1,510 research outputs found
H∞ based control of a DC/DC buck converter feeding a constant power load in uncertain DC microgrid system
DC microgrids are gaining more and more popularity and are becoming a more viable alternative to AC microgrids (MGs) due to their advantages in terms of simpler power converter stages, flexible control algorithms and the absence of synchronization and reactive power. However, DC-MGs are prone to instability issues associated with the presence of nonlinear loads such as constant power loads (CPL) known by their incremental negative impedance (INI), which may lead to voltage collapse of the main DC Bus. In this paper, -based controller of a source side buck converter is designed to avoid the instability issues caused by the load-side converter acting as a CPL. Besides, the proposed controller allows a perfect rejection of all perturbations that may arise from parameter variations, input voltage and CPL current fluctuations. The design process of H-based controller is based on the Golver Doyle Optimization Algorithm (GDOA), which requires an augmented system extracted from the small-signal model of the DC/DC converter including the mathematical model of parameter variations and overall external perturbations. The based controller involves the use of weight functions in order to get the desired performances. The proposed controller is easy to implement and lead to reducing the implementation cost and avoid the use of current measurement that may have some disadvantages. The derived controller is validated by simulation performed in Psim software and experimental setup
Control of AC/DC microgrids with renewables in the context of smart grids including ancillary services and electric mobility
Microgrids are a very good solution for current problems raised by the constant growth
of load demand and high penetration of renewable energy sources, that results in grid
modernization through “Smart-Grids” concept. The impact of distributed energy sources
based on power electronics is an important concern for power systems, where natural
frequency regulation for the system is hindered because of inertia reduction. In this context,
Direct Current (DC) grids are considered a relevant solution, since the DC nature of power
electronic devices bring technological and economical advantages compared to Alternative
Current (AC). The thesis proposes the design and control of a hybrid AC/DC Microgrid
to integrate different renewable sources, including solar power and braking energy recovery
from trains, to energy storage systems as batteries and supercapacitors and to loads like
electric vehicles or another grids (either AC or DC), for reliable operation and stability.
The stabilization of the Microgrid buses’ voltages and the provision of ancillary services
is assured by the proposed control strategy, where a rigorous stability study is made.
A low-level distributed nonlinear controller, based on “System-of-Systems” approach is
developed for proper operation of the whole Microgrid. A supercapacitor is applied to
deal with transients, balancing the DC bus of the Microgrid and absorbing the energy
injected by intermittent and possibly strong energy sources as energy recovery from the
braking of trains and subways, while the battery realizes the power flow in long term.
Dynamical feedback control based on singular perturbation analysis is developed for
supercapacitor and train. A Lyapunov function is built considering the interconnected
devices of the Microgrid to ensure the stability of the whole system. Simulations highlight
the performance of the proposed control with parametric robustness tests and a comparison
with traditional linear controller. The Virtual Synchronous Machine (VSM) approach is
implemented in the Microgrid for power sharing and frequency stability improvement. An
adaptive virtual inertia is proposed, then the inertia constant becomes a system’s state
variable that can be designed to improve frequency stability and inertial support, where
stability analysis is carried out. Therefore, the VSM is the link between DC and AC side
of the Microgrid, regarding the available power in DC grid, applied for ancillary services
in the AC Microgrid. Simulation results show the effectiveness of the proposed adaptive
inertia, where a comparison with droop and standard control techniques is conducted.As Microrredes são uma ótima solução para os problemas atuais gerados pelo constante crescimento
da demanda de carga e alta penetração de fontes de energia renováveis, que resulta na modernização
da rede através do conceito “Smart-Grids”. O impacto das fontes de energia distribuídas baseados
em eletrônica de potência é uma preocupação importante para o sistemas de potência, onde a
regulação natural da frequência do sistema é prejudicada devido à redução da inércia. Nesse
contexto, as redes de corrente contínua (CC) são consideradas um progresso, já que a natureza
CC dos dispositivos eletrônicos traz vantagens tecnológicas e econômicas em comparação com a
corrente alternada (CA). A tese propõe o controle de uma Microrrede híbrida CA/CC para integrar
diferentes fontes renováveis, incluindo geração solar e frenagem regenerativa de trens, sistemas de
armazenamento de energia como baterias e supercapacitores e cargas como veículos elétricos ou
outras (CA ou CC) para confiabilidade da operação e estabilidade. A regulação das tensões dos
barramentos da Microrrede e a prestação de serviços anciliares são garantidas pela estratégia
de controle proposta, onde é realizado um rigoroso estudo de estabilidade. Um controlador não
linear distribuído de baixo nível, baseado na abordagem “System-of-Systems”, é desenvolvido para
a operação adequada de toda a rede elétrica. Um supercapacitor é aplicado para lidar com os
transitórios, equilibrando o barramento CC da Microrrede, absorvendo a energia injetada por fontes
de energia intermitentes e possivelmente fortes como recuperação de energia da frenagem de trens
e metrôs, enquanto a bateria realiza o fluxo de potência a longo prazo. O controle por dynamical
feedback baseado numa análise de singular perturbation é desenvolvido para o supercapacitor e
o trem. Funções de Lyapunov são construídas considerando os dispositivos interconectados da
Microrrede para garantir a estabilidade de todo o sistema. As simulações destacam o desempenho
do controle proposto com testes de robustez paramétricos e uma comparação com o controlador
linear tradicional. O esquema de máquina síncrona virtual (VSM) é implementado na Microrrede
para compartilhamento de potência e melhoria da estabilidade de frequência. Então é proposto o
uso de inércia virtual adaptativa, no qual a constante de inércia se torna variável de estado do
sistema, projetada para melhorar a estabilidade da frequência e prover suporte inercial. Portanto,
o VSM realiza a conexão entre lado CC e CA da Microrrede, onde a energia disponível na rede CC
é usada para prestar serviços anciliares no lado CA da Microrrede. Os resultados da simulação
mostram a eficácia da inércia adaptativa proposta, sendo realizada uma comparação entre o
controle droop e outras técnicas de controle convencionais
Contributions to impedance shaping control techniques for power electronic converters
El conformado de la impedancia o admitancia mediante control para convertidores electrónicos de potencia permite alcanzar entre otros objetivos: mejora de la robustez de los controles diseñados, amortiguación de la dinámica de la tensión en caso de cambios de carga, y optimización del filtro de red y del controlador en un solo paso (co-diseño). La conformación de la impedancia debe ir siempre acompañada de un buen seguimiento de referencias. Por tanto, la idea principal es diseñar controladores con una estructura sencilla que equilibren la consecución de los objetivos marcados en cada caso. Este diseño se realiza mediante técnicas modernas, cuya resolución (síntesis del controlador) requiere de herramientas de optimización. La principal ventaja de estas técnicas sobre las clásicas, es decir, las basadas en soluciones algebraicas, es su capacidad para tratar problemas de control complejos (plantas de alto orden y/o varios objetivos) de una forma considerablemente sistemática. El primer problema de control por conformación de la impedancia consiste en reducir el sobreimpulso de tensión ante cambios de carga y minimizar el tamaño de los componentes del filtro pasivo en los convertidores DC-DC. Posteriormente, se diseñan controladores de corriente y tensión para un inversor DC-AC trifásico que logren una estabilidad robusta del sistema para una amplia variedad de filtros. La condición de estabilidad robusta menos conservadora, siendo la impedancia de la red la principal fuente de incertidumbre, es el índice de pasividad. En el caso de los controladores de corriente, el impacto de los lazos superiores en la estabilidad basada en la impedancia también se analiza mediante un índice adicional: máximo valor singular. Cada uno de los índices se aplica a un rango de frecuencias determinado. Finalmente, estas condiciones se incluyen en el diseño en un solo paso del controlador de un convertidor back-to-back utilizado para operar generadores de inducción doblemente alimentados (aerogeneradores tipo 3) presentes en algunos parques eólicos. Esta solución evita los problemas de oscilación subsíncrona, derivados de las líneas de transmisión con condensadores de compensación en serie, a los que se enfrentan estos parques eólicos. Los resultados de simulación y experimentales demuestran la eficacia y versatilidad de la propuesta.Impedance or admittance shaping by control for power electronic converters allows to
achieve among other objectives: robustness enhancement of the designed controls, damped
voltage dynamics in case of load changes, and grid filter and controller optimization in
a single step (co-design). Impedance shaping must always be accompanied by a correct
reference tracking performance. Therefore, the main idea is to design controllers with a
simple structure that balance the achievement of the objectives set in each case. This
design is carried out using modern techniques, whose resolution (controller synthesis)
requires optimization tools. The main advantage of these techniques over the classical
ones, i.e. those based on algebraic solutions, is their ability to deal with complex control
problems (high order plants and/or several objectives) in a considerably systematic way.
The first impedance shaping control problem is to reduce voltage overshoot under load
changes and minimize the size of passive filter components in DC-DC converters. Subsequently,
current and voltage controllers for a three-phase DC-AC inverter are designed
to achieve robust system stability for a wide variety of filters. The least conservative
robust stability condition, with grid impedance being the main source of uncertainty, is
the passivity index. In the case of current controllers, the impact of higher loops on
impedance-based stability is also analyzed by an additional index: maximum singular
value. Each of the indices is applied to a given frequency range. Finally, these conditions
are included in the one-step design of the controller of a back-to-back converter used
to operate doubly fed induction generators (type-3 wind turbines) present in some wind
farms. This solution avoids the sub-synchronous oscillation problems, derived from transmission
lines with series compensation capacitors, faced by these wind farms. Simulation
and experimental results demonstrate the effectiveness and versatility of the proposa
Plug-and-play and coordinated control for bus-connected AC islanded microgrids
This paper presents a distributed control architecture for voltage and
frequency stabilization in AC islanded microgrids. In the primary control
layer, each generation unit is equipped with a local controller acting on the
corresponding voltage-source converter. Following the plug-and-play design
approach previously proposed by some of the authors, whenever the
addition/removal of a distributed generation unit is required, feasibility of
the operation is automatically checked by designing local controllers through
convex optimization. The update of the voltage-control layer, when units plug
-in/-out, is therefore automatized and stability of the microgrid is always
preserved. Moreover, local control design is based only on the knowledge of
parameters of power lines and it does not require to store a global microgrid
model. In this work, we focus on bus-connected microgrid topologies and enhance
the primary plug-and-play layer with local virtual impedance loops and
secondary coordinated controllers ensuring bus voltage tracking and reactive
power sharing. In particular, the secondary control architecture is
distributed, hence mirroring the modularity of the primary control layer. We
validate primary and secondary controllers by performing experiments with
balanced, unbalanced and nonlinear loads, on a setup composed of three
bus-connected distributed generation units. Most importantly, the stability of
the microgrid after the addition/removal of distributed generation units is
assessed. Overall, the experimental results show the feasibility of the
proposed modular control design framework, where generation units can be
added/removed on the fly, thus enabling the deployment of virtual power plants
that can be resized over time
IMPROVEMENT OF POWER SYSTEM QUALITY USING VSC-BASED HVDC TRANSMISSION
The HVDC technology can be represented by the combination of a Direct Current (DC) circuit with two power electronics converters, each one at a link terminal, for AC/DC and DC/AC conversion The principal characteristic of VSC-HVDC transmission is its ability to independently control the reactive and real power flow at each of the AC systems via the Point of Common Coupling (PCC). The active and reactive power is related to the power angle and the magnitude of voltage in the reference -frame selected such that the quadrature component will result in the ratio between the maximum fundamental peak phase voltage and the DC total voltagehttp://dx.doi.org/10.4314/njt.v36i3.3
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