Multiphase Design and Control Techniques Applied to a Forward Micro-Inverter

En la última década la potencia instalada de energía solar fotovoltaica ha crecido una media de un 49% anual y se espera que alcance el 16%del consumo energético mundial en el año 2050. La mayor parte de estas instalaciones se corresponden con sistemas conectados a la red eléctrica y un amplio porcentaje de ellas son instalaciones domésticas o en edificios. En el mercado ya existen diferentes arquitecturas para este tipo de instalaciones, entre las que se encuentras los módulos AC. Un módulo AC consiste en un inversor, también conocido como micro-inversor, que se monta en la parte trasera de un panel o módulo fotovoltaico. Esta tecnología ofrece modularidad, redundancia y la extracción de la máxima potencia de cada panel solar de la instalación. Además, la expansión de esta tecnología posibilitará una reducción de costes asociados a las economías de escala y a la posibilidad de que el propio usuario pueda componer su propio sistema. Sin embargo, el micro-inversor debe ser capaz de proporcionar una ganancia de tensión adecuada para conectar el panel solar directamente a la red, mientras mantiene un rendimiento aceptable en un amplio rango de potencias. Asimismo, los estándares de conexión a red deber ser satisfechos y el tamaño y el tiempo de vida del micro-inversor son factores que han de tenerse siempre en cuenta. En esta tesis se propone un micro-inversor derivado de la topología “forward” controlado en el límite entre los modos de conducción continuo y discontinuo (BCM por sus siglas en inglés). El transformador de la topología propuesta mantiene la misma estructura que en el convertidor “forward” clásico y la utilización de interruptores bidireccionales en el secundario permite la conexión directa del inversor a la red. Asimismo el método de control elegido permite obtener factor de potencia cercano a la unidad con una implementación sencilla. En la tesis se presenta el principio de funcionamiento y los principales aspectos del diseño del micro-inversor propuesto. Con la idea de mantener una solución sencilla y de bajo coste, se ha seleccionado un controlador analógico que está originalmente pensado para controlar un corrector del factor de potencia en el mismo modo de conducción que el micro-inversor “forward”. La tesis presenta las principales modificaciones necesarias, con especial atención a la detección del cruce por cero de la corriente (ZCD por sus siglas en inglés) y la compatibilidad del controlador con la inclusión de un algoritmo de búsqueda del punto de máxima potencia (MPPT por sus siglas en inglés). Los resultados experimentales muestran las limitaciones de la implementación elegida e identifican al transformador como el principal contribuyente a las pérdidas del micro-inversor. El principal objetivo de esta tesis es contribuir a la aplicación de técnicas de control y diseño de sistemas multifase en micro-inversores fotovoltaicos. En esta tesis se van a considerar dos configuraciones multifase diferentes aplicadas al micro-inversor “forward” propuesto. La primera consiste en una variación con conexión paralelo-serie que permite la utilización de transformadores con una relación de vueltas baja, y por tanto bien acoplados, para conseguir una ganancia de tensión adecuada con un mejor rendimiento. Esta configuración emplea el mismo control BCM cuando la potencia extraída del panel solar es máxima. Este método de control implica que la frecuencia de conmutación se incrementa considerablemente cuando la potencia decrece, lo que compromete el rendimiento. Por lo tanto y con la intención de mantener unos bueno niveles de rendimiento ponderado, el micro-inversor funciona en modo de conducción discontinuo (DCM, por sus siglas en inglés) cuando la potencia extraía del panel solar es menor que la máxima. La segunda configuración multifase considerada en esta tesis es la aplicación de la técnica de paralelo con entrelazado. Además se han considerado dos técnicas diferentes para decidir el número de fases activas: dependiendo de la potencia continua extraída del panel solar y dependiendo de la potencia instantánea demandada por el micro-inversor. La aplicación de estas técnicas es interesante en los sistemas fotovoltaicos conectados a la red eléctrica por la posibilidad que brindan de obtener un rendimiento prácticamente plano en un amplio rango de potencia. Las configuraciones con entrelazado se controlan en DCM para evitar la necesidad de un control de corriente, lo que es importante cuando el número de fases es alto. Los núcleos adecuados para todas las configuraciones multifase consideradas se seleccionan usando el producto de áreas. Una vez seleccionados los núcleos se ha realizado un diseño detallado de cada uno de los transformadores. Con la información obtenida de los diseños y los resultados de simulación, se puede analizar el impacto que el número de transformadores utilizados tiene en el tamaño y el rendimiento de las distintas configuraciones. Los resultados de este análisis, presentado en esta tesis, se utilizan posteriormente para comparar las distintas configuraciones. Muchas otras topologías se han presentado en la literatura para abordar los diferentes aspectos a considerar en los micro-inversores, que han sido presentados anteriormente. La mayoría de estas topologías utilizan un transformador de alta frecuencia para solventar el salto de tensión y evitar problemas de seguridad y de puesta a tierra. En cualquier caso, es interesante evaluar si topologías sin aislamiento galvánico son aptas para su utilización como micro-inversores. En esta tesis se presenta una revisión de inversores con capacidad de elevar tensión, que se comparan bajo las mismas especificaciones. El objetivo es proporcionar la información necesaria para valorar si estas topologías son aplicables en los módulos AC. Las principales contribuciones de esta tesis son: • La aplicación del control BCM a un convertidor “forward” para obtener un micro-inversor de una etapa sencillo y de bajo coste. • La modificación de dicho micro-inversor con conexión paralelo-series de transformadores que permite reducir la corriente de los semiconductores y una ganancia de tensión adecuada con transformadores altamente acoplados. • La aplicación de técnicas de entrelazado y decisión de apagado de fases en la puesta en paralelo del micro-inversor “forward”. • El análisis y la comparación del efecto en el tamaño y el rendimiento del incremento del número de transformadores en las diferentes configuraciones multifase. • La eliminación de las medidas y los lazos de control de corriente en las topologías multifase con la utilización del modo de conducción discontinuo y un algoritmo MPPT sin necesidad de medida de corriente. • La recopilación y comparación bajo las mismas especificaciones de topologías inversoras con capacidad de elevar tensión, que pueden ser adecuadas para la utilización como micro-inversores. Esta tesis está estructurada en seis capítulos. El capítulo 1 presenta el marco en que se desarrolla la tesis así como el alcance de la misma. En el capítulo 2 se recopilan las topologías existentes de micro-invesores con aislamiento y aquellas sin aislamiento cuya implementación en un módulo AC es factible. Asimismo se presenta la comparación entre estas topologías bajo las mismas especificaciones. El capítulo 3 se centra en el micro-inversor “forward” que se propone originalmente en esta tesis. La aplicación de las técnicas multifase se aborda en los capítulos 4 y 5, en los que se presentan los análisis en función del número de transformadores. El capítulo está orientado a la propuesta paralelo-serie mientras que la configuración con entrelazado se analiza en el capítulo 5. Por último, en el capítulo 6 se presentan las contribuciones de esta tesis y los trabajos futuros. ABSTRACT In the last decade the photovoltaic (PV) installed power increased with an average growth of 49% per year and it is expected to cover the 16% of the global electricity consumption by 2050. Most of the installed PV power corresponds to grid-connected systems, with a significant percentage of residential installations. In these PV systems, the inverter is essential since it is the responsible of transferring into the grid the extracted power from the PV modules. Several architectures have been proposed for grid-connected residential PV systems, including the AC-module technology. An AC-module consists of an inverter, also known as micro-inverter, which is attached to a PV module. The AC-module technology offers modularity, redundancy and individual MPPT of each module. In addition, the expansion of this technology will enable the possibility of economies of scale of mass market and “plug and play” for the user, thus reducing the overall cost of the installation. However, the micro-inverter must be able to provide the required voltage boost to interface a low voltage PV module to the grid while keeping an acceptable efficiency in a wide power range. Furthermore, the quality standards must be satisfied and size and lifetime of the solutions must be always considered. In this thesis a single-stage forward micro-inverter with boundary mode operation is proposed to address the micro-inverter requirements. The transformer in the proposed topology remains as in the classic forward converter and bidirectional switches in the secondary side allows direct connection to the grid. In addition the selected control strategy allows high power factor current with a simple implementation. The operation of the topology is presented and the main design issues are introduced. With the intention to propose a simple and low-cost solution, an analog controller for a PFC operated in boundary mode is utilized. The main necessary modifications are discussed, with the focus on the zero current detection (ZCD) and the compatibility of the controller with a MPPT algorithm. The experimental results show the limitations of the selected analog controller implementation and the transformer is identified as a main losses contributor. The main objective of this thesis is to contribute in the application of control and design multiphase techniques to the PV micro-inverters. Two different multiphase configurations have been applied to the forward micro-inverter proposed in this thesis. The first one consists of a parallel-series connected variation which enables the use of low turns ratio, i.e. well coupled, transformers to achieve a proper voltage boost with an improved performance. This multiphase configuration implements BCM control at maximum load however. With this control method the switching frequency increases significantly for light load operation, thus jeopardizing the efficiency. Therefore, in order to keep acceptable weighted efficiency levels, DCM operation is selected for low power conditions. The second multiphase variation considered in this thesis is the interleaved configuration with two different phase shedding techniques: depending on the DC power extracted from the PV panel, and depending on the demanded instantaneous power. The application of interleaving techniques is interesting in PV grid-connected inverters for the possibility of flat efficiency behavior in a wide power range. The interleaved variations of the proposed forward micro-inverter are operated in DCM to avoid the current loop, which is important when the number of phases is large. The adequate transformer cores for all the multiphase configurations are selected according to the area product parameter and a detailed design of each required transformer is developed. With this information and simulation results, the impact in size and efficiency of the number of transformer used can be assessed. The considered multiphase topologies are compared in this thesis according to the results of the introduced analysis. Several other topological solutions have been proposed to solve the mentioned concerns in AC-module application. The most of these solutions use a high frequency transformer to boost the voltage and avoid grounding and safety issues. However, it is of interest to assess if the non-isolated topologies are suitable for AC-module application. In this thesis a review of transformerless step-up inverters is presented. The compiled topologies are compared using a set benchmark to provide the necessary information to assess whether non-isolated topologies are suitable for AC-module application. The main contributions of this thesis are: • The application of the boundary mode control with constant off-time to a forward converter, to obtain a simple and low-cost single-stage forward micro-inverter. • A modification of the forward micro-inverter with primary-parallel secondary-series connected transformers to reduce the current stress and improve the voltage gain with highly coupled transformers. •The application of the interleaved configuration with different phase shedding strategies to the proposed forward micro-inverter. • An analysis and comparison of the influence in size and efficiency of increasing the number of transformers in the parallel-series and interleaved multiphase configurations. • Elimination of the current loop and current measurements in the multiphase topologies by adopting DCM operation and a current sensorless MPPT. • A compilation and comparison with the same specifications of suitable non-isolated step-up inverters. This thesis is organized in six chapters. In Chapter 1 the background of single-phase PV-connected systems is discussed and the scope of the thesis is defined. Chapter 2 compiles the existing solutions for isolated micro-inverters and transformerless step-up inverters suitable for AC-module application. In addition, the most convenient non-isolated inverters are compared using a defined benchmark. Chapter 3 focuses on the originally proposed single-stage forward micro-inverter. The application of multiphase techniques is addressed in Chapter 4 and Chapter 5, and the impact in different parameters of increasing the number of phases is analyzed. In Chapter 4 an original primary-parallel secondary-series variation of the forward micro-inverter is presented, while Chapter 5 focuses on the application of the interleaved configuration. Finally, Chapter 6 discusses the contributions of the thesis and the future work.

Antisense oligonucleotides against rat brain α1E DNA and its atrial homologue decrease T-type calcium current in atrial myocytes

Low voltage-activated, or T-type, calcium currents are important regulators of neuronal and muscle excitability, secretion, and possibly cell growth and differentiation. The gene (or genes) coding for the pore-forming subunit of low voltage-activated channel proteins has not been unequivocally identified. We have used reverse transcription–PCR to identify partial clones from rat atrial myocytes that share high homology with a member of the E class of calcium channel genes. Antisense oligonucleotides targeting one of these partial clones (raE1) specifically block the increase in T-current density that normally results when atrial myocytes are treated with insulin-like growth factor 1 (IGF-1). Antisense oligonucleotides targeting portions of the neuronal rat α1E sequence, which are not part of the clones detected in atrial tissue, also block the IGF-1-induced increase in T-current, suggesting that the high homology to α1E seen in the partial clone may be present in the complete atrial sequence. The basal T-current expressed in these cells is also blocked by antisense oligonucleotides, which is consistent with the notion that IGF-1 up-regulates the same gene that encodes the basal current. These results support the hypothesis that a member of the E class of calcium channel genes encodes a low voltage-activated calcium channel in atrial myocytes.

Efficient and regulated erythropoietin production by naked DNA injection and muscle electroporation

We show that an electric treatment in the form of high-frequency, low-voltage electric pulses can increase more than 100-fold the production and secretion of a recombinant protein from mouse skeletal muscle. Therapeutical erythopoietin (EPO) levels were achieved in mice with a single injection of as little as 1 μg of plasmid DNA, and the increase in hematocrit after EPO production was stable and long-lasting. Pharmacological regulation through a tetracycline-inducible promoter allowed regulation of serum EPO and hematocrit levels. Tissue damage after stimulation was transient. The method described thus provides a potentially safe and low-cost treatment for serum protein deficiencies.

Altering the biochemical state of individual cultured cells and organelles with ultramicroelectrodes

We describe an efficient technique for the selective chemical and biological manipulation of the contents of individual cells. This technique is based on the electric-field-induced permeabilization (electroporation) in biological membranes using a low-voltage pulse generator and microelectrodes. A spatially highly focused electric field allows introduction of polar cell-impermeant solutes such as fluorescent dyes, fluorogenic reagents, and DNA into single cells. The high spatial resolution of the technique allows for design of, for example, cellular network constructions in which cells in close contact with each other can be made to possess different biochemical, biophysical, and morphological properties. Fluorescein, and fluo-3 (a calcium-sensitive fluorophore), are electroporated into the soma of cultured single progenitor cells derived from adult rat hippocampus. Fluo-3 also is introduced into individual submicrometer diameter processes of thapsigargin-treated progenitor cells, and a plasmid vector cDNA construct (pRAY 1), expressing the green fluorescent protein, is electroporated into cultured single COS 7 cells. At high electric field strengths, observations of dye-transfer into organelles are proposed.

Estudo sobre a interação de métodos anti-ilhamento para sistemas fotovoltaicos conectados à rede de distribuição de baixa tensão com múltiplos inversores.

Este trabalho estuda a interação entre os métodos anti-ilhamento aplicados em sistemas fotovoltaicos residenciais, operando simultaneamente em uma rede de distribuição de baixa tensão. Os sistemas fotovoltaicos em geral interagem entre si, com a rede de distribuição da concessionária e com outras fontes de geração distribuída. Uma consequência importante dessa interação é a ocorrência do ilhamento, que acontece quando as fontes de geração distribuída fornecem energia ao sistema elétrico de potência mesmo quando esta se encontra eletricamente isolada do sistema elétrico principal. A função anti-ilhamento é uma proteção extremamente importante, devendo estar presente em todos os sistemas de geração distribuída. Atualmente, são encontradas diversas técnicas na literatura. Muitas delas oferecem proteção adequada quando um inversor está conectado à linha de distribuição, mas podem falhar quando dois ou mais funcionam simultaneamente, conectados juntos ou próximos entre si. Dois destes métodos são analisados detalhadamente nesse estudo, avaliados em uma rede de distribuição residencial de baixa tensão. Os resultados obtidos mostram que a influência de um método sobre o outro é dependente da predominância de cada um deles dentro do sistema elétrico. Contudo, nas condições analisadas o ilhamento foi detectado dentro do limite máximo estabelecido pelas normas pertinentes.

Evidence for non-conservative current-induced forces in the breaking of Au and Pt atomic chains

This experimental work aims at probing current-induced forces at the atomic scale. Specifically it addresses predictions in recent work regarding the appearance of run-away modes as a result of a combined effect of the non-conservative wind force and a ‘Berry force’. The systems we consider here are atomic chains of Au and Pt atoms, for which we investigate the distribution of break down voltage values. We observe two distinct modes of breaking for Au atomic chains. The breaking at high voltage appears to behave as expected for regular break down by thermal excitation due to Joule heating. However, there is a low-voltage breaking mode that has characteristics expected for the mechanism of current-induced forces. Although a full comparison would require more detailed information on the individual atomic configurations, the systems we consider are very similar to those considered in recent model calculations and the comparison between experiment and theory is very encouraging for the interpretation we propose.

Análise do regime de produção distribuída fotovoltaica para autoconsumo

Tese de mestrado integrado em Engenharia da Energia e do Ambiente, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2016

Electrophysiologic assessment of conduction abnormalities and atrial arrhythmias associated with amyloid cardiomyopathy.

BACKGROUND Arrhythmias in cardiac amyloidosis (CA) result in significant comorbidity and mortality but have not been well characterized. OBJECTIVE The purpose of this study was to define intracardiac conduction, atrial arrhythmia substrate, and ablation outcomes in a group of advanced CA patients referred for electrophysiologic study. METHODS Electrophysiologic study with or without catheter ablation was performed in 18 CA patients. Findings and catheter ablation outcomes were compared to age- and gender-matched non-CA patients undergoing catheter ablation of persistent atrial fibrillation (AF). RESULTS Supraventricular tachycardias were seen in all 18 CA patients (1 AV nodal reentrant tachycardia, 17 persistent atrial tachycardia [AT]/AF). The HV interval was prolonged (>55 ms) in all CA patients, including 6 with normal QRS duration (≤100 ms). Thirteen supraventricular tachycardia ablations were performed in 11 patients. Of these, 7 underwent left atrial (LA) mapping and ablation for persistent AT/AF. Compared to non-CA age-matched comparator AF patients, CA patients had more extensive areas of low-voltage areas LA (63% ± 22% vs 34% ± 22%, P = .009) and a greater number of inducible ATs (3.3 ± 1.9 ATs vs 0.2 ± 0.4 ATs, P <.001). The recurrence rate for AT/AF 1 year after ablation was greater in CA patients (83% vs 25%), and the hazard ratio for postablation AT/AF recurrence in CA patients was 5.4 (95% confidence interval 1.9-35.5, P = .007). CONCLUSION In this group of patients with advanced CA and atrial arrhythmias, there was extensive conduction system disease and LA endocardial voltage abnormality. Catheter ablation persistent AT/AF in advanced CA was associated with a high recurrence rate and appears to have a limited role in control of these arrhythmias.

Ground-fault protection on construction sites.

"1992 (Reprinted)."

Guidelines for the installation of MSHA accepted ground wire monitors /

"July 1996."

Power quality improvements for LV networks

In order to increase the capacity of the existing Low Voltage grid, one solution is to increase the nominal residential network voltage from 230 V to 300 V, which is easily accommodated within the voltage rating of existing infrastructure such as cabling. A power electronic AC-AC converter would then be used to step the voltage back down to 230 V at an individual property. Such equipment could also be used to provide power quality improvements on both the utility and customer side of the converter depending on its topology. This paper provides an overview of a project which is looking at the development of such a device. The project is being carried out in collaboration with the local UK, Distribution Network Operator (DNO).

Small-signal input admittance characteristics of a brushless DC motor drive

To carry out stability studies on more electric systems in which there is a preponderance of motor drive equipment, input admittance expressions are required for the individual pieces of equipment. In this paper the techniques of averaging and small-signal linearisation will be used to derive a simple input admittance model for a low voltage, trapezoidal back EMF, brushless, DC motor drive system.

Controlling the properties of microstructured plastic optical fiber Bragg gratings using acousto-optic excitation

A fine control of the mPOF Bragg grating spectrum properties, such as maximum reflected power and 3dB bandwidth, through acousto-optic modulation (AOM) using flexural regime is presented. A numerical comparison of the strain field along mPOFBG - AOM and the similar structure with SMFBG-AOM was presented, showing that the strain field amplitude is higher along the mPOFBG due to its smaller mechanical stiffness. The obtained results can be used in the development of fine-tuned optical filters using low voltage sources and low frequency regimes, to obtain tunable optical filters and to control the shape of the spectrum. Studies of the behavior in different gratings (such as phase shifted and long period gratings) for photonic applications, such as tunable notch filters or tunable cavities, are in progress. It can potentially be applied on tunable optical filters for POF transmission. © 2012 IEEE.

Indicador de inconformidades em instalações elétricas de baixa tensão

With the progress of devices technology, generation and use of energy ways, power quality parameters start to influence more significantly the various kinds of power consumers. Currently, there are many types of devices that analyze power quality. However, there is a need to create devices, and perform measurements and calculate parameters, find flaws, suggest changes, and to support the management of the installation. In addition, you must ensure that such devices are accessible. To maintain this balance, one magnitude measuring method should be used which does not require great resources processing or memory. The work shows that application of the Goertzel algorithm, compared with the commonly used FFT allows measurements to be made using much less hardware resources, available memory space to implement management functions. The first point of the work is the research of troubles that are more common for low voltage consumers. Then we propose the functional diagram indicate what will be measured, calculated, what problems will be detected and that solutions can be found. Through the Goertzel algorithm simulation using Scilab, is possible to calculate frequency components of a distorted signal with satisfactory results. Finally, the prototype is assembled and tests are carried out by adjusting the parameters necessary for one to maintain a reliable device without increasing its cost.

Integración de aerogeneradores de baja potencia en la red de baja tensión

[EN] The current energy model is unsustainable economically, environmentally and socially forcing develop renewable technologies that modify the concept to the responsible use. The use of low-power wind turbines has generally been limited to isolated which has slowed their development systems. The study addresses this technological niche to deepen their study of integration of electricity on a low voltage network. For this first wind potential with Homer software is evaluated. Various scenarios are simulated in "The IEEE European Test Low Voltage Feeder" network using the software OpenDSS widely used in studies of Generation Distributed by EPRI (Electric Power Research Institute).