Recursion and Sequentiality in Categories of Sheaves

We present a fully abstract model of a call-by-value language with higher-order functions, recursion and natural numbers, as an exponential ideal in a topos. Our model is inspired by the fully abstract models of O'Hearn, Riecke and Sandholm, and Marz and Streicher. In contrast with semantics based on cpo's, we treat recursion as just one feature in a model built by combining a choice of modular components

Concrete sheaf models of higher-order recursion

This thesis studies denotational models, in the form of sheaf categories, of functional programming languages with higher-order functions and recursion. We give a general method for building such models and show how the method includes examples such as existing models of probabilistic and differentiable computation. Using our method, we build a new fully abstract sheaf model of higher-order recursion inspired by the fully abstract logical relations models of O’Hearn and Riecke. In this way, we show that our method for building sheaf models can be used both to unify existing models that have so far been studied separately and to discover new models. The models we build are in the style of Moggi, namely, a cartesian closed category with a monad for modelling non termination. More specifically, our general method builds sheaf categories by specifying a concrete site with a class of admissible monomorphisms, a concept which we define. We combine this approach with techniques from synthetic and axiomatic domain theory to obtain a lifting monad on the sheaf category and to model recursion. We then prove the models obtained in this way are computationally adequate

An improved DC fault protection scheme independent of boundary components for MMC based HVDC grids

For Modular Multilevel Converter (MMC) based DC grids, current-limiting reactors (CLRs) are mainly employed to suppress the fault current and provide boundary effects to detect internal faults. Thus, most existing protection schemes are highly dependent on the larger CLRs to guarantee high selectivity. However, in existing MMC based HVDC projects, the size of CLRs is restrained by the cost, weight and system stability under normal state. Thus, boundary protections may fail to detect high-resistance faults and pole-to-ground faults. To overcome these shortcomings, this paper proposes a fast and selective DC fault detection algorithm independent of boundary components. The propagation characteristics of line-mode backward traveling-waves (TW) are analyzed to identify external and internal faults. The polarities of zero-mode backward TWs are employed to select faulted poles. To detect remote faults, a pilot protection scheme based on the directional overcurrent is adopted as the complementary criterion. The detection speed of the proposed protection is fast, with a delay less than 1.1ms. Besides, it is robust to fault resistance and immune to noise. Various simulation results in PSCAD/EMTDC demonstrate that the proposed method is not affected by AC faults, fault distances and fault type

Conductive bridging RAM devices inspired on solid-state biopolymer electrolytes

This work reports the design, fabrication and characterization of metal-insulator-metal (MIM) structures acting as conductive bridging random access memory (CBRAM) devices using biopolymer insulator. Chitosan and hydroxypropyl cellulose (HPC) were deposited by spin coating in between evaporated Pt and Ag electrodes. CBRAM devices fabricated using chitosan as the insulating layer demonstrated retention times of up to 105 s with an on/off ratio of 102 as well as enduring several program/erase cycles. Devices fabricated with HPC showed retention times of up to 104 s with an on/off ratio of approximately 106, and also showed stable device operation over several cycles. Furthermore, the functionalization of chitosan with silver nanoparticles and its integration in the MIM structures were investigated, as well as the substitution of the e-beam evaporated Ag electrode by a screen printed Ag electrode

Algebraic Algorithm Design and Local Search

Formal, mathematically-based techniques promise to play an expanding role in the development and maintenance of the software on which our technological society depends. Algebraic techniques have been applied successfully to algorithm synthesis by the use of algorithm theories and design tactics, an approach pioneered in the Kestrel Interactive Development System (KIDS). An algorithm theory formally characterizes the essential components of a family of algorithms. A design tactic is a specialized procedure for recognizing in a problem specification the structures identified in an algorithm theory and then synthesizing a program. Design tactics are hard to write, however, and much of the knowledge they use is encoded procedurally in idiosyncratic ways. Algebraic methods promise a way to represent algorithm design knowledge declaratively and uniformly. We describe a general method for performing algorithm design that is more purely algebraic than that of KIDS. This method is then applied to local search. Local search is a large and diverse class of algorithms applicable to a wide range of problems; it is both intrinsically important and representative of algorithm design as a whole. A general theory of local search is formalized to describe the basic properties common to all local search algorithms, and applied to several variants of hill climbing and simulated annealing. The general theory is then specialized to describe some more advanced local search techniques, namely tabu search and the Kernighan-Lin heuristic

The technical and economic benefits of utility sponsored renewable energy integration

In recent years, Australian energy consumers have begun to install large amounts of distributed generation (DG), particularly residential solar photovoltaic (PV) systems. This rapid increase in DG, has led to the flow of power throughout low voltage (LV) networks to become bidirectional. This reverse power flow, along with the intermittent nature of solar PV and the inability for distribution network service providers (DNSPs) to control where this DG is installed, has led to voltage regulation issues throughout LV networks. Along with solar PV, energy storage (ES) is also becoming more prevalent among energy consumers. The combination of solar PV and ES allows customers to become more energy independent, relying less on utilising energy from the grid. This poses a major risk to DNSPs who rely on the income generated from customers based on kWh sales to build and maintain network infrastructure. This paper presents a coordinated reactive power control scheme to reduce voltage rise along LV distribution feeders with high penetrations of solar PV. The value of privately investing in solar PV and ES for the years 2015 and 2020 from the perspective of an average residential customer is determined. Finally, a business model is proposed outlining how utility sponsored residential solar PV and ES could be implemented by a DNSP. The business model is then evaluated from a technical and economic standpoint

Resistive switching in ALD metal-oxides with engineered interfaces

L'abstract è presente nell'allegato / the abstract is in the attachmen

Lightning Pin Injection Testing on MOSFETS

Lightning transients were pin-injected into metal-oxide-semiconductor field-effect transistors (MOSFETs) to induce fault modes. This report documents the test process and results, and provides a basis for subsequent lightning tests. MOSFETs may be present in DC-DC power supplies and electromechanical actuator circuits that may be used on board aircraft. Results show that unprotected MOSFET Gates are susceptible to failure, even when installed in systems in well-shielded and partial-shielded locations. MOSFET Drains and Sources are significantly less susceptible. Device impedance decreased (current increased) after every failure. Such a failure mode may lead to cascading failures, as the damaged MOSFET may allow excessive current to flow through other circuitry. Preliminary assessments on a MOSFET subjected to 20-stroke pin-injection testing demonstrate that Breakdown Voltage, Leakage Current and Threshold Voltage characteristics show damage, while the device continues to meet manufacturer performance specifications. The purpose of this research is to develop validated tools, technologies, and techniques for automated detection, diagnosis and prognosis that enable mitigation of adverse events during flight, such as from lightning transients; and to understand the interplay between lightning-induced surges and aging (i.e. humidity, vibration thermal stress, etc.) on component degradation