Lyapunov Functions Family Approach to Transient Stability Assessment

Analysis of transient stability of strongly nonlinear post-fault dynamics is one of the most computationally challenging parts of Dynamic Security Assessment. This paper proposes a novel approach for assessment of transient stability of the system. The approach generalizes the idea of energy methods, and extends the concept of energy function to a more general Lyapunov Functions Family (LFF) constructed via Semi-Definite-Programming techniques. Unlike the traditional energy function and its variations, the constructed Lyapunov functions are proven to be decreasing only in a finite neighborhood of the equilibrium point. However, we show that they can still certify stability of a broader set of initial conditions in comparison to the traditional energy function in the closest-UEP method. Moreover, the certificates of stability can be constructed via a sequence of convex optimization problems that are tractable even for large scale systems. We also propose specific algorithms for adaptation of the Lyapunov functions to specific initial conditions and demonstrate the effectiveness of the approach on a number of IEEE test cases

Randomized methods to characterize large-scale vortical flow networks.

We demonstrate the effective use of randomized methods for linear algebra to perform network-based analysis of complex vortical flows. Network theoretic approaches can reveal the connectivity structures among a set of vortical elements and analyze their collective dynamics. These approaches have recently been generalized to analyze high-dimensional turbulent flows, for which network computations can become prohibitively expensive. In this work, we propose efficient methods to approximate network quantities, such as the leading eigendecomposition of the adjacency matrix, using randomized methods. Specifically, we use the Nyström method to approximate the leading eigenvalues and eigenvectors, achieving significant computational savings and reduced memory requirements. The effectiveness of the proposed technique is demonstrated on two high-dimensional flow fields: two-dimensional flow past an airfoil and two-dimensional turbulence. We find that quasi-uniform column sampling outperforms uniform column sampling, while both feature the same computational complexity

Randomized methods to characterize large-scale vortical flow network

We demonstrate the effective use of randomized methods for linear algebra to perform network-based analysis of complex vortical flows. Network theoretic approaches can reveal the connectivity structures among a set of vortical elements and analyze their collective dynamics. These approaches have recently been generalized to analyze high-dimensional turbulent flows, for which network computations can become prohibitively expensive. In this work, we propose efficient methods to approximate network quantities, such as the leading eigendecomposition of the adjacency matrix, using randomized methods. Specifically, we use the Nystr\"om method to approximate the leading eigenvalues and eigenvectors, achieving significant computational savings and reduced memory requirements. The effectiveness of the proposed technique is demonstrated on two high-dimensional flow fields: two-dimensional flow past an airfoil and two-dimensional turbulence. We find that quasi-uniform column sampling outperforms uniform column sampling, while both feature the same computational complexity.Comment: 18 pages, 8 figure

Performance improvement of electrochemical capacitors through the integration of advanced materials and the cell configuration assessment

209 p.The electrochemical capacitors or supercapacitors are envisioned as potential next-generation energy storage systems because of their excellent storage capacity, power density, and long-term durability. However, all these advantages are overshadowed by their poor energy density. Thus, this thesis aims to achieve a high-energy supercapacitor device without compromising its power performance to make them more commercially viable for many applications. The research work is associated with the improvement of the supercapacitors in different device configurations, such as EDL, asymmetric, and hybrid LIC systems by integration of advanced material and cell design. The results obtained from the studies of different supercapacitor systems demonstrate that the variation in electrode mass, cell voltage, and electrolyte has a huge impact on the overall electrochemical performance, stability, life expectancy, and safety of the device. Therefore, careful optimization of cell design and advancement in electrode materials retains the high importance driving factors of the supercapacitors for the development of future energy storage technology

Performance improvement of electrochemical capacitors through the integration of advanced materials and the cell configuration assessment

209 p.The electrochemical capacitors or supercapacitors are envisioned as potential next-generation energy storage systems because of their excellent storage capacity, power density, and long-term durability. However, all these advantages are overshadowed by their poor energy density. Thus, this thesis aims to achieve a high-energy supercapacitor device without compromising its power performance to make them more commercially viable for many applications. The research work is associated with the improvement of the supercapacitors in different device configurations, such as EDL, asymmetric, and hybrid LIC systems by integration of advanced material and cell design. The results obtained from the studies of different supercapacitor systems demonstrate that the variation in electrode mass, cell voltage, and electrolyte has a huge impact on the overall electrochemical performance, stability, life expectancy, and safety of the device. Therefore, careful optimization of cell design and advancement in electrode materials retains the high importance driving factors of the supercapacitors for the development of future energy storage technology

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

Molecular sorption of carbon-based porous structures: a study on water harvesting and carbon dioxide capture

Carbon (CO2) capture using regenerable sorbents is an effective means of mitigating green-house gas emission to the environment. The first-generation sorbents, based on liquid amines, suffer from instability, toxicity and high-energy penalty for regeneration. Solid sorbents, e.g. based on porous silica or active carbon, offer the potential of long cyclability and low cost. However, the sorption capacity is limited due to low surface area and pore volume, particularly if only physisorption mechanism dominates. The challenge and main aim of this study is to identify an effective porous carbon-based solid sorbent that can possess high capacity and low regeneration energy (hence cost) penalty. The structures should offer enhanced physisorption (e.g. van der Waals binding at slit pores, with a binding energy ~-10-20 kJ mol-1) and moderate chemisorption (e.g. binding at graphene edges, point defects or carbon-supported amine-groups, with a binding energy ~-20-50 kJ mol-1) so that adsorption and desorption “window” for CO2 can be narrow and at relatively low temperature. The porous structure must show high specific surface area and well-connected pores, so that the capacity can be maximised. To achieve such goals, the project first studied graphene oxide (GO) with various degrees of oxidation, ranging from 30 at% oxygen content, to enhance surface area and defect density; secondly, highly hierarchical porous graphene networks were derived through GO via moderate temperature thermal shock (300 °C), thermal annealing (600 °C) and/or KOH activation, to promote micro-pores and porosity hierarchy; and finally, for comparison and porosity improvement, another type of porous carbon structures were derived from carbonised metal-organic frameworks (MOFs), namely MOF-5 and MOF-74. The chemical and structural properties of synthesized materials were characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption-desorption isotherms (the BET method) and Raman spectroscopy. Sorption capacity and kinetics were assessed by CO2 adsorption isotherms, thermogravimetric-differential scanning calorimetry (TG-DSC) and in-house water saturation apparatus. GO demonstrated a stacked layered structure with oxygenated functional groups such as hydroxyl, epoxy and carbonyl groups on its basal planes and edges, resulting in a hybrid structure comprising a mixture of sp2 and sp3 hybridized carbon atoms. GO is hydrophilic due to the presence of the oxygenated functional groups and the laminated structure can allow slow water diffusion into the layers. As water exist in practical cases of CO2 capture, the sorption of water was studied separately and together with CO2. From the study of highly porous GO derived exfoliated GO (exfGO), it was identified that the resulting materials possessed ultrahigh surface area and total pore volume up to 853 m2 g-1 and 6.68 cm3 g-1. The structures were applied as solid sorbents with chemical modification by TEPA and PEHA polyamines wet impregnation to incorporate amine-based sorption sites. The solid-amine system exhibited ultrahigh selective flue-gas CO2 capture of 6.16 mmol g-1 at 75 °C. The desorption occurred at 100 °C, giving a desirable narrow temperature-swing window. Further testing showed the cycling stability under simulated flue-gas stream conditions had moderate decay of ~7 wt% over 40 adsorption-desorption cycles and demonstrated stable CO2 uptake ~25 wt%. From the study on MOF carbons, it was found that the carbonisation process of MOF precursors led to loss of local metal centres and produced defective carbon structures with mainly sp2 bonding. By varying the synthesis conditions and solvents, micrometre to millimetre-sized MOF-5 crystals can be synthesized. Carbonisation process retained both meso- and macro-pores and yielded MOF carbons with high surface area up to 2237 m2 g-1 and total pore volume up to 4.6 cm3 g-1. The resulting amine-impregnated MOF carbons achieved CO2 adsorption of 4.37 mmol g-1. In summary, the project has developed highly porous carbon-based solid sorbents, which are stable and environmentally benign, with high specific surface area to offer a CO2 adsorption capacity > 6 mmol g-1. The binding energy is typically controlled at -50 kJ mol-1, which allows CO2 adsorption and desorption to be carried out between 25 °C and 100 °C. The developed sorbents have met the identified challenges of flue-gas conditions CO2 capture (>50 °C, humid), high thermal stability, chemical resistance and potential for large-scale production at low-cost, as well as offering great potential for practical applications in industry. The results also show great potential for the development of high capacity carbon-based sorbents for effective pre-combustion CO2 capture and energy storage applications