Renewable energy sources are one key enabler to decrease greenhouse gas emissions and to cope with the anthropogenic climate change. Their intermittent behavior and limited storage capabilities present a new challenge to power system operators to maintain power quality and reliability. Additional technical complexity arises from the large number of small distributed generation units and their allocation within the power system. Market liberalization and changing regulatory framework lead to additional organizational complexity. As a result, the design and operation of the future electric energy system have to be redefined. Sophisticated information and communication architectures, automation concepts, and control approaches are necessary in order to manage the higher complexity of so-called smart grids. This paper provides an overview of the state of the art and recent developments enabling higher intelligence in future smart grids. The integration of renewable sources and storage systems into the power grids is analyzed. Energy management and demand response methods and important automation paradigms and domain standards are also reviewed.

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A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems

Selective harmonic elimination pulse width modulation (SHE-PWM) offers tight control of the harmonic spectrum of a given voltage and/or current waveform generated by a power electronics converter. Owing to its formulation and focus on elimination of low-order harmonics, it is highly beneficial for high-power converters operating with low switching frequencies. Over the last decade, the application of SHE-PWM has been extended to include multilevel converters. This paper provides a comprehensive review of the SHE-PWM modulation technique, aimed at its application to multilevel converters. This review focuses on various aspects of multilevel SHE-PWM, including different problem formulations, solving algorithms, and implementation in various multilevel converter topologies. An overview of current and future applications of multilevel SHE-PWM is also provided.

A Review of Multilevel Selective Harmonic Elimination PWM: Formulations, Solving Algorithms, Implementation and Applications

The cost reduction of power-electronic devices, the increase in their reliability, efficiency, and power capability, and lower development times, together with more demanding application requirements, has driven the development of several new inverter topologies recently introduced in the industry, particularly medium-voltage converters. New more complex inverter topologies and new application fields come along with additional control challenges, such as voltage imbalances, power-quality issues, higher efficiency needs, and fault-tolerant operation, which necessarily requires the parallel development of modulation schemes. Therefore, recently, there have been significant advances in the field of modulation of dc/ac converters, which conceptually has been dominated during the last several decades almost exclusively by classic pulse-width modulation (PWM) methods. This paper aims to concentrate and discuss the latest developments on this exciting technology, to provide insight on where the state-of-the-art stands today, and analyze the trends and challenges driving its future.

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The Essential Role and the Continuous Evolution of Modulation Techniques for Voltage-Source Inverters in the Past, Present, and Future Power Electronics

In this paper, a new structure for cascade multilevel converters is presented. The proposed structure is based on a cascaded connection of submultilevel converters. The proposed cascade structure can generate a large number of levels with reduced numbers of insulated-gate bipolar transistors, gate drivers, antiparallel diodes, dc voltage sources, and blocked voltage by switches. For the proposed cascade converter, a new algorithm to determine dc source values is presented. In addition, the optimal structures are presented for different goals. The suggested structure is compared with conventional cascade and other topologies. The performance and operation of the suggested submultilevel and cascade structures is verified by experimental and simulation results. Validation of the analytical conclusions is done using MATLAB/Simulink software.

Reduction of Power Electronic Elements in Multilevel Converters Using a New Cascade Structure

In this paper, a new cascade switch-ladder multilevel inverter topology is presented which can generate a large number of output voltage levels. First, a fundamental switch-ladder multilevel inverter structure is described. Then, the structure of recommended cascade topology based on series connection of fundamental switch-ladder topologies is presented. To generate maximum number of levels with minimum number of switching elements, dc sources, and voltage on switches, the proposed cascade topology is optimized. Comparison results prove that the presented cascade topology requires fewer numbers of components. Also, the value of voltage rating on switches is less than other structures. Experimental results for two topologies are analyzed to verify the performance of the proposed topology.

Optimal Design of New Cascaded Switch-Ladder Multilevel Inverter Structure

This paper proposes an analytical procedure for computation of all pairs of valid switching angles used in pattern generation in five-level H-bridge cascaded inverters. The proposed procedure eliminates harmonic components from inverter output voltage and, for each harmonic, returns the exact boundaries of all valid modulation index intervals. Due to its simple mathematical formulation, it can be easily implemented in real time using a digital signal processor or a field-programmable gate array. In this paper, after a detailed description of the method, simulation and experimental results demonstrate the high quality of achievable results.

Analytical Method for Pattern Generation in Five-Level Cascaded H-Bridge Inverter Using Selective Harmonic Elimination

In distributed generation, high power capability and high efficiency are very important requirements. For this reason, this paper proposes a new analytical procedure for selective harmonic elimination in five-level inverters operating at fundamental frequency. For each modulation index, the proposed method calculates all possible switching angles eliminating a specified odd harmonic from the output voltage. The method has limited computational complexity, simple and cost-effective real-time implementation, and full capability of integration with closed loop control. Simulation and experimental results highlight full elimination of selected harmonic.

A Selective Harmonic Elimination Method for Five-Level Converters for Distributed Generation

This paper proposes a new pulse amplitude width modulation (PAWM) procedure for cascaded H-bridge multilevel inverters fed by dc voltage sources with unequal amplitudes. With proposed procedure, the generated output voltage is obtained modulating a sinusoidal reference signal at the desired fundamental frequency with equally spaced switching angles. It has been analytically demonstrated that, under these assumptions, all harmonics except those of order $n=2kl\pm 1$ , $k=1,2,\ldots$ are deleted from the output voltage waveform. After a detailed description of the method and a comparative analysis with others existing in the literature, its harmonics elimination capability has been experimentally verified with 5-, 7-, 9-, and 11-level cascaded H-bridge inverters, moreover, it has been mathematically demonstrated that it reduces total harmonic distortion below 5% with a 17-level inverter and it is capable of eliminating the first 49 harmonics considered by standards with a 27-level inverter.

A New Pulse Active Width Modulation for Multilevel Converters

One of the big concerns in power conversion systems is the amount of harmonic content. This paper considers a seven level cascaded inverter and proposes a simple graphical approach, useful to obtain three switching angles which, if applied to the inverter, eliminates two harmonics. The method has been further improved for mitigating some additional harmonics. The effect of modulation index on variation of angles is also investigated and total harmonic distortion (THD) is calculated for selected modulation indexes. The proposed modulation operates at fundamental frequency, which means high inverter efficiency due to reduced power losses. Simulation and experimental results are obtained to show the effectiveness of the proposed procedure.

Selective harmonic elimination in a seven level cascaded multilevel inverter based on graphical analysis

A new procedure to mitigate some harmonics is presented for a 5-level cascaded inverter, varying the modulation index. The two switching angles α
 1 
and α
 2 
are computed by using a procedure based on graphycal analysis by using Chebyshev polynomials and the Waring's formulae. The proposed mitigation technique is applied to reduce a pair of harmonics among third, fifth and seventh in inverter output voltage according to standard code requirements. Experimental results are also extracted by the prototype built at the laboratory. The measured results confirm the quality of the presented procedure.

Maria Gabriella Cimoroni

Papers

Analytical Method for Pattern Generation in Five-Level Cascaded H-Bridge Inverter Using Selective Harmonic Elimination

A Selective Harmonic Elimination Method for Five-Level Converters for Distributed Generation

A New Pulse Active Width Modulation for Multilevel Converters

Selective harmonic elimination in a seven level cascaded multilevel inverter based on graphical analysis

Harmonic mitigation technique for multilevel inverters in power systems