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  • Conference paper
    Falugi P, 2014,

    Model predictive Control: a passive scheme

    , 19th IFAC World Congress, Publisher: International Federation of Automatic Control, Pages: 1017-1022

    This note studies the formulation of model predictive control exploiting passivity properties. The introduction of passive constraints in model predictive control schemes is particularly appealing since robustness against model uncertainty is inherently guaranteed. The potential of the discussed control scheme is shown on the regulation problem of a robot manipulator.
  • Journal article
    Yan S, Tan S-C, Lee C-K, Chaudhuri B, Hui SYRet al., 2014,

    Electric Springs for Reducing Power Imbalance in Three-Phase Power Systems

    , IEEE Transactions on Power Electronics, Vol: 30, Pages: 3601-3609, ISSN: 0885-8993
  • Conference paper
    Ye Y, Papadaskalopoulos D, Strbac G, 2014,

    Pricing Flexible Demand Non-Convexities in Electricity Markets

    , 18th Power Systems Computation Conference (PSCC), Publisher: IEEE, Pages: 1-7, ISSN: 1944-9933

    Uniform marginal prices cannot generally support competitive equilibrium solutions in markets with non-convexities and yield schedules' inconsistency and surplus sub-optimality effects. Previous work has identified non-convexities associated with the generation side of electricity markets and proposed a generalized uplift approach to eliminate these effects. This paper examines the above issues from the perspective of the flexible demand (FD) side. FD non-convexities are identified, including its ability to forgo demand activities and minimum power levels, and resulting inconsistency and surplus sub-optimality effects are demonstrated through simple examples. Generalized uplift functions for FD participants are proposed, including quadratic pricing terms to limit their tendency to concentrate at the lowest-priced periods, and binary terms associated with their ability to forgo activities. Finally, a new rule is proposed for the equitable distribution of the total surplus loss among the market participants. These contributions are supported by case studies on a market with a day-ahead horizon and hourly resolution.
  • Journal article
    Tortelli OL, Laurenco EM, Garcia AV, Pal BCet al., 2014,

    Fast Decoupled Power Flow to Emerging Distribution Systems via Complex pu Normalization

    , IEEE Transactions on Power Systems, Vol: PP, Pages: 1-8, ISSN: 0885-8950

    This paper proposes a generalized approach of the per unit normalization, named complex per unit normalization (cpu), to improve the performance of fast decoupled power flow methods applied to emerging distribution networks. The proposed approach takes into account the changes envisaged and also already faced by distribution systems, such as high penetration of generation sources and more interconnection between feeders, while considering the typical characteristics of distribution systems, as the high R/ X ratios. These characteristics impose difficulties on the performance of both backward-forward sweep and decoupled-based power flow methods. The cpu concept is centred on the use of a complex volt-ampere base, which overcomes the numerical problems raised by the high R/ X ratios of distribution feeders. As a consequence, decoupled power flow methods can be efficiently applied to distribution system analysis. The performance of the proposed technique and the simplicity of adapting it to existing power flow programs are addressed in the paper. Different distribution network configurations and load conditions have been used to illustrate and evaluate the use of cpu.
  • Journal article
    Chen HT, Lin DY, Tan SC, Hui SYRet al., 2014,

    Chromatic, photometric and thermal modeling of LED systems with nonidentical LED devices

    , IEEE Transactions on Power Electronics, Vol: 29, Pages: 6636-6647, ISSN: 0885-8993

    With the emergence of new color-mixing LED systems based on LED devices of different color temperatures, the need for a new modeling technique for LED systems with nonidentical LED devices becomes imminent. This paper presents a modeling technique for LED systems with LED arrays comprising nonidentical LED devices that have nonidentical optical-thermal-electrical properties. Based on a general 3-D photo-electro-thermal LED node model, LED devices of different kinds can be arranged in various array forms according to their system construction and design. By linking the system matrix to the correlated-colorerature prediction, the proposed modeling technique provides an accurate prediction of the temperature distribution, luminous flux, and correlated color temperature of the LED systems. The temperature distribution and light output of the LED systems have been measured using an infrared imaging system and a spectrophotocolorimeter with an integrating sphere. The modeling technique has been successfully demonstrated and experimentally verified on several LED systems comprising nonidentical LED devices. It is particularly useful as a modeling tool to study new color-mixing LED systems based on different types of LED devices. © 2014 IEEE.
  • Journal article
    Lueth T, Merlin MMC, Green TC, Hassan F, Barker CDet al., 2014,

    High-frequency operation of a DC/AC/DC system for HVDC applications

    , IEEE Transactions on Power Electronics, Vol: 29, Pages: 4107-4115, ISSN: 0885-8993

    Voltage ratings for HVdc point-to-point connections are not standardized and tend to depend on the latest available cable technology. DC/DC conversion at HV is required for interconnection of such HVdc schemes as well as to interface dc wind farms. Modular multilevel voltage source converters (VSCs), such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), have been shown to incur significantly lower switching losses than previous two- or three-level VSCs. This paper presents a dc/ac/dc system using a transformer coupling two modular multilevel VSCs. In such a system, the capacitors occupy a large fraction of the volume of the cells but a significant reduction in volume can be achieved by raising the ac frequency. Using high frequency can also bring benefits to other passive components such as the transformer but also results in higher switching losses due to the higher number of waveform steps per second. This leads to a tradeoff between volume and losses which has been explored in this study and verified by simulation results with a transistor level model of 30-MW case study. The outcome of the study shows that a frequency of 350 Hz provides a significant improvement in volume but also a penalty in losses compared to 50 Hz.
  • Journal article
    Casagrande D, Astolfi A, Langarica D, Ortega Ret al., 2014,

    Solution to the multi-machine transient stability problem and simulated validation in realistic scenarios

    , IET GENERATION TRANSMISSION & DISTRIBUTION, Vol: 8, Pages: 1392-1405, ISSN: 1751-8687
  • Journal article
    Wu P, Bai F, Xue Q, Liu X, Hui SYRet al., 2014,

    Use of Frequency-Selective Surface for Suppressing Radio-Frequency Interference from Wireless Charging Pads

    , IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, Vol: 61, Pages: 3969-3977, ISSN: 0278-0046
  • Conference paper
    Mylvaganam T, Bauso D, Astolfi A, 2014,

    Mean-field games and two-point boundary value problems

    , Conference on Decision and Control
  • Journal article
    Ariff MAM, Pal BC, Singh AK, 2014,

    Estimating Dynamic Model Parameters for Adaptive Protection and Control in Power System

    , IEEE Transactions on Power Systems, ISSN: 0885-8950

    This paper presents a new approach in estimating important parameters of power system transient stability model such as inertia constant H and direct axis transient reactance x′d in real time. It uses a variation of unscented Kalman filter (UKF) on the phasor measurement unit (PMU) data. The accurate estimation of these parameters is very important for assessing the stability and tuning the adaptive protection system on power swing relays. The effectiveness of the method is demonstrated in asimulated data from 16-machine 68-bus system model. The paper also presents the performance comparison between the UKF and EKF method in estimating the parameters. The robustness of method is further validated in the presence of noise that is likely to be in the PMU data in reality.
  • Conference paper
    Jiang J, Astolfi A, 2014,

    Output-Feedback Shared-Control for Fully Actuated Linear Mechanical Systems

    , American Control Conference, Publisher: IEEE, Pages: 335-340, ISSN: 0743-1619

    This paper presents an output feedback shared-control algorithm for fully-actuated, linear, mechanical systems. The feasible configurations of the system are described by a group of linear inequalities which characterize a convex admissible set. The properties of the shared-control algorithm are established with a Lyapunov-like analysis. Simple numerical examples demonstrate the effectiveness of the strategy.
  • Journal article
    Li S, Chen H, Tan SC, Hui SY, Waffenschmidt Eet al., 2014,

    Power flow analysis and critical design issues of retrofit Light-Emitting Diode (LED) light bulb

    , IEEE Transactions on Power Electronics, Vol: 30, Pages: 3830-3840, ISSN: 0885-8993

    For retrofit applications, some high-brightness light-emitting diode (LED) products have the same form factor restrictions as existing incandescent light bulbs. Such form factor constraints may restrict the design and optimal performance of the LED technology. In this paper, some critical design issues for commercial LED bulbs designed for replacing E27 incandescent lamps are quantitatively analyzed. The analysis involves power audits on such densely packed LED systems so that the amounts of power consumption in: 1) the LED wafer; 2) the phosphor coating; and 3) the lamp translucent cover are quantified. The outcomes of such audits enable R&D engineers to identify the critical areas that need further improvements in a compact LED bulb design. The strong dependence of the luminous output of the compact LED bulb on ambient temperature is also highlighted.
  • Journal article
    Palladino M, Vinter RB, 2014,

    Minimizers That Are Not Also Relaxed Minimizers

    , SIAM Journal on Control and Optimization, Vol: 52, Pages: 2164-2179, ISSN: 1095-7138

    Relaxation is a widely used regularization procedure in optimal control, involving the replacement of velocity sets by their convex hulls, to ensure the existence of a minimizer. It can be an important step in the construction of suboptimal controls for the original, unrelaxed, optimal control problem (which may not have a minimizer), based on obtaining a minimizer for the relaxed problem and approximating it. In some cases the infimum cost of the unrelaxed problem is strictly greater than the infimum cost over relaxed state trajectories; we need to identify such situations because then the above procedure fails. The noncoincidence of these two infima leads also to a breakdown of the dynamic programming method because, typically, solving the Hamilton--Jacobi equation yields the minimum cost of the relaxed, not the original, optimal control problem. Following on from earlier work by Warga, we explore the relation between, on the one hand, noncoincidence of the minimum cost of the optimal control and its relaxation and, on the other, abnormality of necessary conditions (in the sense that they take a degenerate form in which the cost multiplier is set to zero). Two kinds of theorems are proved, depending on whether we focus attention on minimizers of the unrelaxed or the relaxed formulation of the optimal control problem. One kind asserts that a local minimizer which is not also a relaxed local minimizer satisfies an abnormal form of the Hamiltonian inclusion. The other asserts that a relaxed local minimizer that is not also a local minimizer also satisfies an abnormal form of Hamiltonian inclusion.
  • Journal article
    Bettiol P, Frankowska H, Vinter RB, 2014,

    Improved Sensitivity Relations in State Constrained Optimal Control

    , Applied Mathematics and Optimization, Vol: 71, Pages: 353-377, ISSN: 1432-0606

    Sensitivity relations in optimal control provide an interpretation of the costate trajectory and the Hamiltonian, evaluated along an optimal trajectory, in terms of gradients of the value function. While sensitivity relations are a straightforward consequence of standard transversality conditions for state constraint free optimal control problems formulated in terms of control-dependent differential equations with smooth data, their verification for problems with either pathwise state constraints, nonsmooth data, or for problems where the dynamic constraint takes the form of a differential inclusion, requires careful analysis. In this paper we establish validity of both ‘full’ and ‘partial’ sensitivity relations for an adjoint state of the maximum principle, for optimal control problems with pathwise state constraints, where the underlying control system is described by a differential inclusion. The partial sensitivity relation interprets the costate in terms of partial Clarke subgradients of the value function with respect to the state variable, while the full sensitivity relation interprets the couple, comprising the costate and Hamiltonian, as the Clarke subgradient of the value function with respect to both time and state variables. These relations are distinct because, for nonsmooth data, the partial Clarke subdifferential does not coincide with the projection of the (full) Clarke subdifferential on the relevant coordinate space. We show for the first time (even for problems without state constraints) that a costate trajectory can be chosen to satisfy the partial and full sensitivity relations simultaneously. The partial sensitivity relation in this paper is new for state constraint problems, while the full sensitivity relation improves on earlier results in the literature (for optimal control problems formulated in terms of Lipschitz continuous multifunctions), because a less restrictive inward pointing hypothesis is invoked in the proo
  • Journal article
    Bottrell N, Green TC, 2014,

    Comparison of current-limiting strategies during fault ride-through of inverters to prevent latch-up and wind-up

    , IEEE Transactions on Power Electronics, Vol: 29, Pages: 3786-3797, ISSN: 0885-8993

    Transient stability of a power network requires that generators remain synchronized and return to normal power export once a fault is cleared. For inverter-interfaced generators, one must ensure that current and voltage limiters do not latch-up and that controller integrators do not wind-up. A comparison of current-limiting strategies during fault ride-through of inverters to prevent latch-up and wind-up is presented. A voltage-controlled inverter with an inner current controller is used in this paper. Instantaneous limiting (saturation) and latched limiting with a variety of reset strategies are tested to check for correct operation when a fault is applied and cleared. All the cases were tested on an experimental system using 10-kVA inverters and low-impedance three-phase faults. The experimental results showing the current and voltage waveforms of the inverter are presented to test whether each strategy correctly transitioned from current limiting to normal operation once the fault was cleared and to examine the extent to which controller wind-up was a problem. Conclusions are drawn as to which current-limiting strategies provide good performance in ride-through and recovery from faults.
  • Journal article
    Howey DA, Mitcheson PD, Yufit V, Offer GJ, Brandon NPet al., 2014,

    Online measurement of battery impedance using motor controller excitation

    , IEEE Transactions on Vehicular Technology, Vol: 63, Pages: 2557-2566, ISSN: 0018-9545

    This paper presents a fast cost-effective technique for the measurement of battery impedance online in an application such as an electric or hybrid vehicle. Impedance measurements on lithium-ion batteries between 1 Hz and 2 kHz give information about the electrochemical reactions within a cell, which relates to the state of charge (SOC), internal temperature, and state of health (SOH). We concentrate on the development of a measurement system for impedance that, for the first time, uses an excitation current generated by a motor controller. Using simple electronics to amplify and filter the voltage and current, we demonstrate accurate impedance measurements obtained with both multisine and noise excitation signals, achieving RMS magnitude measurement uncertainties between 1.9% and 5.8%, in comparison to a high-accuracy laboratory impedance analyzer. Achieving this requires calibration of the measurement circuits, including measurement of the inductance of the current sense resistor. A statistical correlation approach is used to extract the impedance information from the measured voltage and current signals in the presence of noise, allowing a wide range of excitation signals to be used. Finally, we also discuss the implementation challenges of an SOC estimation system based on impedance.
  • Conference paper
    Mylvaganam T, Astolfi A, 2014,

    Approximate solutions to a class of nonlinear Stackelberg differential games

    , Conference on Decision and Control
  • Report
    Strbac G, Moreno Vieyra R, Konstantelos I, Aunedi M, Pudjianto Det al., 2014,

    Strategic Development of North Sea Grid Infrastructure to Facilitate Least-Cost Decarbonisation

    , Strategic Development of North Sea Grid Infrastructure to Facilitate Least-Cost Decarbonisation, Publisher: E3G

    Offshore wind power is expected to make a significant contribution towards de-carbonisingthe European energy system. It is envisaged that today’s installed capacity levels of about 5GW of offshore wind generation may reach 150GW by 2030, with approximately half of thiscapacity located in the North Seas. Given Europe’s goal of increased integration of the powermarkets by expanding cross-border interconnectors, there is a significant opportunity tointegrate offshore wind generation and interconnector projects in the North Seas in order totake advantage of potentially significant economies of scale and thus reduce network costs.
  • Journal article
    Carnevale D, Astolfi A, 2014,

    Semi-Global Multi-Frequency Estimation in the Presence of Deadzone and Saturation

    , IEEE TRANSACTIONS ON AUTOMATIC CONTROL, Vol: 59, Pages: 1913-1918, ISSN: 0018-9286
  • Journal article
    Lin D, Hui SYR, Chua LO, 2014,

    Gas Discharge Lamps Are Volatile Memristors

    , IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS I-REGULAR PAPERS, Vol: 61, Pages: 2066-2073, ISSN: 1549-8328
  • Journal article
    Chen H-T, Tan S-C, Hui SYR, 2014,

    Color Variation Reduction of GaN-Based White Light-Emitting Diodes Via Peak-Wavelength Stabilization

    , IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol: 29, Pages: 3709-3719, ISSN: 0885-8993
  • Report
    Schofield J, Carmichael R, Tindemans S, Woolf M, Bilton M, Strbac Get al., 2014,

    Residential consumer responsiveness to time-varying pricing

    , Report A3 for the “Low Carbon London” LCNF project
  • Conference paper
    Kerrigan EC, 2014,

    Co-design of Hardware and Algorithms for Real-time Optimization

    , 2014 European Control Conference (ECC), Publisher: IEEE, Pages: 2484-2489

    It is difficult or impossible to separate the performance of an optimization solver from the architecture of the computing system on which the algorithm is implemented. This is particularly true if measurements from a physical system are used to update and solve a sequence of mathematical optimization problems in real-time, such as in control, automation, signal processing and machine learning. In these real-time optimization applications the designer has to trade off computing time, space and energy against each other, while satisfying constraints on the performance and robustness of the resulting cyber-physical system. This paper is an informal introduction to the issues involved when designing the computing hardware and a real-time optimization algorithm at the same time, which can result in systems with efficiencies and performances that are unachievable when designing the sub-systems independently. The co-design process can, in principle, be formulated as a sequence of uncertain and non-smooth optimization problems. In other words, optimizers might be used to design optimizers. Before this can become a reality, new systems theory and numerical methods will have to be developed to solve these co-design problems effectively and reliably.
  • Journal article
    Mueller MA, Angeli D, Allgoewer F, 2014,

    On the performance of economic model predictive control with self-tuning terminal cost

    , Journal of Process Control, Vol: 24, Pages: 1179-1186, ISSN: 1873-2771

    In this paper, we analyze the closed-loop performance of a recently introduced economic model predictive control (MPC) scheme with self-tuning terminal cost. To this end, we propose to use a generalized terminal region constraint instead of a generalized terminal equality constraint within the repeatedly solved optimization problem, which allows us to obtain improved closed-loop asymptotic average performance bounds. In particular, these bounds can be obtained a priori. We discuss how the necessary parameters for the generalized terminal region setting can be calculated, and we illustrate our findings with two numerical examples.
  • Conference paper
    Padoan A, Astolfi A, 2014,

    Model reduction by moment matching for ZIP systems

    , 53rd IEEE Conference on Decision and Control, Pages: 3631-3636, ISSN: 0191-2216
  • Journal article
    Angeli D, Enciso GA, Sontag ED, 2014,

    A small-gain result for orthant-monotone systems under mixed feedback

    , Systems and Control Letters, Vol: 68, Pages: 9-19, ISSN: 0167-6911

    This paper introduces a small-gain result for interconnected orthant-monotone systems for which no matching condition is required between the partial orders in input and output spaces. Previous results assumed that the partial orders adopted would be induced by positivity cones in input and output spaces and that such positivity cones should fulfill a compatibility rule: namely either be coincident or be opposite. Those two configurations correspond to positive feedback or negative feedback cases. We relax those results by allowing arbitrary orthant orders. A linear example is provided showing that the small-gain iteration used for the negative feedback case is not sufficient for global attractivity under mixed feedback. An algebraic characterization is given of the new small-gain condition, generalizing a result known in the negative feedback case. An application is given to nonlinear protein networks with one positive and one negative feedback loop. © 2014 Elsevier B.V. All rights reserved.
  • Journal article
    Pin G, Chen B, Parisini T, Bodson Met al., 2014,

    Robust Sinusoid Identification With Structured and Unstructured Measurement Uncertainties

    , IEEE TRANSACTIONS ON AUTOMATIC CONTROL, Vol: 59, Pages: 1588-1593, ISSN: 0018-9286
  • Journal article
    Zhang Q, Zhang X, Polycarpou MM, Parisini Tet al., 2014,

    Distributed sensor fault detection and isolation for multimachine power systems

    , INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL, Vol: 24, Pages: 1403-1430, ISSN: 1049-8923
  • Journal article
    Pudjianto D, Castro M, Strbac G, Liu Z, van der Sluis L, Papaefthymiou Get al., 2014,

    Asymmetric impacts of European transmission network development towards 2050: Stakeholder assessment based on IRENE-40 scenarios

    , Energy Economics, Vol: 53, Pages: 261-269, ISSN: 1873-6181

    This paper presents the assessment of stakeholder impacts of European electricity transmission network investments for a set of future system development scenarios. A techno-economic analysis is adopted, which quantifies the economic impacts on different stakeholders including electricity producers, consumers, and network investors, under the future de-carbonisation pathways described in the IRENE-40 scenarios, ranging from 2010 to 2050. To quantify the impact of transmission infrastructure development, for each pathway, two distinct scenarios of future European transmission development are assessed: “low” (no new transmission between 2010 and 2050) and “high” (optimal transmission development between 2010 and 2050 to accommodate the generation pathway).The geographical scope of the analysis covers the EU 27+2 nations. A detailed case study of Spain and France is presented to illustrate the so-called ‘asymmetric’ impacts towards different stakeholders in different importing and exporting zones. The resulting arbitrage trades shift the market equilibriums, which eventually affect asymmetrically the welfare of stakeholders.
  • Conference paper
    Ge M, Kerrigan E, 2017,

    Noise Covariance Estimation for Time-varying and Nonlinear Systems

    , The 19th World Congress of the International Federation of Automatic Control

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