Control directo de potencia de convertidores electrónicos conectados a la red

Los convertidores electrónicos de potencia se han consolidado como un elemento fundamental en los sistemas eléctricos, ya que hacen posible una transmisión más flexible de la potencia por el sistema eléctrico y pueden actuar como compensadores. El constante incremento del número de plantas de generación a partir de fuentes renovables de energía, principalmente eólica y solar fotovoltaica, que requieren convertidores electrónicos para evacuar a la red la potencia generada, ha propiciado un creciente interés en sus técnicas de control. Para que su funcionamiento sea óptimo, es necesario diseñar sistemas de control robustos y con una respuesta dinámica muy rápida. El Control Directo de Potencia se presenta como una técnica de control muy adecuada para cumplir estos requisitos, ya que las variables de control son directamente las potencias intercambiadas. Además, el continuo incremento de la potencia unitaria de los sistemas de generación ha dado lugar a la aparición de los convertidores multinivel como la solución idónea para su conexión a redes de tensiones mayores, como la red de distribución o la red de transmisión. El objetivo principal de esta tesis es desarrollar una nueva estrategia de Control Directo de Potencia para convertidores multinivel. Esta nueva estrategia supone un cambio respecto a métodos anteriores de control directo, ya que no se basa en tablas de diseño para la obtención de los vectores de tensión del convertidor, sino en criterios de decisión a partir de consideraciones geométricas. Además, el nuevo método integra el control de la tensión en los puntos intermedios del enlace de continua, evitándose así la inclusión de reguladores adicionales. La implementación en tiempo real de este control en un banco de ensayos en el laboratorio confirma la validez del método. Por otro lado, el incremento de la potencia generada a partir de fuentes renovables ha provocado la aparición de nuevas normativas más estrictas para la conexión a la red, ya que una pérdida repentina de esas cantidades de potencia podría inestabilizar el sistema. En relación a esto, en esta tesis se propone una modificación del Control Directo de Potencia para poder inyectar intensidades sinusoidales ante desequilibrios en la red. Nuevamente, el método se valida experimentalmente en tiempo real en el banco de ensayos del laboratorio. Con los resultados obtenidos se ha conseguido dar mayor versatilidad al Control Directo de Potencia, que se puede aplicar a convertidores multinivel sin necesidad de diseñar nuevas tablas y que puede inyectar intensidades sinusoidales durante un desequilibrio en las tensiones de la red. ____________________________________________Power electronic converters have become a fundamental component in modern utilities. The increasing number of renewable energy generation plants, mainly wind farms and solar photovoltaic power plants, which must be connected to the grid through converters, has caused great interest in their control methods. Besides, power converters make a more flexible power transmission possible and can act as compensators. In order to get an optimal performance, very fast and robust control methods must be designed. Direct Power Control appears to be very adequate to fulfil these requirements, as active and reactive powers are their control variables. Moreover, the increasing amount of power from renewable sources has yielded multilevel converters as a solution for connecting to higher voltage grids, such as distribution or transmission. The main objective of this thesis is to develop a new strategy of Direct Power Control for multilevel converters. This new strategy is no longer based on tables and sector division, but on decision criteria based on geometrical considerations. Besides, this new method includes middle point voltage control, avoiding the use of more controllers. Real-time implementation in a laboratory set-up has validated the proposed control. Likewise, due to the increasing amount of power from renewable sources, new grid codes have appeared, so that a sudden loss of power caused by a transient fault would not make the system unstable. Regarding this, a new Direct Power Control strategy under unbalanced grid voltages has been proposed. The aim is to inject sinusoidal currents in an unbalanced grid. Again, real-time implementation has validated the proposed control method. With theses results, a more versatile Direct Power Control method has been achieved, being applicable to multilevel converters, as well as under voltage imbalance

Sequence Control Strategy for Grid-Forming Voltage Source Converters Based on the Virtual-Flux Orientation under Balanced and Unbalanced Faults

Renewable power generation has increased in recent years, which has led to a decrease in the use of synchronous generators (SGs). These power plants are mainly connected to the power system through electronic converters. One of the main differences between electronic converters connected to power systems and SGs connected to the grid is the current contribution during faults, which can have an impact on protection systems. New grid codes set requirements for fast current injection, but the converters' maximum current limitation during faults make it challenging to develop control strategies for such current contribution. This paper presents a positive and negative sequence current injection strategy according to the new Spanish grid code requirements for the novel grid-forming converter control algorithm based on virtual-flux orientation. The behavior of the proposed strategy is tested in a hardware in the loop (HiL) experimental set-up under balanced faults, meaning that the fault is symmetrically distributed among the three phases, and unbalanced faults, where the fault current is distributed asymmetrically between the phases.This paper was supported by the Spanish Research Agency under project reference PID2019-106028RB-I00/AEI/10.13039/501100011033

Improving the inertial response of a Grid-Forming voltage source converter

This article belongs to the Special Issue Power Converter Design, Control and Applications.In recent years, the use of synchronous generators (SGs) has been displaced due to the increased use of renewable energy sources. These types of plants mostly use power electronic converters to connect to power grids, which, due to their mode of operation, cannot provide the same services. This paper analyzes the synchronization of Grid-Forming converters (GFM) without phase-locked loop (PLL) through the active power control loop. Stability analysis shows that when increasing the emulated moment of inertia in a voltage source converter (VSC) using grid-forming control, the system becomes oscillatory. The paper proposes a novel compensation mechanism in order to damp the system oscillation, allowing the implementation of inertia emulation. Finally, the real-time implementation is executed using a Hardware in the Loop experimental set-up. The response of VSC under grid disturbances is simulated in a real time simulator, while the proposed control system is implemented in a real-time controller platform.This paper was supported by the Spanish Research Agency under project reference PID2019-106028RB-I00/AEI/10.13039/501100011033

A Voltage and Frequency Control Strategy for Stand-Alone Full Converter Wind Energy Conversion Systems

This paper addresses the design and analysis of a voltage and frequency control (VFC) strategy for full converter (FC)-based wind energy conversion systems (WECSs) and its applicability for the supply of an isolated load. When supplying an isolated load, the role of the back-to-back converters in the FC must change with respect to a grid-connected application. Voltage and frequency are established by the FC line side converter (LSC), while the generator side converter (GSC) is responsible for maintaining constant voltage in the DC link. Thus, the roles of the converters in the WECS are inverted. Under such control strategies, the LSC will automatically supply the load power and hence, in order to maintain a stable operation of the WECS, the wind turbine (WT) power must also be controlled in a load-following strategy. The proposed VFC is fully modelled and a stability analysis is performed. Then, the operation of the WECS under the proposed VFC is simulated and tested on a real-time test bench, demonstrating the performance of the VFC for the isolated operation of the WECS

A Voltage and Frequency Control Strategy for Stand-Alone Full Converter Wind Energy Conversion Systems

This paper addresses the design and analysis of a voltage and frequency control (VFC) strategy for full converter (FC)-based wind energy conversion systems (WECSs) and its applicability for the supply of an isolated load. When supplying an isolated load, the role of the back-to-back converters in the FC must change with respect to a grid-connected application. Voltage and frequency are established by the FC line side converter (LSC), while the generator side converter (GSC) is responsible for maintaining constant voltage in the DC link. Thus, the roles of the converters in the WECS are inverted. Under such control strategies, the LSC will automatically supply the load power and hence, in order to maintain a stable operation of the WECS, the wind turbine (WT) power must also be controlled in a load-following strategy. The proposed VFC is fully modelled and a stability analysis is performed. Then, the operation of the WECS under the proposed VFC is simulated and tested on a real-time test bench, demonstrating the performance of the VFC for the isolated operation of the WECS

Contemporary use of cefazolin for MSSA infective endocarditis: analysis of a national prospective cohort

Objectives: This study aimed to assess the real use of cefazolin for methicillin-susceptible Staphylococcus aureus (MSSA) infective endocarditis (IE) in the Spanish National Endocarditis Database (GAMES) and to compare it with antistaphylococcal penicillin (ASP). Methods: Prospective cohort study with retrospective analysis of a cohort of MSSA IE treated with cloxacillin and/or cefazolin. Outcomes assessed were relapse; intra-hospital, overall, and endocarditis-related mortality; and adverse events. Risk of renal toxicity with each treatment was evaluated separately. Results: We included 631 IE episodes caused by MSSA treated with cloxacillin and/or cefazolin. Antibiotic treatment was cloxacillin, cefazolin, or both in 537 (85%), 57 (9%), and 37 (6%) episodes, respectively. Patients treated with cefazolin had significantly higher rates of comorbidities (median Charlson Index 7, P <0.01) and previous renal failure (57.9%, P <0.01). Patients treated with cloxacillin presented higher rates of septic shock (25%, P = 0.033) and new-onset or worsening renal failure (47.3%, P = 0.024) with significantly higher rates of in-hospital mortality (38.5%, P = 0.017). One-year IE-related mortality and rate of relapses were similar between treatment groups. None of the treatments were identified as risk or protective factors. Conclusion: Our results suggest that cefazolin is a valuable option for the treatment of MSSA IE, without differences in 1-year mortality or relapses compared with cloxacillin, and might be considered equally effective