Optimum SHE for cascaded H-bridge multilevel inverters using: NR-GA-PSO, comparative study

Selective Harmonic Elimination (SHE) is very widely applied technique in the control of multilevel inverters that can be used to eliminate the low order dominant harmonics. This is considered a low frequency technique, in which the switching angles are predetermined based on solving a system of transcendental equations. Iterative techniques such as NR and Heuristic techniques such as GA and PSO have been used widely in literatures for the problem of SHE. This paper presents a detailed comparative study of these three techniques when applied for a 7-level CHB-MLI

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

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

TIME DOMAIN EVALUATION OF MULTILEVEL CONVERTERS VOLTAGE AND CURRENT QUALITY

Multilevel converters are power conversion devices consisting of voltage sources and semiconductor switches. This work was motivated by the fact that majority of the conducted research estimates converter quality using frequency-domain approach, which is requires tones of calculation. As an alternative for frequency-domain approach, time-domain evaluation method, developed in a recent time, was selected. This method brings novelty in deep-seated method of convertor evaluation, based on frequency-domain. General goal of this thesis is to demonstrate competency and applicability of time-domain optimization method on online adaptive inverter systems. Work will be focused on development of algorithm for calculation of optimal parameters for single phase CHB converter. Modulation optimization and local minimum finding algorithms are developed and discussed in this work. Moreover, combination of SHE and THD minimization techniques was designed and simulated. Obtained results coincide with those presented so far. However, majority of the results are novel, and has not presented to community yet. Future work will focus on development of the optimization algorithm for three-phase inverters

Real-Time Selective Harmonic Minimization for Multilevel Inverters Using Genetic Algorithm and Artificial Neural Network Angle Generation

This work approximates the selective harmonic elimination problem using Artificial Neural Networks (ANN) to generate the switching angles in an 11-level full bridge cascade inverter powered by five varying DC input sources. Each of the five full bridges of the cascade inverter was connected to a separate 195W solar panel. The angles were chosen such that the fundamental was kept constant and the low order harmonics were minimized or eliminated. A non-deterministic method is used to solve the system for the angles and to obtain the data set for the ANN training. The method also provides a set of acceptable solutions in the space where solutions do not exist by analytical methods. The trained ANN is a suitable tool that brings a small generalization effect on the angles\u27 precision and is able to perform in real time (50/60Hz time window)

A Real-Time and Closed-Loop Control Algorithm for Cascaded Multilevel Inverter Based on Artificial Neural Network

In order to control the cascaded H-bridges (CHB) converter with staircase modulation strategy in a real-time manner, a real-time and closed-loop control algorithm based on artificial neural network (ANN) for three-phase CHB converter is proposed in this paper. It costs little computation time and memory. It has two steps. In the first step, hierarchical particle swarm optimizer with time-varying acceleration coefficient (HPSO-TVAC) algorithm is employed to minimize the total harmonic distortion (THD) and generate the optimal switching angles offline. In the second step, part of optimal switching angles are used to train an ANN and the well-designed ANN can generate optimal switching angles in a real-time manner. Compared with previous real-time algorithm, the proposed algorithm is suitable for a wider range of modulation index and results in a smaller THD and a lower calculation time. Furthermore, the well-designed ANN is embedded into a closed-loop control algorithm for CHB converter with variable direct voltage (DC) sources. Simulation results demonstrate that the proposed closed-loop control algorithm is able to quickly stabilize load voltage and minimize the line current’s THD (<5%) when subjecting the DC sources disturbance or load disturbance. In real design stage, a switching angle pulse generation scheme is proposed and experiment results verify its correctness

A Simple THD Minimization Technique for Transistor-Clamped H-Bridge-Based Cascaded Multilevel Inverter

This paper presents a simple modulation technique that minimizes the output voltage total harmonic distortion (THD) without eliminating the lowest order harmonics. It uses the voltage-angle equal concept of sinusoidal reference waveform to generate the step output voltage of a single-phase transistor-clamped H-bridge (TCHB)-based cascaded multilevel inverter. The real implementation of the modulation technique for a various range of modulation indices is built using an Altera field-programmable gate array (FPGA). It is found that the proposed modulation method resulted in a dramatic decrease in the inverter’s output voltage THD when increasing the number of output steps up to thirteen levels

Theoretical and Experimental Analysis of Output Power Quality in Single-Phase Cascaded H-Bridge Multilevel Inverters

In this thesis simple closed-form asymptotic solutions for estimating the output power quality in single-phase cascaded H-bridge multilevel inverters are presented for staircase modulation technique and pulse-width modulation (PWM) technique. The analysis is carried out in the time domain considering the whole harmonic content and being used for an arbitrary inverter level count. In case of the staircase modulation technique, the voltage and current ripple normalized means square (NMS) expressions are obtained in time domain considering the fundamental period. Voltage and current total harmonic distortions (THDs) as a function of the corresponding NMS values are defined as constrained optimization ones. Optimizing the voltage and current THDs determines the voltage and current optimal switching angles over the modulation index range. The current THD is understood as voltage frequency weighted THD that assumes a pure inductive load, but it is practically accurate for inductively dominant RL-loads. The same approach for estimating the current quality is given for a grid-connected inverter. In the case of the PWM technique, the voltage and current THDs are estimated supposing that the ratio between switching and fundamental frequencies is (infinitely) large (asymptotic assumption). The voltage and current ripple normalized mean square (NMS) values are obtained in time domain by double integration of their normalized squared ripples over the switching and fundamental periods. They present piecewise continuously differentiable analytical solutions with only elementary functions and can be understood as the time-domain equivalent of the frequency-domain double Fourier transformation. The direct relation between the voltage and current NMS values and their qualities is presented. Considering the same approach, the current THD evaluation in case of a grid-connected system is presented. Besides analytical developments, simulation and experimental verifications for three-level (one H-bridge), five-level (two cascaded H-bridges) and seven-level (three cascaded H-bridges) single-phase inverters are analysed, presented and compared in details

Losses and cost optimisation of PV multilevel voltage source inverter with integrated passive power filters

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonNowadays, the need for more contributions from renewable energy sources is rapidly growing. This is forced by many factors including the requirements to meet the targeted reductions of greenhouse gas emissions as well as improving the security of energy supply. According to the International Renewable Energy Agency (IRENA) report 2016, the total installed capacity of solar energy was at least 227 GWs worldwide by the end of 2015 with an annual addition of about 50 GWs in 2015, making solar power the world’s fastest growing energy source. The majority of these are grid-connected photo voltaic (PV) solar power plants, which are required be integrated efficiently into the power grids to meet the requirements of power quality standards at the minimum total investment cost. For this, multilevel voltage source inverters (VSI) have been applied extensively in recent years. In practice, there is a trade-off between the inverter’s number of levels and the required size of output filter, which is a key optimisation area. The aim of this research is to propose a generic model to optimise the design number of levels for the Cascaded H-Bridge Multilevel Inverter (CHB-MLI) and the size of output filter for medium voltage – high power applications. The model is based on key measures, including inverter power loss minimisation, efficient control for minimum total harmonic distortion (THD), minimisation of total system cost and proposing the optimum size of output filter. This research has made a contribution to knowledge in the optimisation of CHB-MLI for medium-voltage high-power applications, in particular, the trade-off optimisation of the inverter’s number of levels and the size of the output filter. The main contribution is the establishment and demonstration of a sound methodology and model based on multi-objective optimisation for the considered key measures of the trade-off model. Furthermore, this study has developed a generic precise model for conduction and switching loss calculation in multilevel inverters. Moreover, it applied Genetic Algorithm (GA) optimisation to provide a complete optimum solution for the problem of selective harmonic elimination (SHE) and suggests the optimum size of output passive power filter (PPF) for different levels CHB-MLIs. The proposed trade-off optimisation model presents an efficient tool for finding the optimum number of the inverter’s levels and the size of output filter, in which the integration system is at its lowest cost, based on optimisation dimensions and applied system constraints. The trade-off optimisation model is generic and can be applied to any multilevel inverter topologies and different power applications.Taif University (TU

Development of alternative pulse width modulation methods for conventional and multilevel voltage source inverters

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonMultilevel inverters have attracted wide interest in both the academic community and the industry for the past decades. Therefore, the investigation and development of modulation strategies in multilevel inverters emerges as a necessity for the industry and researchers. In this doctoral thesis, alternative modulation methods suitable for three-level conventional single-phase inverters and especially for cascade H-bridge multilevel inverters are discussed and proposed. The theory of Equal Areas is reformed and presented and its modifications are proposed. These modifications are compared with other well-known modulation schemes, such as carrier-based modulation schemes and programmed pulse width modulation techniques. The advantage of the modified Equal Areas Pulse Width Modulation (EAPWM) is its algorithmic simplicity due to simple algebraic relationships, which results in less computational effort. A fully mathematical formulation for the Equal Areas modulation is proposed for both conventional and multilevel inverters. The EAPWM is shown to produce well-formed switched output voltages that have low total harmonic distortion at even low switching frequencies. The importance of this thesis is complimented by the results, produced after the implementation of EAPWM in multilevel inverters, which can be used as a more accurate reference when compared with other modulation strategies. Moreover, this direct modulation strategy has been extended to work on higher amplitude modulation ratios, in a linear manner, while entering the over modulation region. In this context, modified algorithms have been developed using different criteria for the calculation of the pulses’ width and their placement inside the time interval. The equal areas method, implemented in conventional single-phase inverters, uses odd pulse numbers per half cycle, holding integer frequency ratios in contrast to its implementation in multilevel inverters, where non-integer frequency ratios occur due to the level-by-level application. The application of the method is verified by simulations together with experimental work using a full-scale prototype inverter

Modular Multilevel Converters for Medium Voltage Applications: Low Switching Frequency Modulation Strategies and Circulating Current Control Techniques.

233 p.El objetivo de la presente tesis ha sido el aumento de la eficiencia y la mejora del funcionamiento de convertidores multinivel modulares (MMCs) en aplicaciones de media tensión (drives, STATCOMs, redes de media tensión en DC o colectores de energía en parques eólicos). Para ello se ha propuesto la utilización de una modulación de baja frecuencia de conmutación como la Eliminación Selectiva de Armónicos (SHE-PWM). De esta forma se reducen las pérdidas de conmutación significativamente. Las contribuciones de la tesis son:- Nueva formulación para implementar SHE-PWM: Esta nueva formulación, a diferencia de las existentes, proporciona un sistema único de ecuaciones que es válido para cualquier forma de onda. De esta forma, es posible buscar los ángulos de disparo y los patrones de conmutación, que resuelven el problema de SHE-PWM, sin necesidad de predefinir ninguna forma de onda. Por lo tanto, la búsqueda de ángulos de disparo se simplifica significativamente y se puede encontrar un alto número de soluciones diferentes, pudiendo optimizar el diseño de la forma de onda. Además, esta formulación es válida con simetrías de cuarto de onda y de media onda.- Controles de la corriente circulante en MMCs cuando se utiliza SHE-PWM: estos controles, a diferencia de los existentes, no distorsionan la tensión de fase de salida cuando se utiliza SHE-PWM y permiten mantener equilibradas las tensiones de los condensadores de los sub-módulos del MMC, además de reducir rizado de la corriente circulante. En concreto, se han propuesto dos controles, uno con (N+1) SHE-PWM y otro con (2N+1) SHE-PWM