Power Converter Resonant Control for an Unbalanced and Non-Constant Frequency Supply.

Distorted voltage supplied as unbalanced and/or non-constant frequency can be found in weak grids, such as microgrids, or systems working in islanding mode. These kinds of systems are more sensitive under load changes. Particularly, an unbalanced voltage supply may be produced for large, single-phase loads. On the other hand, the connection/disconnection of high current loads may lead to important frequency variation, especially in weak grids where the short circuit current capacity is reduced. These conditions make the control of the power converter a more difficult task, because of the variations in the frequency and unbalancing. To address these issues, this paper proposes a resonant control algorithm to deal with variations in the voltage amplitude as well as grid frequency when a distorted power supply is considered. The frequency variation is an important drawback for resonant control because the resonance must be tuned at the grid frequency. This issue is overcome by using a variable sampling frequency in order to avoid re-tuning the controller parameters. On the other hand, under unbalanced conditions, the proposed method relaxes the phase with lower voltage amplitude by taking more power from the other phases in order to help the stability of the grid supply. To corroborate the mathematical analysis and the proposed control, a stability study is performed, including experimental and simulated results.

Stability Analysis of Two Power Converters Control Algorithms Connected to Micro-Grids with Wide Frequency Variation.

Distributed power generation, micro-grids, and networks working in islanding mode have strong deviations in voltage quantities. These deviations can be divided into amplitude and frequency. Amplitude deviations are well-known and studied, as they are common in small and big grids. However, deviations on the ac mains frequency have not been widely studied. The literature shows control schemes capable of bearing these variations, but no systematic analysis has been performed to ensure stability. As the majority of power converters are designed for big grids, their analysis and design neglect frequency disturbances, therefore those devices allow a very small frequency operating window. For instance, in power converters that need to be synchronized to the grid, the standard deviation does not go beyond 0.5 Hz, and for grid-tied inverters it does not go beyond 1 Hz, whereas variations of around 8 Hz can be expected in micro-grids. This work presents a comprehensive analysis of the control system's stability, where two different control schemes for a back-to-back static converter topology are implemented and studied under a wide variable grid frequency. Because the behavior of power converters is nonlinear and coupled, dynamic and static decouplers are usually introduced in the controller, being a key element on the scheme according to the findings. The results show that using just a static decoupler does not guarantee stability under frequency variations; meanwhile, when a dynamic decoupler is used, the operating window can be greatly extended. The procedure shown in this paper can also be extended to other control algorithms, making it possible to carefully choose the control system for a variable frequency condition. Simulated and experimental results confirm the theoretical approach.

Incidencia del alcoholismo y drogadiccion en jovenes pre-bachilleres de la Ciudad de Oruro / Incidence of alcoholism and drugadicction in youngstere of the city of Oruro

Se efectuo una encuesta en 1042 jovenes pre-bachilleres para conocer los verdaderos indices de alcoholismo y drogadiccion en la ciudad de Oruro, donde se encontro resultados alarmantes ya que el 30 por ciento mas o menos de los encuestados consumen alcohol actualmente y el 8.15 por ciento lo hace rutinariamente (diario o semanal), demostrando que el alcoholismo empieza en edades verdaderamente tempranas y antes de la amyoria de edad, haciendose claro el vacio legal que hay en este sentido. Se ha confirmado que las otras drogas tales como la marihuana, cocaina, sedantes, gasolina y otros tienen un porcentaje de 2.40 por ciento del universo cuestionado (1). No se ha podido demostrar que la presencia de alcoholismo en la familia sea un factor determinante en la induccion de los jovenes yaque los amigos huegasn un papel muy importante en este sentido. Los varones son porcentalmente mayoria en el consumo de alcohol y otras drogas. En los colegios privados tambien este porcentaje es mayor en cuanto al consumo de los mismos.