic distortion. This paper proposes a space-vector modulation scheme to reduce common mode voltage for cascaded multilevel inverters. The proposed scheme can increase the voltage range of operation by about 17% and can produce lower total harmonic distortion than the previously proposed schemes. The scheme is explained for five-level inverter. The scheme can be easily extended to a n-level inverter. Both experimental and simulation results are provided.     

高压大功率变频器的谐波分析

当电动机容量较大时，人功率变频器的输入谐波对电网的影响以及输出谐波对电动机的影响成为了交流变频系统中突出的问题。为了减小大功率变频器的谐波，普遍采用多脉动整流、变压器耦合输出、多电平和单元级联技术，形成了以多脉动整流拓扑或多电平拓扑为输入级、以变压器耦合输出或多电平输出拓扑为输出级的大功率变颁器主电路，以及多重化结构大功率变频器主电路。本文对目前几种有代表性的高压变频器主电路拓扑及输入输出谐波进行了分忻，评勺IEEE-519标准进行比较，研究了变频器的谐波特性。     

A simple indirect field-oriented control scheme for multiconverter-fed induction motor

In multiconverter topology, several voltage-source pulsewidth modulation (PWM) inverters using common sinusoidal modulating signals and phase-shifted triangular carrier are operated in tandem with their outputs coupled through current sharing inductors, with an objective of increasing the overall rating to megawatt range, reducing the harmonic current and torque ripples, increasing the reliability with phase-redundant operation under various fault conditions. The individual PWM voltage-source inverter (VSI) operates with the devices switching at their optimum switching frequency and behaves as a linear power amplifier (LPA). The overall multiconverter exhibits an improved quality of alternating currents (ac) due to the multiplied carrier effects in PWM. This paper, therefore, presents a simple indirect field-oriented control scheme for the multiconverter-fed induction machine. Necessary experimental and simulation results are presented to show the effectiveness of the proposed indirect field-oriented control scheme. In the study, on-line analysis has been performed using C/sup ++/, while MATLAB/Simulink has been used to perform the off-line analysis. The paper also discusses the design philosophy of the two well-known topologies of the converters used for high-voltage and high-current applications to facilitate the design and development of near-future megavolt ampere rating power converters/conditioners.     

High-Performance Torque and Flux Control for Multilevel Inverter Fed Induction Motors

This paper presents a high-performance torque and flux control strategy for high-power induction motor drives. The control method uses the torque error to control the load angle, obtaining the appropriate flux vector trajectory from which the voltage vector is directly derived based on direct torque control principles. The voltage vector is then generated by an asymmetric cascaded multilevel inverter without need of modulation and filter. Due to the high output quality of the inverter, the torque response presents nearly no ripple. In addition, switching losses are greatly reduced since 80% of the power is delivered by the high-power cell of the asymmetric inverter, which commutates at fundamental frequency. Simulation and experimental results for 81-level inverter are presented.     

Input characteristics of variable modulation index controlled current source inverters

Standard PWM current source inverters (CSIs) usually operate at fixed modulation index. The proposed modified current source inverter (MCSI) can operate with most pulse width modulation (PWM) techniques and with a variable modulation index, since the DC link inductor current freewheels on itself and not through the CSI. The use of variable modulation index control results in faster response times with no penalty on input power factor as compared to other variable modulation index schemes. This paper confirms this by investigating the input characteristics of the MCSI as seen from the AC mains. The quality of the input AC line currents is examined, and a design procedure for the input filters is given. Power factor and efficiency are discussed. Results are compared to those of other current source inverter topologies. Experimental results obtained from a 5 kVA converter confirm the theoretical considerations.<>     

Fundamental Frequency Switching Strategies of a Seven-Level Hybrid Cascaded H-Bridge Multilevel Inverter

This paper presents a cascaded H-bridge multilevel inverter that can be implemented using only a single dc power source and capacitors. Standard cascaded multilevel inverters require $n$ dc sources for 2$n + hbox{1}$ levels. Without requiring transformers, the scheme proposed here allows the use of a single dc power source (e.g., a battery or a fuel cell stack) with the remaining $n-1$ dc sources being capacitors, which is referred to as hybrid cascaded H-bridge multilevel inverter (HCMLI) in this paper. It is shown that the inverter can simultaneously maintain the dc voltage level of the capacitors and choose a fundamental frequency switching pattern to produce a nearly sinusoidal output. HCMLI using only a single dc source for each phase is promising for high-power motor drive applications as it significantly decreases the number of required dc power supplies, provides high-quality output power due to its high number of output levels, and results in high conversion efficiency and low thermal stress as it uses a fundamental frequency switching scheme. This paper mainly discusses control of seven-level HCMLI with fundamental frequency switching control and how its modulation index range can be extended using triplen harmonic compensation.