1,332 research outputs found
Battery charging station for electric vehicles based on bipolar dc power grid with grid-to-vehicle, vehicle-to-grid and vehicle-to-vehicle operation modes
This paper proposes an electric vehicle (EV) battery charging station (EV-BCS) based on a bipolar dc power grid with the capabilities of returning energy back to the power grid (vehicle-to-grid – V2G mode), as well as to perform power transfer between different EVs connected to the EV-BCS without drawing power from the power grid (vehicle-to-vehicle – V2V mode), besides the traditional battery charging operation (grid-to-vehicle – G2V mode). The proposed EV-BCS is modular, using three-level bidirectional dc-dc converters. In this paper, for simplicity reasons, only two converters, and hence two EVs, are considered in order to validate the previously referred operation modes. Furthermore, unbalanced operation from the EVs side is also considered for all the operation modes, aiming to consider a real scenario of operation. Simulation results verify the correct operation of the EV-BCS in all cases, with balanced and unbalanced current consumption from the EVs resulting always in balanced currents from the bipolar dc power grid side
Modelling and power-voltage control in unbalanced bipolar multi-terminal HVDC grids
Questa tesi investiga il comportamento della strategia di controllo distribuito di tensione (droop control) in una rete VSC-HVDC bipolare multi-terminale e magliata. In particolare sono studiate le conseguenze de fuori servizio di uno dei convertitori della rete. Infine, è matematicamente descritta l’influenza dei parametri di rete, quali resistenza del percorso di richiusura della corrente, lunghezza delle linee e guadagno di droop, sull’accoppiamento tra i poli del sistema bipolare
Variable-Angle Phase-Shifted PWM for Multilevel Three-Cell Cascaded H-bridge Converters
Multilevel cascaded H-bridge converters have become a mature technology for applications where high-power medium ac voltages are required. Normal operation of multilevel cascaded H-bridge converters assumes that all power cells have the same dc voltage, and each power cell generates the same voltage averaged over a sampling period using a conventional phase-shifted pulse width modulation (PWM) technique. However, this modulation method does not achieve good results under unbalanced operation per H-bridge in the power converter, which may happen in grid-connected applications such as photovoltaic or battery energy storage systems. In the paper, a simplified mathematical analysis of the phase-shifted PWM technique is presented. In addition, a modification of this conventional modulation method using variable shift angles between the power cells is introduced. This modification leads to the elimination of harmonic distortion of low-order harmonics due to the switching (triangular carrier frequency and its multiples) even under unbalanced operational conditions. The analysis is particularized for a three-cell cascaded H-bridge converter, and experimental results are presented to demonstrate the good performance of the proposed modulation method
Ancillary Services in Hybrid AC/DC Low Voltage Distribution Networks
In the last decade, distribution systems are experiencing a drastic transformation
with the advent of new technologies. In fact, distribution networks are no longer passive
systems, considering the current integration rates of new agents such as distributed generation,
electrical vehicles and energy storage, which are greatly influencing the way these systems are
operated. In addition, the intrinsic DC nature of these components, interfaced to the AC system
through power electronics converters, is unlocking the possibility for new distribution topologies
based on AC/DC networks. This paper analyzes the evolution of AC distribution systems,
the advantages of AC/DC hybrid arrangements and the active role that the new distributed agents
may play in the upcoming decarbonized paradigm by providing different ancillary services.Ministerio de Economía y Competitividad ENE2017-84813-RUnión Europea (Programa Horizonte 2020) 76409
Analysis of heterogeneously configured converter stations in HVDC grids under asymmetrical DC operation
Additional technologies different from classical high voltage alternating current (HVAC)
transmission are necessary to deal with the higher renewable energy integration in the
current energetic framework. High voltage direct current (HVDC) transmission based on
modular multilevel voltage source converters (MMC-VSC) is a promising alternative for
some applications. Thus, the number of HVDC projects is increasing worldwide. This
makes possible their future gradual interconnection to constitute an overlay DC grid that
offers numerous additional advantages but still many challenges.
Even if the development of the HVDC technology overcomes all the present challenges
in the future, the lack of standardisation will lead to a DC grid integrated by different
HVDC station topologies, grounding schemes, DC-DC converters, or control strategies.
During normal operation, the DC grid is assumed to work symmetrically, and some
aspects, such as the topology or the grounding scheme, do not intervene in the system
response. However, in case of working asymmetrically due to a fault or outage affecting
a single pole of the DC network, all the aspects mentioned above affect the system
operation.
However, such a heterogeneous DC grid under asymmetrical DC operation has yet to be
addressed in the literature. Thus, it constitutes the general objective of this thesis. To
achieve this objective, the asymmetrical DC operation in different heterogeneous DC
systems is studied using load flow, dynamic EMT simulation, and small-signal stability
analysis. The analysis of a system of these characteristics under asymmetrical DC
operation is an original contribution of the thesis.
First, a DC grid connecting different AC zones and formed by different HVDC station
topologies and DC-DC converters is modelled to perform the load-flow assessment. The
asymmetrical DC operation is examined by causing an asymmetrical contingency in the
DC network. The analysis is carried out considering different grounding resistances,
control strategies, control parameters, and galvanic isolation ability of the DC-DC
converters. The results obtained regarding DC current and voltage asymmetry, which are
related to the overloading of elements and excessive voltage deviation, allow for
assessing the impact of the asymmetrical operation under different circumstances.
Second, the dynamic assessment aims to identify the main aspects involved in the
transient response during asymmetrical DC operation. The connection of a symmetrical
monopolar station to a bipolar system is modelled, and the outage of one of the converters
of a bipolar station is simulated. The effect of the grounding impedance and the control
strategy on the dynamic response of the system is assessed. Therefore, the main system
parameters and issues that may appear are identified. Furthermore, the effect of the
connection of the symmetrical monopole station over the existing protections of the
bipolar system is assessed by considering different grounding impedances in the
monopolar station. Finally, the small-signal analysis of a system composed of different topologies focuses
on the asymmetrical DC operation. A new suitable model is developed and validated
against EMT simulations. The small-signal analysis is carried out, and the main aspects
that impact the small-signal stability during asymmetrical operation are identified.
Furthermore, a new controller that enhances the system stability during asymmetrical DC
operation is developed.Para hacer frente a la mayor integración de energías renovables en el marco energético
actual se necesitan tecnologías adicionales distintas de la transmisión clásica en corriente
alterna en alta tensión (HVAC). La transmisión de corriente continua en alta tensión
(HVDC) basada en convertidores multinivel modulares de fuente de tensión (MMCVSC)
es una alternativa prometedora para algunas aplicaciones. Por tanto, el número de
proyectos HVDC está aumentando en todo el mundo. Esto hace posible que se
interconecten gradualmente en el futuro para formar una red de corriente continua (CC)
que ofrece numerosas ventajas adicionales, pero todavía muchos retos.
Aunque el desarrollo de la tecnología HVDC supere todos los retos actuales en el futuro,
la falta de normalización dará lugar a una red de CC integrada por diferentes topologías
de estaciones HVDC, esquemas de puesta a tierra, convertidores CC-CC o estrategias de
control. Durante el funcionamiento normal, la red de CC funciona simétricamente y
algunos aspectos, como la topología o el esquema de puesta a tierra, no intervienen en la
respuesta del sistema. Sin embargo, en caso de funcionamiento asimétrico, debido a una
falta o desconexión que afecte a un solo polo de la red de CC, todos los aspectos
mencionados anteriormente afectan al funcionamiento del sistema.
Este tipo de red de CC heterogénea en funcionamiento asimétrico aún no se ha abordado
en el estado del arte. Por ello, constituye el objetivo general de esta tesis. Para lograr este
objetivo, se estudia el funcionamiento asimétrico de CC en diferentes sistemas
heterogéneos de CC utilizando diferentes enfoques como el flujo de cargas, la simulación
dinámica EMT y el análisis de estabilidad de pequeña señal. El análisis de un sistema de
estas características en funcionamiento asimétrico en CC constituye la principal
contribución de la tesis.
Para realizar la evaluación del flujo de cargas, se modela una red de CC que conecta
diferentes zonas de CA y está formada por diferentes topologías de estaciones HVDC y
convertidores CC-CC. A continuación, se examina el funcionamiento asimétrico de CC
provocando una contingencia asimétrica en la red de CC. El análisis se lleva a cabo
considerando diferentes resistencias de puesta a tierra, estrategias de control, parámetros
de control y capacidad de aislamiento galvánico de los convertidores CC-CC. Los
resultados obtenidos sobre la asimetría de corriente y tensión en CC, relacionados con la
sobrecarga de los elementos y la desviación excesiva de la tensión, permiten evaluar el
impacto del funcionamiento asimétrico en distintas circunstancias.
La evaluación dinámica pretende identificar los principales aspectos que intervienen en
la respuesta transitoria durante el funcionamiento asimétrico en CC. En primer lugar, se
modela la conexión de una estación monopolar simétrica a un sistema bipolar. A
continuación, se simula la interrupción de uno de los convertidores de una estación
bipolar y se evalúa el efecto de la impedancia de puesta a tierra y de la estrategia de
control en la respuesta dinámica del sistema. Por último, se identifican los principales parámetros del sistema y los problemas que pueden aparecer. Además, se evalúa el efecto
de la conexión de la estación monopolar simétrica sobre las protecciones existentes del
sistema bipolar, considerando diferentes impedancias de puesta a tierra en la estación
monopolar.
Por último, se realiza el análisis de pequeña señal de un sistema compuesto por diferentes
topologías centrándose en el funcionamiento asimétrico en CC. Para ello, primero se
desarrolla un nuevo modelo adecuado para este análisis y se valida con simulaciones
EMT. A continuación, se lleva a cabo el análisis de pequeña señal y se identifican los
principales aspectos que afectan a la estabilidad de pequeña señal durante el
funcionamiento asimétrico. Además, se desarrolla un nuevo controlador que mejora la
estabilidad del sistema durante el funcionamiento asimétrico en CC.Programa de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de MadridPresidente: José Luis Rodríguez Amenedo.- Secretario: Eduardo Prieto Araujo.- Vocal: Dunixe Marene Larruskain Escoba
Effects of grounding configurations on post-contingency performance of MTDC system: a 3-terminal example
The grounding system is extremely important, as it affects the performance of the MTDC system virtually in any possible mode: normal (asymmetrical operation) and abnormal operation (faults), steady-state and dynamic. The objective of this paper is to introduce a simple approach to assess the steady-state post-contingency of multi-Terminal HVDC System and uses it order to illustrate the effects of grounding configurations on steady-state post-contingency performance. A 3-terminal HVDC system is used to formulate the main theoretical framework for performance prediction on post-contingency steady-state of MTDC system as well as for demonstrative purposes
Power Quality Enhancement in Electricity Grids with Wind Energy Using Multicell Converters and Energy Storage
In recent years, the wind power industry is experiencing a rapid growth and more wind farms with larger size wind turbines are being connected to the power system. While this contributes to the overall security of electricity supply, large-scale deployment of wind energy into the grid also presents many technical challenges. Most of these challenges are one way or another, related to the variability and intermittent nature of wind and affect the power quality of the distribution grid. Power quality relates to factors that cause variations in the voltage level and frequency as well as distortion in the voltage and current waveforms due to wind variability which produces both harmonics and inter-harmonics. The main motivation behind work is to propose a new topology of the static AC/DC/AC multicell converter to improve the power quality in grid-connected wind energy conversion systems. Serial switching cells have the ability to achieve a high power with lower-size components and improve the voltage waveforms at the input and output of the converter by increasing the number of cells. Furthermore, a battery energy storage system is included and a power management strategy is designed to ensure the continuity of power supply and consequently the autonomy of the proposed system. The simulation results are presented for a 149.2 kW wind turbine induction generator system and the results obtained demonstrate the reduced harmonics, improved transient response, and reference tracking of the voltage output of the wind energy conversion system.Peer reviewedFinal Accepted Versio
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