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  • Public defence: 2018-08-17 14:00 F3, Stockholm
    Mahmood, Farhan
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Synchrophasor based Steady State Model Synthesis of Active Distribution Networks2018Doctoral thesis, monograph (Other academic)
    Abstract [en]

    With the increased penetration of distributed energy resources (DERs) at lower voltage levels, distribution networks (DNs) are being transformed into active grids. This has led to a paradigm shift in the operation, planning and control of DNs. Traditional monitoring infrastructure is unlikely to satisfy the requirements that active distribution networks (ADNs) pose in terms of higher speed networking, time synchronization and signal resolution, precision and accuracy, scope, etc. As a result, high performance monitoring infrastructures are needed to fully utilize the potential of sensing devices at DNs, capable of monitoring ADNs in real-time. In this context, phasor measurement units (PMUs) have emerged as one of the most promising alternatives for ADNs monitoring in real-time.

    The focus of this thesis is to exploit PMU measurements to perform real-time steady state model synthesis (SSMS) of ADNs. To this end, methods for pre-processing PMU data are developed in this thesis. As the focus of this thesis is the development of a steady state PMU application, the methods presented herein extract the quasi-steady state component in PMU measurements and feeds them to the SSMS application. In addition, the methods are capable of filtering noise, compensating for missing data, and removing the outliers in PMU signals in real-time.

    The synthesis method can be applied to multiple sections of unbalanced ADNs requiring measurements from multiple PMUs. The proposed approach is generic and can be applied to any portion of a DN with any feeder configuration. The performance and the effectiveness of the proposed methodology have been illustrated in details by using real-time hardware-in-the-loop (HIL) experiments.

    A detailed sensitivity analysis of the SSMS application is performed in order to show how sensitive the output of the SSMS method is to changes in its inputs. An extended version of the total vector error (TVE) was developed as an evaluation metric. The location of PMUs, system operating point and the occurrence of different disturbances are considered when evaluating the SSMS method. The sensitivity analysis is performed through several case studies as discussed above.

    Finally, the thesis provides extensive experimental validation experiments on the SSMS application. Syncrophasor measurements acquired from real PMUs installed at an actual active distribution feeder in a university campus were used for this purpose. A detailed performance assessment of the SSMS method is conducted for different conditions. Additionally, a comprehensive analysis is performed to help power system operators to determine how to configure the SSMS application.

  • Public defence: 2018-08-29 14:00 Salongen, Stockholm
    Buckley, Jeffrey
    KTH, School of Industrial Engineering and Management (ITM).
    Investigating the role of spatial ability as a factor of human intelligence in technology education: Towards a causal theory of the relationship between spatial ability and STEM education2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Education is a particularly complex discipline due to the numerous variables which impact on teaching and learning. Due to the large effect of human intelligence on the variance in student educational achievement, there is a substantial need to further contemporary understandings of its role in education. Multiple paradigms exist regarding the study of human intelligence. One in particular, the psychometric tradition, has offered many critical findings which have had a substantial impact on STEM education. One of the most significant offerings of this approach is the wealth of empirical evidence which demonstrates the importance of spatial ability in STEM education. However, while categorically identified as important, a causal relationship between spatial ability and STEM is yet to be confirmed

    As there is insufficient evidence to support a causal investigation, this thesis aims to develop an empirically based causal theory to make this possible. Five studies were conducted to achieve this aim and are described in the appended papers. As the research explores spatial ability in technology education, Paper I examines the epistemological position of technology education within STEM education. Based on the evidence showing spatial ability is important in Science, Engineering and Mathematics, Paper II explores its relevance to Technology. Paper III offers an empirically based definition for spatial ability through a synthesis of contemporary research and illustrates empirically where it has been observed as important to STEM learning. Paper IV examines the perceived importance of spatial ability relative to intelligence in STEM education from the perspective of technology education. Finally, Paper V examines the psychometric relationship between spatial ability and fluid intelligence (Gf) based on a hypothesis generated throughout the preceding papers.

    The main results of this thesis illustrate the predictive capacity of visualization (Vz), memory span (MS), and inductive reasoning (I) on fluid intelligence (Gf) which is posited to offer a causal explanation based on the creative, innovative, and applied nature of STEM. Additional findings include the observation that learners use problem solving strategies which align with their cognitive strengths, that external representations of problems can scaffold the use of spatial ability or alleviate the need for it, that the variability of knowledge types across STEM sub-disciplines may affect the nature of reasoning within disciplines, and that for technology education specifically, acquiring an explicit knowledge base is not perceived to denote intelligence while the capacity to reason abstractly to solve novel problems is. This epistemological fluidity and focus on reasoning highlights the unique way in which technology education can provide insight into intelligence in STEM education. The implications of these results are discussed with specific focus on their theoretical validity and potential application in applied educational contexts.

  • Public defence: 2018-09-03 13:15 Sal Ka-208, Kista, Stockholm
    Castañeda Lozano, Roberto
    KTH, School of Electrical Engineering and Computer Science (EECS), Software and Computer systems, SCS. RISE SICS (Swedish Institute of Computer Science).
    Constraint-Based Register Allocation and Instruction Scheduling2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Register allocation (mapping variables to processor registers or memory) and instruction scheduling (reordering instructions to improve latency or throughput) are central compiler problems. This dissertation proposes a combinatorial optimization approach to these problems that delivers optimal solutions according to a model, captures trade-offs between conflicting decisions, accommodates processor-specific features, and handles different optimization criteria.

    The use of constraint programming and a novel program representation enables a compact model of register allocation and instruction scheduling. The model captures the complete set of global register allocation subproblems (spilling, assignment, live range splitting, coalescing, load-store optimization, multi-allocation, register packing, and rematerialization) as well as additional subproblems that handle processor-specific features beyond the usual scope of conventional compilers.

    The approach is implemented in Unison, an open-source tool used in industry and research that complements the state-of-the-art LLVM compiler. Unison applies general and problem-specific constraint solving methods to scale to medium-sized functions, solving functions of up to 647 instructions optimally and improving functions of up to 874 instructions. The approach is evaluated experimentally using different processors (Hexagon, ARM and MIPS), benchmark suites (MediaBench and SPEC CPU2006), and optimization criteria (speed and code size reduction). The results show that Unison generates code of slightly to significantly better quality than LLVM, depending on the characteristics of the targeted processor (1% to 9.3% mean estimated speedup; 0.8% to 3.9% mean code size reduction). Additional experiments for Hexagon show that its estimated speedup has a strong monotonic relationship to the actual execution speedup, resulting in a mean speedup of 5.4% across MediaBench applications.

    The approach contributed by this dissertation is the first of its kind that is practical (it captures the complete set of subproblems, scales to medium-sized functions, and generates executable code) and effective (it generates better code than the LLVM compiler, fulfilling the promise of combinatorial optimization). It can be applied to trade compilation time for code quality beyond the usual optimization levels, explore and exploit processor-specific features, and identify improvement opportunities in conventional compilers.

  • Public defence: 2018-09-21 10:00 rum 4301, Stockholm
    Zheng, Weisen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Thermodynamic and kinetic investigation of systems related to lightweight steels2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Lightweight steels have attracted considerable interest for automobile applications due to the weight reduction without loss of high strength and with retained excellent plasticity. In austenitic Fe-Mn-Al-C steels, the nano-precipitation of the κ-carbide within the austenitic matrix significantly contributes to the increase in yield strength. In the present work, the precipitation strengthening simulation has been carried out within the framework of the ICME approach. Thermodynamic assessments of the quaternary Fe-Mn-Al-C system as well as its sub-ternary systems were performed with the CALPHAD method. All available information on phase equilibria and thermochemical properties were critically evaluated and used to optimize the thermodynamic model parameters. By means of the partitioning model, the κ-carbide was described using a five-sublattice model (four substitutional and one interstitial sublattice), which can reflect the ordering between metallic elements and reproduce the wide homogeneity range of the κ-carbide. Based on the present thermodynamic description, a thermodynamic database for lightweight steels was created. Using the database, the phase equilibria evolution in lightweight steels can be satisfactorily predicted, as well as the partition of alloying elements. In order to accelerate the development of a kinetic database for multicomponent systems, a high-throughput optimization method was adopted to optimize the diffusion mobilities. This method may largely reduce the necessary diffusion-couple experiments in multicomponent systems. Based on the developed thermodynamic and kinetic databases for lightweight steels, the precipitation of the κ-carbide was simulated using TC-PRISMA. The volume fraction and particle size were reasonably reproduced. Finally, the precipitation strengthening contribution to the yield strength was predicted. The calculation results show that the anti-phase boundary effect is predominant in the precipitation strengthening. Overall, the relationship between the composition, processing parameters, microstructure and mechanical properties are established in the thesis.