A New Approach to Optimization of Dynamic Reactive Power Sources Addressing FIDVR Issues

Dynamic reactive power (var) sources, e.g. SVCs and STATCOMs, can effectively mitigate fault-induced delayed voltage recovery (FIDVR) issues or even voltage collapse. However, their optimal allocation in a power grid is a complicated nonlinear optimization problem since the post-fault voltage trajectories have to be solved to check constraints on voltage responses. Thus, solvers of both nonlinear optimization and power system differential and algebraic equations (DAEs) are required. Currently, most of existing methods merely achieve dynamic optimization locally with great dependence on the initial operation point. Also, complicated algorithm and time consuming are obstacles for the practical implementation. This thesis proposes a new approach to optimize the sizes of dynamic var sources at candidate locations by efficiently interfacing a heuristic linear programming based searching algorithm with power system simulation software. Within several iterative search steps, the optimal size of dynamic var can be achieved. In order to verify the result obtained from the proposed approach, Voronoi diagram is applied to tackle the feasible solution area, and then to demonstrate the result of heuristic linear programming is the global optimal. Case studies on a 9-bus system and the IEEE 39-bus system have benchmarked the new approach with an existing representative approach and demonstrated that the new approach can quickly converge to an optimal solution. Voronoi diagram is implemented to tackle non-convex feasible solution area of both cases and it shows that the result is global optimal

Enhance OF SOGI-FLL and SOGI-PLL response to voltage sags and swells perturbations

The scope of this thesis is related to the enhancement of the operation of distributed generators (DGs) based on renewable energies (REs) when connected to the conventional grid network. Synchronization of those DGs, i.e. their front-end inverters, with the grid is critical for injecting power into the grid. Internal control loops of voltage source inverters (VSIs) monitor parameters such as the utility voltage's phase, amplitude, and frequency to achieve proper synchronization and inject power into the grid. Therefore, an appropriate estimation of the grid parameters is needed for completing the mentioned goals. The different existing monitoring techniques still face technical challenges when appearing faults on the grid, particularly voltage sags and voltage swells. From those techniques, SOGI-FLL and SOGI-PLL are widely spread in power inverters. In the mentioned faulty conditions, a degradation of the estimated parameters occurs due to the impact of the grid defaults on the dynamic performance of the SOGI-FLL and SOGI-PLL estimators. In the literature, it was not found contributions to those problems. This research aimed to contribute by providing a fast and accurate detection of the faults, yielding a robust dynamical response of the VSIs control structure in front of such perturbations, thanks to a fast and precise estimation of grid parameters. This faults detection strategy was accompanied by minimization of the monitored parameters drift due to inherent harmonic pollution of the faults. Then a first contribution has been the design and implementation of a Finite State Machine on a synchronization structure known as SOGI-PLL and in SOGI-FLL, enhancing their dynamical response regarding transient time and steady-state response in front of voltage sags. A second contribution has been implementing the strategy to face the problems derived from voltage swells with satisfactory results. The research was carried out in the E3PACS laboratory facilities of EEBE-UPC. This work is organized as follows: In Section 1, the study is contextualized. Section 2 describes Power Quality indexes benchmarking for assessing the problem to solve and the obtained results. It is then followed by Section 3, where the SOGI-FLL response against voltage sags and swells is analyzed. Therefore, Section 4 summarizes the different approaches and results for mitigating the grid faults studied. Section 5 is devoted to showing SOGI-PLL amelioration by applying SOGI Error-Based algorithm. Finally, Section 6 summarizes the main contributions of this work, along with the general conclusions. Section 7 lists future work, in Section 8 are listed the published JCR indexed papers.L'abast d'aquesta tesi està relacionat amb la millora del funcionament dels generadors distribuïts (DG, distributed generation en anglès) basats en energies renovables quan es connecten a la xarxa elèctrica convencional. La sincronització d'aquests DG amb la xarxa és fonamental per injectar energia a aquesta, especialment quan es tracta d'inversors de potència. Els llaços de control interns dels inversors de font de tensió (VSI, Voltage Source Inverters en anglès) controlen paràmetres com ara la fase, l'amplitud i la freqüència de la tensió de la xarxa per aconseguir una sincronitzaciórequerida. Per tant, es necessita una estimació adequada dels paràmetres del bus de tensió per assolir els objectius esmentats. Les diferents tècniques de monitorització existents encara s'enfronten a reptes tècnics a l'hora d'aparèixer fallades a la xarxa, especialment caigudes i augments abruptes de tensió ("Voltage sags" i "Voltage swells" en anglès, respectivament). En el cas dels inversors, les tècniques basades en estructures SOGI-FLL i SOGI-PLL estan àmpliament esteses i, en les esmentades condicions de fallada, es produeix una degradació dels paràmetres estimats a causa de l'impacte dels defectes de la xarxa en el seu comportament dinàmic . A la literatura, no es van trobar contribucions rellevants per mitigar aquests problemes. Aquesta investigació pretén doncs contribuir proporcionant una detecció ràpida i precisa de les fallades, donant una resposta dinàmica robusta de l'estructura de control dels VSI davant d'aquestes pertorbacions, gràcies a una estimació ràpida i precisa dels paràmetres de la xarxa. Aquesta estratègia de detecció de fallades s'acompanya de la minimització de la deriva dels paràmetres monitoritzats a causa de la contaminació harmònica inherent de les fallades. Així doncs, una primera contribució ha estat el disseny i la implementació d'una màquina d'estats finits en una estructura de sincronització coneguda com SOGI-PLL i en SOGI-FLL, millorant la seva resposta dinàmica pel que fa al temps transitori i la resposta en estat estacionari davant les caigudes de tensió. Una segona aportació ha estat la implementació de l'estratègia per afrontar amb resultats satisfactoris els problemes derivats de les pujades de tensió (voltage swells). La investigació es va dur a terme a les instal·lacions del laboratori E3PACS de l'EEBE-UPC. Aquest treball s'organitza de la següent manera: A l'apartat 1 es contextualitza l'estudi. La secció 2 descriu el la referenciació dels índexs de qualitat de l'energia per avaluar el problema a resoldre i els resultats obtinguts. A continuació, segueix la Secció 3, on s'analitza la resposta SOGI-FLL contra les caigudes i les inflors de tensió. Per tant, la Secció 4 resumeix els diferents enfocaments i resultats per mitigar les falles de la xarxa estudiades. La secció 5 està dedicada a mostrar la millora SOGI-PLL aplicant l'algorisme SOGI basat en errors. Finalment, l'apartat 6 resumeix les principals aportacions d'aquest treball, juntament amb les conclusions generals. La secció 7 enumera els treballs futurs, a la secció 8 s'enumeren els articles indexats JCR publicats.Postprint (published version

Role of stress-assisted martensite in the design of strong ultrafine-grained duplex steels

This work explains the occurrence of transformation-induced plasticity via stress-assisted martensite, when designing ultrafine-grained duplex steels. It is found that, when the austenite is reduced to a fine scale of about 300 nm, the initial deformation-induced microstructure can be dominated by parallel lamellae of epsilon martensite or mechanical twinning, which cannot efficiently provide nucleation sites for strain-induced martensite. Hence, alpha martensite nucleation occurs independently by a stress-assisted process that enhances transformation-induced plasticity in ultrafine-grained austenite. This metallurgical principle was validated experimentally by using a combination of transmission Kikuchi diffraction mapping, transmission electron microscopy, and atom probe microscopy, and demonstrated theoretically by the thermodynamics model of stress-assisted martensite.The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility (ammrf.org.au) node at Sydney Microscopy & Microanalysis, at the University of Sydney.This is the accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S1359645414006958

Voltage and Frequency Recovery in Power System and MicroGrids Using Artificial Intelligent Algorithms

This thesis developed an advanced assessment tools to recover the power system voltage margin to the acceptable values during the disturbance. First, the effect of disturbance in islanded microgrids are analyzed using power factor-based power-voltage curves and a comprehensive under voltage-frequency load shedding(UVFLS) method is proposed as a last resort in order to restore the system voltage and frequency. The effect of disturbance in conventional power system is investigated by introducing a phenomenon called fault induced delayed voltage recovery(FIDVR) and comprehensive real-time FIDVR assessments are proposed to employ appropriate emergency control approaches as fast as possible to maintain the system voltage margins within the desired range. Then, polynomial regression techniques have been used for predicting the FIDVR duration. Next, advanced FIDVR assessment is implemented which simultaneously predicts whether the event can be classified as FIDVR or not and also predicts the duration of FIDVR with high accuracy

A Hybrid Method of Performing Electric Power System Fault Ride-Through Evaluations on Medium Voltage Multi-Megawatt Devices

This dissertation explores the design and analysis of a Hybrid Method of performing electrical power system fault ride-through evaluations on multi-megawatt, medium voltage power conversion equipment. Fault ride-through evaluations on such equipment are needed in order to verify and validate full scale designs prior to being implemented in the field. Ultimately, these evaluations will help in reducing the deployment risks associated with bringing new technologies into the marketplace. This is especially true for renewable energy and utility scale energy storage systems, where a significant amount of attention in recent years has focused on their ever increasing role in power system security and stability. The Hybrid Method couples two existing technologies together - a reactive voltage divider network and a power electronic variable voltage source - in order to overcome the inherent limitation of both methods, namely the short circuit duty required for implementation. This work provides the background of this limitation with respect to the existing technologies and demonstrates that the Hybrid Method can minimize the fault duty required for fault evaluations. The physical system, control objectives, and operation cycle of the Hybrid Method are analyzed with respect to the overall objective of reducing the fault duty of the system. A vector controller is designed to incorporate the time variant nature of the Hybrid Method operation cycle, limit the fault current seen by the power electronic variable voltage source, and provide regulation of the voltage at the point of common coupling with the device being evaluated. In order to verify the operation of both the Hybrid Method physical system and vector controller, a controller hardware-in-the-loop experiment is created in order to simulate the physical system in real-time against the prototype implementation of the vector controller. The physical system is simulated in a Real Time Digital Simulator and is controlled with the Hybrid Method vector controller implemented on a National Instruments FPGA. In order to evaluate the complete performance of the Hybrid Method, both a synchronous generator and a doubly-fed induction generator are modeled as the device under test in the simulations of the physical system. Finally, the results of the controller hardware-in-the-loop experiments are presented which demonstrate that the Hybrid Method is a viable solution to performing fault ride-through evaluations on multi-megawatt, medium voltage power conversion equipment