Nonlinear Control Approaches of Several Power Converters

The exploitation and utilization of renewable energy have become the important measures taken by countries all over the world to solve the contradiction among energy shortage, economic development and ecological environment. As an important bridge and energy conversion channel between renewable energy and electric energy, power converter is a basic form of transformation and control of electric energy, which plays a vital role in the exploitation and utilization of renewable energy. In the past few decades, due to the significance of both theory and practical applications, the study of power converter has become one of the hotspots of research in the field of power electronics and automation. The study of power converter control strategies is an important study research of power converter. The control strategy as the core of the control system directly determines the dynamic and static performance of the power converter. The power converter system is a typical nonlinear system. However, most of power converter control strategies are designed based on linearization control methods, which makes the control system sensitive to system parameter variations, slow dynamic response speed and poor steady-state performance, etc. Thus, in order to further improve the dynamic and static performances of power converters, the investigation of using nonlinear control methods for power converters is a challenging and meaningful work. Based on modern control theory utilizing nonlinear control approaches, this dissertation investigates the nonlinear control strategies design for several typical power converters, and the main contributions are as follows: (1) The important role of power converters in renewable energy power generation systems is introduced. Then the internal and overseas research situations of the control strategies design for the several typical power converters are classified and summarized, where the theoretical significance and the practical application backgrounds are given, and the study structures and contents of this dissertation are presented. (2) Two system models are built for the DC-DC Buck converters, respectively, i.e., the nominal system model and uncertain system model. Based on the nominal system without considering parametric uncertainties, the single-loop adaptive control strategy is built by adaptive and back-stepping control approach, and the double-loop adaptive control strategy is set up by adaptive and sliding mode control approach. Based on the uncertain system model, the single-loop disturbance observer based control strategy is developed using designed disturbance observer and back-stepping control technique, and the double-loop disturbance observer based control strategy is synthesized using designed disturbance observer and sliding mode control method. (3) The control strategy of voltage regulation and current tracking for three phase two-level grid-connected power rectifiers is presented. By using power-invariant Park’s transformation, an averaged mathematical model of power converters is obtained in synchronous reference frame. Then a novel control strategy using adaptive control and technique is proposed to regulate the dc-link output voltage as well as track a desired current reference. More specifically, an efficient adaptive controller is established in the external loop for regulating dc-link output voltage in the presence of external disturbances. A set of controllers are designed in the internal loop to force the input currents track their desired values. (4) A novel robust control strategy is proposed for three-level neutral-point-clamped power rectifiers. The proposed control scheme consists of three control loops, i.e., instantaneous power tracking control loop, voltage regulation loop and voltage balancing loop. First, in the power tracking control loop, a set of adaptive sliding mode controllers are established to drive the active and reactive power tracking their desired values via radial basis function neural network technology. In the voltage regulation loop, an efficient but simple adaptive controller is designed to regulate dc-link output voltage where the load is considered as an external disturbance. Moreover, a composite controller is developed in the voltage balancing loop to ensure imbalance voltages between two dc-link capacitors close to zero, in which a reduced-order observer is used to estimate sinusoidal disturbance improving the converter performance. (5) Based on the second order sliding mode control technique, a novel control strategy is proposed for three-phase power rectifiers under unbalanced grid conditions to achieve cooperative control between power and current. A consolidated control objective which can be flexibly adjust among the degree of oscillation in active and reactive powers and balance of three-phase current is obtained in the stationary frame. Based on the dynamic of the converter and control objective, a control scheme in a cascaded framework is presented, in which an adaptive observer is applied to estimate the positiveand negative-sequence of grid voltage without complex filtering process. In the current tracking loop, the super–twisting algorithm current controller coupled with super-twisting differentiator is implemented to force the currents to their references, featuring a fast dynamic and an improved robustness. Also, in the voltage regulation loop, an effective composite controller is developed for regulation of the output voltage, where a supertwisting observer is used to estimate load disturbance. (6) The problem of regulation output voltage of three-phase two level filtered voltage source inverters is presented using disturbance observer-based integral sliding mode control approach. First, the dynamics of the inverter are reformulated to facilitate the use of the proposed control strategy, which consider the parametric uncertainties of filter. A disturbance observer is designed to estimate the parametric uncertainties and external disturbances. Then, an integral sliding mode surface is established considering the voltage tracking error, its integral and the estimations of the parametric uncertainties and external disturbances. A sliding mode controller is proposed such that the systems are robustness to the admissible uncertainties and disturbances and satisfy the reaching condition. The stability of the closed-loop system is proved based on the Lyapunov theory

eXtended Variational Quasicontinuum Methodology for Lattice Networks with Damage and Crack Propagation

Lattice networks with dissipative interactions are often employed to analyze materials with discrete micro- or meso-structures, or for a description of heterogeneous materials which can be modelled discretely. They are, however, computationally prohibitive for engineering-scale applications. The (variational) QuasiContinuum (QC) method is a concurrent multiscale approach that reduces their computational cost by fully resolving the (dissipative) lattice network in small regions of interest while coarsening elsewhere. When applied to damageable lattices, moving crack tips can be captured by adaptive mesh refinement schemes, whereas fully-resolved trails in crack wakes can be removed by mesh coarsening. In order to address crack propagation efficiently and accurately, we develop in this contribution the necessary generalizations of the variational QC methodology. First, a suitable definition of crack paths in discrete systems is introduced, which allows for their geometrical representation in terms of the signed distance function. Second, special function enrichments based on the partition of unity concept are adopted, in order to capture kinematics in the wakes of crack tips. Third, a summation rule that reflects the adopted enrichment functions with sufficient degree of accuracy is developed. Finally, as our standpoint is variational, we discuss implications of the mesh refinement and coarsening from an energy-consistency point of view. All theoretical considerations are demonstrated using two numerical examples for which the resulting reaction forces, energy evolutions, and crack paths are compared to those of the direct numerical simulations.Comment: 36 pages, 23 figures, 1 table, 2 algorithms; small changes after review, paper title change

A Supervisor for Control of Mode-switch Process

Many processes operate only around a limited number of operation points. In order to have adequate control around each operation point, and adaptive controller could be used. When the operation point changes often, a large number of parameters would have to be adapted over and over again. This makes application of conventional adaptive control unattractive, which is more suited for processes with slowly changing parameters. Furthermore, continuous adaptation is not always needed or desired. An extension of adaptive control is presented, in which for each operation point the process behaviour can be stored in a memory, retrieved from it and evaluated. These functions are co-ordinated by a ¿supervisor¿. This concept is referred to as a supervisor for control of mode-switch processes. It leads to an adaptive control structure which quickly adjusts the controller parameters based on retrieval of old information, without the need to fully relearn each time. This approach has been tested on experimental set-ups of a flexible beam and of a flexible two-link robot arm, but it is directly applicable to other processes, for instance, in the (petro) chemical industry

Thermodynamic costs of information processing in sensory adaption

Biological sensory systems react to changes in their surroundings. They are characterized by fast response and slow adaptation to varying environmental cues. Insofar as sensory adaptive systems map environmental changes to changes of their internal degrees of freedom, they can be regarded as computational devices manipulating information. Landauer established that information is ultimately physical, and its manipulation subject to the entropic and energetic bounds of thermodynamics. Thus the fundamental costs of biological sensory adaptation can be elucidated by tracking how the information the system has about its environment is altered. These bounds are particularly relevant for small organisms, which unlike everyday computers operate at very low energies. In this paper, we establish a general framework for the thermodynamics of information processing in sensing. With it, we quantify how during sensory adaptation information about the past is erased, while information about the present is gathered. This process produces entropy larger than the amount of old information erased and has an energetic cost bounded by the amount of new information written to memory. We apply these principles to the E. coli's chemotaxis pathway during binary ligand concentration changes. In this regime, we quantify the amount of information stored by each methyl group and show that receptors consume energy in the range of the information-theoretic minimum. Our work provides a basis for further inquiries into more complex phenomena, such as gradient sensing and frequency response.Comment: 17 pages, 6 figure