Control Based Soft Switching Three-phase Micro-inverter: Efficiency And Power Density Optimization

In the field of renewable energy, solar photovoltaic is growing exponentially. Grid-tied PV micro-inverters have become the trend for future PV system development because of their remarkable advantages such as enhanced energy production due to MPPT implementation for each PV panel, high reliability due to redundant and distributed system architecture, and simple design, installation, and management due to its plug-and-play feature. Conventional approaches for the PV micro-inverters are mainly in the form of single-phase grid connected and they aim at the residential and commercial rooftop applications. It would be advantageous to extend the micro-inverter concept to large size PV installations such as MW-class solar farms where threephase AC connections are used. The relatively high cost of the three-phase micro-inverter is the biggest barrier to its large scale deployment. Increasing the switching frequency may be the best way to reduce cost by shrinking the size of reactive components and heat-sink. However, this approach could cause conversion efficiency to drop dramatically without employing soft switching techniques or using costly new devices. This dissertation presents a new zero voltage switching control method that is suitable for low power applications such as three-phase micro-inverters. The proposed hybrid boundary conduction mode (BCM) current control method increases the efficiency and power density of the micro-inverters and features both reduced number of components and easy digital implementation. Zero voltage switching is achieved by controlling the inductor current bidirectional in every switching cycle and results in lower switching losses, higher operating frequency, and reduced size and cost of passive components, especially magnetic cores. Some iv practical aspects of hybrid control implementation such as dead-time insertion can degrade the performance of the micro-inverter. A dead-time compensation method that improves the performance of hybrid BCM current control by decreasing the output current THD and reducing the zero crossing distortion is presented. Different BCM ZVS current control modulation schemes are compared based on power losses breakdown, switching frequency range, and current quality. Compared to continuous conduction mode (CCM) current control, BCM ZVS control decreases MOSFET switching losses and filter inductor conduction losses but increases MOSFET conduction losses and inductor core losses. Based on the loss analysis, a dual-mode current modulation method combining ZVS and zero current switching (ZCS) schemes is proposed to improve the efficiency of the micro-inverter. Finally, a method of maintaining high power conversion efficiency across the entire load range of the three-phase micro-inverter is proposed. The proposed control method substantially increases the conversion efficiency at light loads by minimizing switching losses of semiconductor devices as well as core losses of magnetic components. This is accomplished by entering a phase skipping operating mode wherein two phases of an inverter are disabled and three inverters are combined to form a new three-phase system with minimal grid imbalance. A 400W prototype of a three-phase micro-inverter and its hybrid control system have been designed and tested under different conditions to verify the effectiveness of the proposed controller, current modulation scheme, and light load efficiency enhancement method

High Power Density, High Efficiency Single Phase Transformer-less Photovoltaic String Inverters

abstract: Two major challenges in the transformer-less, single-phase PV string inverters are common mode leakage currents and double-line-frequency power decoupling. In the proposed doubly-grounded inverter topology with innovative active-power-decoupling approach, both of these issues are simultaneously addressed. The topology allows the PV negative terminal to be directly connected to the neutral, thereby eliminating the common-mode ground-currents. The decoupling capacitance requirement is minimized by a dynamically-variable dc-link with large voltage swing, allowing an all-film-capacitor implementation. Furthermore, the use of wide-bandgap devices enables the converter operation at higher switching frequency, resulting in smaller magnetic components. The operating principles, design and optimization, and control methods are explained in detail, and compared with other transformer-less, active-decoupling topologies. A 3 kVA, 100 kHz single-phase hardware prototype at 400 V dc nominal input and 240 V ac output has been developed using SiC MOSFETs with only 45 μF/1100 V dc-link capacitance. The proposed doubly-grounded topology is then extended for split-phase PV inverter application which results in significant reduction in both the peak and RMS values of the boost stage inductor current and allows for easy design of zero voltage transition. A topological enhancement involving T-type dc-ac stage is also developed which takes advantage of the three-level switching states with reduced voltage stress on the main switches, lower switching loss and almost halved inductor current ripple. In addition, this thesis also proposed two new schemes to improve the efficiency of conventional H-bridge inverter topology. The first scheme is to add an auxiliary zero-voltage-transition (ZVT) circuit to realize zero-voltage-switching (ZVS) for all the main switches and inherent zero-current-switching (ZCS) for the auxiliary switches. The advantages include the provision to implement zero state modulation schemes to decrease the inductor current THD, naturally adaptive auxiliary inductor current and elimination of need for large balancing capacitors. The second proposed scheme improves the system efficiency while still meeting a given THD requirement by implementing variable instantaneous switching frequency within a line frequency cycle. This scheme aims at minimizing the combined switching loss and inductor core loss by including different characteristics of the losses relative to the instantaneous switching frequency in the optimization process.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Decentralized control techniques applied to electric power distributed generation in microgrids

Distributed generation of electric energy has become part of the current electric power system. In this context a new scenario is arising in which small energy sources make up a new supply system: The microgrid.The most recent research projects show the technical difficulty of controlling the operation of microgrids, because they are complex systems in which several subsystems interact: energy sources, power electronic converters, energy storage systems, local, linear and non-linear loads and of course, the main grid. In next years, the electric grid will evolve from the current very centralized model toward a more distributed one. At the present time the generation, consumption and storage points are very far away one from each other. Under these circumstances, relatively frequent failures of the electric supply and important losses take place in the transport and distribution of energy, so that it can be stated that the efficiency of the supply system is low.In another context, electric companies are aiming at an electric grid, formed in a certain proportion by distributed generators, where the consumption points are near the generation points, avoiding high losses in the transmission lines and reducing the rate of shortcomings. Summing up, it is pursued the generation of small quantities of electric power by the users (this concept is called microgeneration in the origin), considering them not only as electric power consumers but also as responsible for the generation, becoming this way an integral part of the grid.In this context it is necessary to develop a new concept of flexible grid, i.e., with reconfiguration capability for operation with or without connection to the mains. The future microgrids should incorporate supervision and control systems that allow the efficient management of various kinds of energy generators, such as photovoltaic panels, energy storage systems, and local loads. Hence, we are dealing with intelligent flexible Microgrids capable of import and export power from/to the grid reconfiguring its operation modes and making decisions in real time.The researching lineas that have been introduced in this thesis are focused on the innovation in this kind of systems, the integration of several renewable energy sources, the quality of the power supply, security issues, and the system behavior during faults.In order to carry out some solutions related within these characteristics, the main goal of this thesis is the application on new control stretegies and a power management analysis of a microgrid. Thus, thanks to the emerging of renewable energy, is possible to give an alternative to the decoupling of generation units connected to the utility grid.Likewise, a work methodology has been analyzed and developed based on the modeling, control parameters design, and power management control starting from a single voltage source inverter to a number of interconnected DG units forming flexible Microgrids. In addition, all the mencioned topics have been studied giving new system performances, viability and safe functioning, thanks to the small-signal analysis and introducing control loop design algorithms, improving the import/export of electric power and operating both grid connected mode and an island.This thesis has presented an analysis, simulation and experimental results focusing on modeling, control, and analysis of DG units, giving contributions according to the following steps:- Control-oriented modeling based on active and reactive power analysis- Control synthesis based on enhanced droop control technique.- Small-signal stability study to give guidelines for properly adjusting the control system parameters according to the desired dynamic responseThis methodology has been extended to microgrids by using hierarchical control applied to droop-controlled line interactive UPSs showing that:- Droop-controlled inverters can be used in islanded microgrids.- By using multilevel control systems the microgrid can operate in both grid-connected and islanded mode, in a concept called flexible microgrid.The proposed hierarchical control required for flexible Microgrids consisted of different control levels, as following:- Primary control is based on the droop method allowing the connection of different AC sources without any intercommunication.- Secondary control avoids the voltage and frequency deviation produced by the primary control. Only low bandwidth communications are needed to perform this control level. A synchronization loop can be added in this level to transfer from islanding to grid connected modes.- Tertiary control allows the import/export of active and reactive power to the grid

Photovoltaic based electric and plug-in hybrid electric vehicle battery charging infrastructure using a modified z-converter topology

There currently exist numerous uncertainties about the future of energy consumption in the industrialized world. It is safe to claim with confidence that fossil fuel reserves are proving to be inadequate, highly difficult to extract and refine, and less attractive to the community, given their environmental and socio-political impact. More recently, this has lead to the natural conclusion that renewable energy technologies should be preferred over more traditional ones. Replacing carbon as an economic engine can be achieved in numerous ways. However, it is still not clear, as to which renewable options will be more successful. On the other hand, it is quite safe to expect that the trend towards distributed and local power generation, using wind and photovoltaic sources will continue. At the same time, the enormous transportation sector will rely more heavily on electricity and related infrastructure needed for storage and distribution. All the above issues point towards the realization of public and private facilities to generate electricity locally, to recharge electric and plug-in hybrid electric vehicles. In this thesis, a photovoltaic (PV) source is proposed for electric/plug-in hybrid electric vehicle battery charging, due to the fact that solar panels can be conveniently placed above the vehicle parking space and can double as a shade provider. In fact, such a feature is so desirable that indeed, several installations exist today, that use a carport PV array to generate power for purposes other than EV recharging. Determining the technical goals for such facilities and discussing some relevant solutions is, broadly, the scope of this thesis. More particularly, the thesis aims at establishing application oriented technical differences between regular PV-grid-tied systems and PV systems that are specifically adapted to public or semi-public EV charging, noting that the former arrangement has enjoyed much attention in literature. This goal will be accomplished by presenting the design process for one such system, starting with the definition of technical specifications that take into account all the real constraints dictated by the state of the art in PV and battery technologies, grid interface requirements, safety standards, and market demands. The second part of this thesis focuses on the power converter topology, with strong emphasis on the analysis of the Z-loaded/sourced converter, as a fairly suitable and practical topology. At the same time, other possible topologies will also be considered for comparison purposes, especially with regards to reliability, efficiency, and cost

Modelización de una micro red fotovoltaica aplicada a equipos de cocción por inducción para el reemplazo de GLP

Due to the increasing use of electricity, cooking food has developed major changes, mainly with respect to fuel use from firewood, coal or fossil fuel derivatives up to the use of electricity. In the case of Ecuador, the government has decided to run a campaign about massive migration from LPG based cookers to induction cookers during the next three years, thanks to the policies of changing the Productive Matrix and also the diversification of the Energy Matrix. This works presents an alternative to feed these devices with electricity from a photovoltaic micro grid, which has the ability to store energy and use the grid energy to cover the peak demand at specific periods of time, following the established conditions in an operation model proposed for this kind of systems. Prior to the design and sizing of the photovoltaic micro grid, the electric induction cooker was characterized thorough an equivalent circuit and as a result a transfer function was obtained that allowed calculating the active power consumption of this equipment. The modeling of this system was performed using the Matlab Simulink tool, for a typical day of the months with highest and lowest solar resource, considering two extreme states of the battery bank at the start of the day. The results show that the use of distributed generation power applied to this equipment could mean for the user to stop using between 57% and 65% of the grid energy, even for the worst weather conditions considered. However, the economic analysis shows that, with the current costs, investment in this kind of facility is not short-term profitable.Debido al creciente uso de la electricidad, la cocción de alimentos ha sufrido grandes transformaciones, principalmente con respecto al combustible empleado, pasando de utilizar leña, carbón o derivados de combustibles fósiles hasta llegar al uso de la electricidad. En el caso de Ecuador, se ha decido realizar una migración masiva de cocinas de GLP a cocinas eléctricas de inducción en los próximos tres años, gracias a las políticas de cambio de la Matriz Productiva y de diversificación de la Matriz Energética. El presente trabajo presenta una alternativa para alimentar a estos equipos con electricidad generada a partir de una micro red fotovoltaica, la misma que posee la capacidad de almacenar energía y recurrir a la red para cubrir picos de demanda en horarios específicos, siguiendo las condiciones contempladas en un modelo de operación propuesto para este tipo de sistemas. Previo al diseño y dimensionamiento de la micro red fotovoltaica, se caracterizó la cocina eléctrica de inducción a través de un circuito equivalente y con ello se obtuvo una función de transferencia que permitió calcular el consumo de potencia activa de este equipo. El modelamiento del sistema se lo hizo empleando la herramienta Simulink de Matlab, para un día tipo de los meses con mayor y menor recurso solar, considerando dos estados extremos del banco de baterías en el inicio del día. Los resultados muestran que el uso de generación distribuida aplicada a la alimentación de este equipo significaría para el usuario dejar de consumir entre un 57% y un 65% de la energía obtenida de la red de distribución, incluso para las peores condiciones climatológicas consideradas. Sin embargo, el análisis económico muestra que, con los niveles de costos actuales, la inversión en este tipo de instalaciones no es rentable en el corto plazo