61 research outputs found
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
Control strategy for direct voltage and frequency stabilityenhancement in HVAC/HVDC grids
Direct voltage fluctuations due to the presence of relatively large DC reactors (as an essen-tial part of HVDC breakers), lack of inertia, and unwanted frequency fluctuations in theAC side of HVDC grids, have major consequences on the stability of HVAC/HVDC grids.The use of the DC Power System Stabilizer (DC-PSS) can damp and eliminate voltageoscillations caused by the presence of the DC reactors. However, DC-PSS cannot addressthe issues of inertia and unwanted frequency fluctuations. A method to improve inertiais proposed here that can operate well with the droop controller, and DC-PSS does notinterfere with power-sharing and does not interact with any of these elements. Since thepresence of a droop controller in HVAC/HVDC grids associates with power and directvoltage, the method proposed here can improve direct voltage fluctuations by eliminatingsevere power peaks. Moreover, this method does not change the voltage level of the entiresystem, so there is no need to change the set-points of controllers. In addition, all param-eters of the controllers are tuned by an intelligent algorithm, and the Participation factor(PF) scheme is used to find the proper placement of the proposed controller
Stability enhancement of HVAC grids using HVDC links.
M. Sc. Eng. University of KwaZulu-Natal, Pietermaritzburg 2016.Eskom is facing challenging times where the national power grid is placed under extreme
pressure, therefore, the long existing poorly damped low frequency inter area oscillations affects
the stability constraints thus reducing the power transfer capacity. Consequently new power
stations are being built in remote locations to reduce the short fall of generation capacity and the
HVDC technology has become appealing to transport large amount of power over long distance.
This research aims to prove that stability enhancement of parallel AC systems can be achieved
with the use of HVDC schemes.
The HVDC system has the rapid ability to control the transmitted power during transient
disturbances and this power system control has a significant effect on the dynamic performance
of the system after a disturbance therefore the dynamic performance is related to the small
signal stability, where the rotor oscillations are minimised and the system is brought back to
steady state after an event or disturbance.The fundamentals of small signal stability in terms of
observability, controllability, residues, network sensitivities and mode shape are explained
together with a dominant oscillation path definition for HVDC links location selection. The key
importance in controlling the power of the HVDC link to affect stability requires that the
oscillation is observable and controllable. Simulation results on a simple four-generator, twoarea
test system are presented, with a view to benchmark the results and develop a fundamental
understanding of how using HVDC links for power transfer can stabilise the grid.
The eigenvalue analysis of the system indicates the frequency of oscillations in the system and
the generator’s participation factors, together with the controllability and observability of the
inter area mode (mode of interest). There are a number of test simulations results from a LCCHVDC
system (First Cigrê benchmark model) integrated into a test network where the influence
on the small signal stability is analysed. Various literature has been reviewed which supports
the basic principles, promoting the benefits of using HVDC systems to enhance stability of a
parallel AC system (Hybrid) and then integrating supplementary control.
This research investigates the use of the HVDC system to enhance the small signal stability
with supplementary control which is termed predictive control. Power Oscillation Damping
(POD) control through LCC HVDC links is studied to ensure secure operation of power
systems. The Power oscillating damper is expressed as a transfer function whereas the MPC
(Model Predictive Controller) is expressed as cost functions of a feedback signal which is a
measured quantity. Two feedback signals are selected and their effectiveness with regard to their contribution to the damping of the system is investigated. The controller feedback signals
are real power and voltage difference across the AC tie lines. Bode plots, root locus plots and
time domain simulation results show the comparison between the different selected controller
inputs and supplementary controls. The voltage angle difference is most effective as it is more
sensitive to changes in the system and assists the controller in bringing the system to steady
state in a shorter period of time when compared to the controller input that uses real power
across the AC tie line.
The controllers with the HVDC integrated, do improve the damping of the system and it is
related to shorter mode decay time, the MPC however has been investigated to reduce the
change of loading levels of the AC tie lines following a change in system operating conditions.
Simulation responses from the research show that this method is more promising and does not
require prior knowledge of the possible contingencies due to its ability to handle complex multi
variable systems with constraints, by using cost function algorithms to perform predictions of
future plant behaviour and calculating the suitable corrective control actions needed to take the
predicted output as close as possible to the target value which is the steady state. This research
however demonstrates the fundamental principle which proves that the HVDC together with
supplementary control can enhance stability of a parallel AC system
Active Power Balancing in Multi-Terminal DC Systems
The present work deals with active power balancing in embedded multi-terminal dc systems. The derived controller and grid control concept is able to achieve a significant improvement in power transmission deviations and excitation of oscillatory modes in the case of a converter outage.
Starting point for the design is the derivation of control requirements based on an analysis of state of the art converter control and dc grid control concepts in the context of active power balancing. The basic requirements are subsequently developed into design principles for a generalized controller and a derived tiered grid control concept.
The controller is designed based on a continuous integration of constant voltage, voltage droop and constant power control. This is achieved by a construction of the control characteristic and the subsequent gain scheduling
of the individual loops as well as the PI control gain.
Finally, the grid concept is described and simulated on a number of benchmark cases in order to show the effectiveness of the proposed controller and grid concept. Results show, that the defined control objectives can be achieved, i.e. the controller implements the grid concept, the system is stable for all operating points and power transmission deviations and the excitation of oscillatory modes is kept at a minimum
Small signal stability analysis of proportional resonant controlled VSCs connected to AC grids with variable X/R characteristic
Capítuos 2,3 y 4 confidenciales por patente.-- Tesis completa 237.p. Tesis censurada 120 p.Para garantizar un futuro energético sostenible, es fundamental la incorporación de energías renovables en la red eléctrica. Sin embargo, con su creciente integración, las redes eléctricas AC se están volviendo cada vez más débiles, más complejas y caóticas. Por ello, se hace imprescindible el estudio de los retos técnicos que dicha integración plantea. Fenómenos como la desconexión de líneas AC, el bloqueo de convertidores, o variaciones de carga debidas a las intermitencias de la generación renovable, están comenzando a producir cambios en los valores de impedancia y en las características inductivo-resistivas de incluso las redes fuertes. Conforme una red AC se debilita su impedancia equivalente aumenta, y esto provoca cambios indeseados en las magnitudes de potencia activa y reactiva, que derivan en variaciones repentinas de tensión en diferentes puntos de la red AC. Esto también conlleva el deterioro de los convertidores y empeoramiento de la calidad de onda. Una solución parcial a este problema es limitar la potencia allí donde se genera, en perjuicio de aumentar las pérdidas locales. Otra solución es introducir controles de convertidores más robustos, para que sean capaces de sortear estos escenarios cada vez más frecuentes. En este contexto, los convertidores de fuente de tensión (VSC), y en especial los convertidores modulares multinivel, presentan una serie de prestaciones que los hacen idóneos para esta clase de escenarios, dado su mejor comportamiento dinámico frente a los convertidores de fuente de corriente, al operar a una frecuencia de conmutación mayor, y presentando capacidades LVRT y control desacoplado de potencia activa y reactiva. Entre los controles internos de corriente de los VSCs, los controladores proporcional resonantes han aparecido como alternativa a los proporcional integrales, debido a su capacidad de manejar operación tanto equilibrada como desequilibrada y a que eliminan lanecesidad de utilizar un phase-locked loop y las transformadas de Park. Muy pocos estudios se han realizado con VSCs con control proporcional resonante sujetos a cambios en la fortaleza de la red AC, y menos aun considerando la variación de su característica inductivo-resistiva. Por lo tanto, en esta tesis doctoral se propone una metodología de parametrización del control proporcional resonante de un VSC conectado a una red AC con fortaleza y característica inductivo-resistiva variables, que asegure su estabilidad en pequeña señal. Con el objetivo de caracterizar dicha estabilidad, se construye un modelo de pequeña señal del sistema compuesto por el VSC conectado a red AC. Posteriormente se valida con simulaciones EMT y se procede con el análisis de escenarios. Los resultados del análisis demuestran que tan solo una desviación del 20% en el ratio X/R de la red AC con respecto a su valor habitual puede hacer perder al sistema su estabilidad en pequeña señal cuando la red AC es débil. La metodología propone nuevas parametrizaciones del control proporcional resonante del VSC que devuelven la estabilidad al sistema en estos escenarios. La validación y verificación de la metodología se realiza a través de un caso de estudio en DIgSILENT PF: una planta de generación eólica marina que evacúa energía a la red AC por medio de un enlace de alta tensión en continua
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