Three-phase Multilevel Converter Research Articles

Space Vector Fundamental Frequency Modulation Compared with the Selective Harmonic Elimination Method

A new modulation strategy for three-phase multilevel-converters, called Space Vector Fundamental Frequency Modulation will be presented. Derived from conventional Space Vector PWM, it was developed to overcome Selective Harmonic Elimination Method in mathematical complexity. Besides the harmonic optimization, the method offers a variable modulation index and is applicable for real-time applications. But in contrast to Selective Harmonic Elimination Method it is an approximation procedure. In this paper, both methods are compared, concerning several criteria such as voltage quality and mathematical complexity. Also, a classification with reference to EN 50160 is presented.

A New Exponential Reaching Law Approach to the Sliding Mode Control: A Multilevel Multifunction Converter Application

This paper presents a new exponential reaching law (ERL) approach for the sliding mode control. Contrary to the existing reaching law methods, the proposed ERL does not employ a sign function and yields a faster reaching time. An expression of reaching time in terms of reaching law gains is derived analytically. The paper will show that the reaching time to the sliding surface is very small provided that the gains are selected properly. The stability of the proposed reaching law will be proven. The proposed reaching law with sliding mode control is employed to control a 5 kW three-phase multi-level multi-function converter. It is shown that the system can operate as a rectifier, as a shunt active power filter (SAPF) or as a combination of both rectifier and SAPF simultaneously. Experimental results corresponding to each operation mode are presented to confirm the efficacy of the proposed reaching law under fast-changing nonlinear load currents.

Modeling and simulation of a stand-alone wind turbine supplying an inductive load through a long cable

This paper considers the principles of modeling, control, and simulation of variable speed wind turbines (VSWTs), based on a stand-alone doubly fed induction generator (DFIG). The DFIG is used to feed a three-phase highly inductive load through a long cable via a three-phase multi-level frequency converter (MLFC). The proposed MLFC increases the number of voltage levels with less power electronic components as compared to the conventional back-to-back AC/DC/AC converter. The length of the cable (transmission line TL) influences a mismatch between the load and TL impedances; therefore, a reflected signal will occur. If the "incident signal" is a "continuous AC waveform", these waveforms will mix with another oncoming "incident waveform", creating stationery waveforms called "standing waves". The simulation process shows that the value of the transient’s over-voltage that appears either on the windings of the DFIG or across the load reaches twice the DC-link voltage periodically, which may cause premature failure of the DFIG windings and cable insulations. Usually, this is produced by generating high harmonics within the loads. Therefore, the main benefit of using the MLFC is the significant reduction of the total harmonic distortion and enhancement of the load voltage waveforms. This was verified, primarily by using the "MATLAB/Simulink, Simplorer 7" simulation software.

Topology, Modeling and Control Scheme for a new Seven-Level Inverter With Reduced DC-Link Voltage

This paper presents a novel single-source three-phase multilevel converter with voltage boosting capability for medium-voltage photovoltaic applications. This converter consists of different switched-capacitor networks, formed by 1 capacitor and 3 active switches, as a basic switching cell to generate variable dc-link voltage that boosts the input dc-source voltage, producing 2n+1 (n2) voltage levels at the output. It can reduce the dc-link voltage requirements by 50% in comparison to the traditional neutral point clamped (NPC), flying capacitors, active NPC (ANPC), hybrid and hybrid clamped ANPC and cascaded H-bridge topologies, and 75% to advanced ANPC topologies. It can also reduce the number of required switches and capacitors as well as their voltage stresses compared to these state-of-the-art topologies reported in the literature so far. A finite control set model predictive control algorithm is derived to control the converter, which has the capability to control the active and reactive power without distorting the generated grid current quality. The capacitor voltage balancing is inherent in the proposed topology, and thus, there is no need for any additional voltage balancing circuit, which reduces the control complexity. As an example, a 7-level version of the proposed topology is compared to other existing 7-level inverter topologies.

Research on Realizing Space Vector Equivalent Modulation Output by Dual Carrier Modulation of Current Source Inverter

In order to achieve equivalent modulation of the multilevel current converter, based on the topology of three-phase multilevel current-source converters, this article presents a method to realize the output characteristic of space vector pulsewidth modulation (SVPWM) with carrier-based PWM, which can be applied in the topologies from five-level to n-level space vector modulation strategy. Different from the strategies of traditional unity theory, the proposed method analyzes the relationship directly according to the switching states of SVPWM in three-phase multilevel current-source converters. By constructing the equivalent relationship between the output level of each phase and switching states, the carriers can control the working status of the switches directly. In addition, this method is generalized to the topologies and modulation strategies under n-level, which fully reflects the correctness and universality of the proposed method. Finally, the results of simulation and experiment verify the correction of the method.

Analysis of Three Phase 3-Level NPC Voltage Source Converter for AC-DC Conversion

The use of power electronic converters influences the generation of harmonics and reactive power flow in power system. Therefore, three-phase multilevel improved power quality AC-DC converters are gaining lot of popularity in power conversion applications. This work deals with critical problem of multilevel structure i.e neutral point potential (NPP) variation. In this paper, a simplified current controlled scheme is presented to ensure unity power factor operation. Neutral point potential (NPP) of three-phase, 3-level NPC AC-DC converter is controlled by modifying control signal in the controller using NPP regulator. An auxiliary circuit is being presented in this paper as an alternative option for controlling the neutral point potential of the converter. Comparison has been carried out between these control techniques in terms of power quality. A complete mathematical model is presented for better understanding of both techniques used for NPP control. The presented control techniques has been verified through simulation investigations and validated

Coupled Three-Phase Converter Concept and an Example: A Coupled Ten-Switch Three-Phase Three-Level Inverter

The bridge arms of traditional three-phase multilevel converters are independent of each other; thus, more active switches and passive diodes are required. In order to reduce the number of devices, a coupled three-phase converter architecture, which is suitable for both dc-ac and ac-dc converters, is presented. Based on this architecture, a ten-switch three-phase three-level (10S-3P-3L) inverter is proposed in this article. The proposed topology employs only ten active switches and no additional diodes, which helps to cut size, weight, and costs of system. The proposed space vector modulation enables 10S-3P-3L inverter to operate full modulation indexes. A comparison between the proposed inverter with some existing topologies in terms of device voltage and current stresses, conduction loss, switching loss, and other parameters is made to highlight its excellent performance. Moreover, the experimental results from a 3-kW experimental prototype reveal that the proposed inverter offers superior power quality.

Review of Five-Level Front-End Converters for Renewable-Energy Applications

With the objective of minimizing environment and energy issues, distributed renewable energy sources have reached remarkable advancements along the last decades, with special emphasis on wind and solar photovoltaic installations, which are deemed as the future of power generation in modern power systems. The integration of renewable energy sources into the power system requires the use of advanced power electronics converters, representing a challenge within the paradigm of smart grids. Therefore, this paper presents a review of front-end converters for renewable energy source applications. Since the objective of this paper is not to cover all types of converters, the focus is on single-phase and three-phase multilevel converters (limited to five voltage levels) with a voltage-source structure and with current or voltage feedback control. The review is performed considering the following classifications: (a) Number of active components; (b) Fault tolerance; (c) Control complexity; (d) Requirements of specific capacitor or inductors; (e) Number of independent or split dc-link voltages. Throughout the paper, several topologies are presented and comparisons are made between them, highlighting the pros and cons of each one of them as a candidate for the interface of renewable energy sources with the power grid.

Three‐level PN cell for multilevel converters

Multilevel converters rely on plurality of DC capacitors switched in specific patterns to obtain the necessary output voltage level. Modular multilevel converter, flying capacitor converter (FLC) and neutral point clamped (NPC) converters require complex control to balance the capacitors’ voltages. Cascaded H-bridge (CHB) converters require plurality of isolated DC sources to energise the capacitors. This study presents a three-level PN cell that modulates the common-mode voltage between the output and input port while simultaneously mirroring the input DC voltage on the output port. Cell's capacitor voltage is naturally balanced and a three-phase multilevel converter requires only one DC power source. The output voltage of a multilevel converter can be higher than the input DC voltage. As a consequence, a single low-voltage DC power source can directly interface a three-phase MV grid.

A fast and flexible nearest-level-equivalent space vector modulation algorithm for three-phase multilevel converters

This paper presents a nearest-level-equivalent space vector modulation (SVM) algorithm for three-phase multilevel converters. The proposed algorithm provides a simple approach to obtain three modulation vectors online based on basic vectors. In addition, the principle of nearest-level modulation (NLM) is applied in the proposed SVM to make its superior performance to the generalized SVM algorithms, which present: (1) Merely one modulation vertex need to be detected and three-phase duty cycles are obtained. Therefore, the unbalanced voltages of inner cell of neutral-point-clamped cascaded multilevel converter (NPC-CMC) can be addressed; (2) Redundant and optimized switching sequences are automatically generated to minimize the number of switching transition; (3) Different modulation modes can be flexibly chosen to optimize the harmonic benefit. The proposed SVM scheme is independent of level numbers and finally verified by simulation and experimental results.

Design and Test of a New Three-Phase Multilevel Inverter for PV System Applications

This paper presents a test of a new three-phase multilevel inverter for PV system applications with reduced number of used DC sources and power switches. The topology of the inverter is designed using an electric assemblage of a two-level dc-dc boost converter (TLBC) with a simplified three-phase multilevel DC-AC converter (THPMC). The TLBC generates two balanced output DC voltages, while the THPMC converts these two DC voltages and generates three-phase AC voltages with five levels per line. Two modulation control techniques are used and tested with the proposed PV system on PSIM and on ISIS Proteus software. The achieved results prove the simplicity and efficiency of the proposed three-phase inverter.

Optimum structure of a generalized three-phase reduced switch multilevel inverter

This study presents a new generalized three-phase multilevel converter based on the combinations of half-bridge modules along with a three-phase T-Type multilevel inverter. The proposed topology reduces the number of switches and associated gate-driver circuits that attains much higher number of output voltage levels. The optimized structure of the proposed three-phase inverter topology has been developed to obtain the maximum number of output voltage levels of the inverter with a minimum number of power electronic switches and the DC voltage sources. The operation, control and performance analysis of the proposed generalized multilevel inverter have been considered here. A nearest level control (NLC) technique is adopted to generate the gating signal for the proposed three-phase hybrid inverter. A laboratory prototype of a specimen three-phase low power fifteen-level inverter have been designed using twenty four switches and nine voltage sources. The conduction losses of the proposed fifteen level inverter is around 14.71watt/phase compared to 23.26watt/phase in an asymmetrical cascaded inverter. The exhaustive simulations of the proposed three-phase inverter are performed using MATLAB/SIMULINK and the results are verified experimentally and presented for the different modulation index.

Performance comparison of a three-phase multilevel matrix converter with three flying capacitors per output phase with a three-phase conventional matrix converter

Multilevel technology is a good solution for medium- and high-voltage power conversion. There are three kinds of multilevel converters, namely diode clamped, capacitor clamped and cascade. Among them, capacitor-clamped multilevel converter is the most suitable topology for direct AC to AC conversion. This paper describes the modelling of a three-phase multilevel matrix converter with a capacitor-clamped topology having three flying capacitors per output phase. Venturini algorithm is used for generating gate pulses for this three-phase multilevel matrix converter, the application of which is different from the conventional three-phase single-cell matrix converter. Simulation results using the software PSCAD are presented. The simulation results for this three-phase multilevel matrix converter is compared with that for a three-phase conventional matrix converter and the total harmonic distortion in the two cases for the line to neutral output voltage, line to line output voltage and input current are tabulated. Simulation results indicate that the three-phase multilevel matrix converter has an improved performance compared to that of the three-phase conventional matrix converter.

A Novel Approach to Generate Effective Carrier-Based Pulsewidth Modulation Strategies for Diode-Clamped Multilevel DC–AC Converters

Several pulsewidth modulation (PWM) strategies have been proposed for three-phase multilevel converters. Typically, these PWM methods can be either space-vector or carrier-based implemented, where the carrier-based implementation usually presents a lower computation complexity. In this paper, a novel approach for the carrier-based implementation of different existing PWM methods is proposed for three- (3L) and four-level (4L) dc–ac converters, which can be easily extended to any number of levels. The main features of the proposed approach are its reduced computation complexity and computation time. On the other hand, the proposed approach suggests the derivation of novel PWM strategies which can potentially present improved performance features. As an example, two new discontinuous PWM (DPWM) strategies are proposed for the 4L converter. One of these DPWM strategies presents lower weighted total harmonic distortion than other previously proposed strategies. Simulation and experimental results verify the effectiveness of the proposed approach and novel DPWM strategies.

A New Fault-Tolerant Strategy Based on a Modified Selective Harmonic Technique for Three-Phase Multilevel Converters With a Single Faulty Cell

This paper proposes a new fault-tolerant strategy to revamp performance of three-phase multilevel converters with a single faulty switch. The proposed fault-tolerant strategy is based on the application of a modified selective harmonic elimination (SHE) technique in conjunction with the fundamental phase-shift compensation method. The proposed fault-tolerant strategy makes the most use of the converter capacity in the faulty condition, generates balanced line voltages, and eliminates a wide range of lower order harmonics to limit the total harmonic distribution of the converter voltages in the faulty condition. In this paper, first, a brief background about cascaded H-bridge multilevel converters and SHE technique is provided. Then, the proposed fault-tolerant strategy and the modified SHE technique are presented. Finally, several experimental results are provided to validate the operation of the proposed strategy.

マルチレベル変換器のための新しい二相PWM 法の提案とその効果－ウェーブレット変換による交流側線間電圧の解析－

– Three-phase multi-level voltage-fed converters operated by a two-phase PWM scheme offers relatively high ac voltage, low switching times, and low switching losses in comparison to that operated by a three-phase PWM scheme.However, the harmonics involved in the ac-side voltages and currents produced in the two-phase PWM scheme are distributed over a wide frequency range, which can be a significant drawback depending on its applications. To overcome this problem,the authors propose novel two-phase PWM scheme in which the harmonics appear in a frequency range as narrow as that in a the conventional three-phase PWM scheme. In this paper, the authors present the validity of the proposed PWM scheme from simulation and wavelet transform results.

A New Active Fault-Tolerant SVPWM Strategy for Single-Phase Faults in Three-Phase Multilevel Converters

Multilevel converters have been well studied for high-voltage high-power renewable energy applications. Owing to the multibranch structure, these converters can be operated under fault conditions in case of a loss of a branch or voltage cell. In this regard, this paper proposes a new active fault-tolerant space vector pulsewidth modulation strategy for single-phase faults in three-phase multilevel converters. The proposed modulation strategy treats the multilevel converter as a two-level converter by introducing an offset vector. A generalized approach is proposed to search for the offset vector and to adjust the modulation of the converter online under different fault operation conditions. Several short- and open-circuit faults have been demonstrated on a seven-level hybrid input-switched converter to prove the effectiveness of the proposed fault-tolerant scheme.

A Flexible Power Electronics Configuration for Coupling Renewable Energy Sources

A combination of series, parallel and multilevel power electronics has been investigated as a potential interface for two different types of renewable energy sources and in order to reach higher power levels. Renewable energy sources are typically dispersed in a territory, and sources, like wind and solar, allow small to medium-scale generation of electricity. The configuration investigated in this article aims at adapting the coupling solution to the specific generation characteristics of the renewable energy source to make it fit the electrical network. The configuration consists of a combination of three-phase multilevel converters and single-phase inverters, which are designed to provide flexibility, high power quality and high efficiency. A detailed analysis and simulation is performed to identify the properties in conjunction with the electrical grid requirements and the potential challenges encountered during operation. An optimized operation example of wind generation combined with solar PV generation is presented to exemplify the flexibility and benefits of the proposed configuration.

A Modified Space Vector Modulation Method for Fault-Tolerant Operation of Multilevel Converters

This paper proposes a new fault-tolerant strategy for the purpose of revamping the performance of three-phase multilevel (ML) converters with faulty switches. The proposed fault-tolerant strategy is based on an adaptation of the modified space vector modulation (SVM) technique to the fundamental phase-shift compensation method. The proposed fault-tolerant strategy generates balanced line-to-line voltages with increased voltage levels in the faulty condition. In this paper, first a brief background about cascaded H-bridge (CHB) ML converters and SVM technique is provided. Then the proposed fault-tolerant strategy and the modified SVM technique are presented. Finally, several experimental results are provided to validate operation of the proposed strategy.

Closed-Form Expressions for Minimizing Total Harmonic Distortion in Three-Phase Multilevel Converters

The total harmonic distortion (THD) of a waveform is a standard way to quantify its deviation from a sinusoid. In three-phase systems, we are interested in minimizing 3THD: the component of THD produced by odd nontriplen harmonics. However, its definition involves an infinite sum, making it difficult to evaluate and analyze. This paper solves the problem of finding equivalent closed-form expressions for the 3THD of a staircase waveform. In particular, two expressions are rigorously derived, which reveal 3THD to be a piecewise differentiable function. One expression is shorter but describes the pieces implicitly. The other is longer but describes the pieces explicitly. We minimize 3THD using the closed-form expression and provide a comparison with previous techniques. Finally, we provide experimental results that show close agreement with the theoretical results.