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Power electronic supplies for public lighting systems with distributed generation capability: solution proposals for power and minimization of the impact in grid quality
Equipos electrónicos de alimentación en sistemas de alumbrado público con capacidad de microgeneración distribuida: propuesta de soluciones para etapas de potencia y control, caracterización y minimización del impacto en la red eléctrica
López Corominas, Emilio Ramón García García, Jorge
Ingeniería Eléctrica, Electrónica, de Computadores y Sistemas, Departamento de
Transmisión y distribución eléctrica Circuitos
This PhD Thesis, entitled “Power Electronic Supplies For Public Lighting Systems With Distributed Generation Capability: Solution Proposals For Power And Control Stages, Characterization And Minimization Of The Impact In Grid Quality”, has been developed as one of the core research activities of the Efficient Energy Conversion, Industrial Electronics and Lighting Engineering group (CE3I2), from the Electrical Engineering Department (DIEECS) of the University of Oviedo. This work targets the enhancement of power electronic solutions for lighting applications, aiming to increase the system performance, the energy efficiency as well as the reliability. The system performance is studied in a top-down approach, focusing on the power and control stages in the lighting system, in order to optimise the final design upon given technical and economic constraints. Nowadays, energy efficiency is a crucial component of the multidisciplinary response to climate change. In this context, a remarkable amount of energy is wasted through inefficient lighting from many sources across the world, including public street/road lighting, commercial, industrial, domestic and office lighting systems. In order to propose solutions, in the present work different key aspects to improve the energy efficiency in power systems will be dealt with. Therefore, it shall be studied the effect of the integration of renewable energy sources and the combined use of modern lighting systems based on LED and the conventional discharge lamps in the distribution grids, both the new microgrids and the traditional ones (ring, radial, etc.). These three points are the key in order to develop this PhD thesis from the aforementioned top-bottom approach. Actually, there are a few works nowadays focused on developing electronic equipment compatible with former and modern lighting systems, including the integration of renewable energy sources into the distribution grids. Accordingly, this work will deal with this issue. Ergo, a combination of LED street lighting with renewable microgeneration capability is studied in this PhD thesis and it is called hybrid system; the hybrid systems that shall be addressed are the ones connected to the AC mains, allowing the reduction of the power consumption from the traditional generators, helping this way to reduce the contaminants particles emission to the atmosphere. On the other hand, these hybrid systems can be connected also to a big distribution grid where there can exist too traditional illumination systems based on discharge lamps and low frequency electromagnetic ballasts. This situation is becoming a trend in many cities due to the fact that the new power LED lamps provide better efficacy and performance than the traditional ones. Hence, the discharge lamps are being substituted slowly but constantly, giving to situations where both illumination systems live together connected to the same distribution grid. The use of conventional lighting system would not be a problem if those lamps were controlled by electronic ballasts with a low harmonic generation and high efficiency. However, discharge lamps are generally driven by low frequency electromagnetic ballasts because they are robust (used since many years ago), and moreover, rather cheap. Their big disadvantage is the great harmonic content they generate in the grids and their poor efficiency. So, one of the main topics of this doctoral work will be the following: • Characterise the effect of the conventional illumination systems (discharge lamps and low frequency electromagnetic ballasts) in the distribution grids. o Develop a distribution grid harmonic mitigation strategy taking advantage of the energy generated by the renewable energy sources installed in the previously mentioned hybrid systems. This proposed solution will be low-cost, avoiding the use of current sensors and the coordination between the different converters associated to the hybrid system of each streetlamp. It must be taken into account that the converter will not be analysed in detail during this section. It just known that connected to the same distribution grid there will be a certain number of low frequency electromagnetic ballasts driving discharge lamps and also several hybrid systems formed by LED lamps and renewable energy sources. The true requirement in these hybrid systems is that the power converter interfacing them with the grid must be bidirectional. On the other hand, it is going to be studied how the non-linear loads affect the new intelligent single-phase distribution grids known as smartgirds, and also the microgrids. Hence, the previous study about the discharge lamps controlled by low frequency electromagnetic ballasts will be taken advantage of. Therefore, about this topic: • Different control strategies to be implemented in active filters, commonly used in this small single-phase distribution grids, will be evaluated and later, compared with the previous low-cost solution. After studying how to compensate the harmonics generated by non-linear loads and how to take advantage of the distributed microgeneration in order to correct them (without going into much detail about the power converter), the subject will change. This time, the topology able to interconnect the hybrid systems with the distribution grids shall be addressed. This topology must be able to operate in two different operation modes. On one hand, the rectifier one, taking energy form the grid and sending it to the lamp; on the other hand, the inverter mode, absorbing the energy from the renewable energy sources and injecting it back into the grid. The harmonics reduction is not implemented in this topology although it will be taken into account for future works. The study of the rectifier mode is the second key point in this PhD thesis. A novel control technique for power factor correctors (PFC) for smart-lighting applications with fast dynamics will be developed, being able to reduce both the total harmonic distortion (THD) and the harmonic content generated. In addition, it allows answering faster to disturbances that might affect the DC bus voltage that interfaces the LED lamps with the grid, like the flicker, harmonics coming from other non-linear loads, etc. This technique is employed in the bidirectional converter that interconnects the grid with the hybrid system, although the combination with the renewable energy sources is not addressed yet in this section. Finally, after deeply studying of the rectifier mode, which basically is studying the bidirectional converter operating as a PFC with a fast dynamics control technique, the inverter mode will be also evaluated, injecting the power coming from the renewables into the grid. Nevertheless, the idea behind this analysis is to interchange between both operation modes, rectifier and inverter, swiftly and automatically, avoiding turning off the power converter that interconnects the grid with the hybrid system, like it was done in past works. Therefore, a control strategy with a unified switching pattern for the semiconductors valid for both operation modes will be evaluated. This unique pattern allows the bidirectional converter (that is actually a modified Flyback connected directly to the AC grid without an input diode bridge) to behave as a full bridge (H-bridge), moving the energy in every direction depending on the current reference sign. In this case, however, a galvanic isolation and the extra degree of freedom of the turns ratio of the high frequency transformer of the Flyback is included, allowing for a low voltage in the DC bus. As a matter of fact, due to the stochastic nature of the renewable energy sources, robust control strategies need to be present in order to reliably integrate them in the utility grid. This control, whether it is done properly, implies: • Lower efforts in the power equipment. • High bandwidth and thus, faster response to disturbances and failures. • Allowance of the LED lamps with non-electrolytic capacitors and a high ripple and hence, longer life span. After explaining all the key issues that will be addressed in this PhD work, its structure is detailed as follows: In Chapter 1, a brief introduction to artificial lighting is outlined. Afterwards, the most common light sources are listed and reviewed. LEDs are then introduced and thoroughly described. In addition, the most important issues about LED lighting will be discussed, paying special attention to their characteristics as power loads. It will be shown how this characteristic greatly differs from LEDs to incandescent or gas-discharge lamps. In Chapter 2, key concepts such as power factor and total harmonic distortion, and standards related to power converters dedicated to lighting systems like the IEC 61000-3-2: 2005 will be covered. Chapter 3 shall deal with a kind of non-linear loads, particularly electromagnetic ballasts, which cause a great distortion in the grid. Real ballasts will be analysed and their models will be obtained in order to perform simulations. Afterwards, different control techniques will be considered in order to compensate the effect of these loads. Then, Chapter 4 is intended to check the effect of non-linear loads in single-phase microgrids and minigrids, mostly operating in island mode. Two control schemes shall be proposed in order to correct the effect of the harmonics into these grids. They will be compared in terms of efficiency, complexity and computational cost. In Chapter 5, a brief introduction to renewable energy sources compatible with street lampposts shall be discussed and the most common ways of interconnection with the AC grid. Chapter 6 will show a brief state of the art about power factor correctors: most known topologies, most common control techniques, operation modes and different architectures. In Chapter 7 it will be studied a new control methodology for power factor correctors. A brief background about the bidirectional Flyback can be found in this chapter, together with the mathematical development of the model and a full description of the control scheme. Finally, experimental results will be shown. Chapter 8 will deal with a unified switching technique of the bidirectional Flyback semiconductors, useful either in rectifier (when the energy is sent from the grid to the loads) and inverter (when the energy is sent from the renewable energy source to the utility mains) mode. A mapping of the switching patterns in both modes will be done and then a common switching map shall be found. Then, simulation and experimental results are shown in order to test the idea. Finally, Chapter 9 gathers the obtained conclusions through the development of this PhD thesis and the possible future developments that would improve the quality of this work. Key words: active filter, bidirectional converters, bidirectional grid interface stages, converter modelling, DCM converters, distributed power generation, electromagnetic ballasts, fast dynamics PFC, grid-tie inverter, multifunctional systems, power electronics, Power Factor Correction, single phase microgrid, street lighting.