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This article presents the design and evaluation of robust controllers, based on linear programming, Kharitonov's theorem and Chebyshev's theorem, in order to enhance the performance of a typical structure of multistage converters present in direct current (dc) systems. In such electric power distribution systems, point-of-load converters act as a constant power load (CPL), which introduces a destabilizing nonlinear effects to their supply bus voltage. The multistage converter system uses a simplified scheme based on a cascaded converter, which is comprised of two dc–dc buck converters in a series connection. The robust controllers evaluated overcome the negative incremental impedance instability problem due to CPL, which causes a high risk of instability in interconnected converters. Thereby, the robust controllers evaluated ensure robust control performance and stability with a minor performance degradation compared to a conventional controller when the cascaded converter system is subjected to parametric uncertainties. The control methodologies evaluated are applied in both dc–dc buck converters. Assessments on the performance of the control methodologies evaluated are conducted. Experimental validations on a 160-W dc–dc cascaded converter system test board are carried out to verify the theoretical claims.

We investigate the state observer design problem for thermoacoustic instabilities in a Rijke tube using an infinite dimensional perspective. The observer, whose design is based on the backstepping methodology with Volterra and full integral terms, consists of a copy of the linearized plant model plus output injection terms and relies only on a single boundary acoustic pressure sensor. The exponential convergence, in the $L_{2}$ sense, of the observed error dynamics is proved, and the analytic expression of the observer gains are derived explicitly. The results are tested in an experimental Rijke tube prototype in order to illustrate the effectiveness of the method.

The present research work aims to study the large signal stability of two parallel-connected voltage-source inverters operating with resistive droop control through bifurcation analysis. This minimal setup can be considered a prototype of a small islanded ac microgrid studying where droop control acts as a synchronization scheme between inverters. Since switching power converters are strong nonlinear circuits, bifurcation analysis helps to understand nonlinear phenomena more deeply than the standard linear approach. Plenty of complicated nonlinear phenomena have been observed through bifurcation analysis, such as multiequilibria, period-doubling bifurcation, Hopf bifurcation, quasiperiodicity, border collision, bistability, and chaos. Thereby, a set of parameters under certain conditions can influence the equilibrium point and its stability, leading to some of the aforementioned nonlinear phenomena. In this sense, the main contribution of this article is to detect the nonlinear phenomena in considering islanded ac microgrid in order to determine a safe droop parameter region for more robust microgrid operation. This information can be summarized in bifurcation diagrams leading to practical rules for choosing the control and droop parameters. Experimental results are performed to validate the main instabilities predicted by the theoretical models.

Resonant sensors are widely employed in several applications, and resonant cavity sensors have advantages when noninvasive and non-destructive measurements are required. In this technology, the resonant frequency of the sensor is used to estimate different measurement parameters, since it varies according to the permittivity of the material under test. In this paper, a resonant frequency tracking system, combining a resonant cavity sensor and a phase-locked loop circuit, is investigated. The system has a quadrature phase detector being able to estimate both, phase and magnitude information, and is well suited for applications involving water as a material under test. A complete description of the measurement system combining experimental results with theoretical models, previously reported in the literature, is accomplished. Such theoretical models are based on the amplitude-phase dynamics and assume that the sensor is modeled as a linear resonator. Hence, a semi-empirical modeling is elaborated and the validation results precisely demonstrate the reliability of the modeling, proving to be very useful for further analysis.

The strive to increase the energy yield of photovoltaic (PV) power systems has made PV module integrated dc–dc converters (dc-MICs) a reality of modern PV plants. These converters regulate their input voltage, and their dynamic behavior is heavily influenced by the non-linear characteristic of the PV module. The regulation of the PV module voltage and average inductor current by means of a linear cascaded controller is a popular control technique, simplifying the converter dynamics, and providing inherent current limiting; however, it is prone to instability depending on the interaction between the PV source and the interfacing converter, as well as the value of the controller parameters. These factors present a clear challenge for control design; moreover, the converter transient response undesirably depends on the PV module operating point. In order to solve these issues, while maintaining regulation of PV module voltage and average inductor current, this paper proposes to adopt a non-linear controller designed with the feedback linearization control (FLC) technique. The control laws are derived and implemented in a non-inverting buck–boost dc module integrated converter, as this is a favorite topology for the PV interfacing application. A digitally controlled converter prototype is built and used to obtain experimental results, where the FLC technique is compared with a linear cascaded control technique. The results confirm the superior performance of the presented FLC technique, which is robust and able to regulate the converter input voltage with fast and consistent dynamics, regardless of the PV module or load operating conditions.

The electronic circuit of a DC–DC boost power converter under a specific sliding mode control strategy is analyzed. This circuit is modeled as a discontinuous piecewise linear three-dimensional dynamical system, whose state space is divided into two open regions by a plane acting as the switching manifold. This system displays sliding motion confined to the switching manifold and limited by two straight lines where lie the points of tangency, one for each involved linear vector field. Such tangency lines can intersect transversally on a invisible two-fold point known as Teixeira singularity. Our goal is to investigate an interesting bifurcation that occurs on the Teixeira singularity, involving both a pseudo-equilibrium point and a crossing periodic orbit. This bifurcation, named TS-bifurcation, occurs when a pseudo-equilibrium point of the discontinuous piecewise linear system, capable of moving between the attractive and repulsive sliding regions as a result of the change in a parameter, collides with the Teixeira singularity. Simultaneously, a periodic orbit arises from the Teixeira singularity crossing the switching manifold in two points. Apart from the TS-bifurcation characterization in the DC–DC converter, we have numerically detected other non-local phenomena like a saddle-node bifurcation of crossing limit cycles. Experimental results to illustrate the effects of the TS-bifurcation on the dynamics of a power electronic converter are also shown.

The non-inverting buck-boost converter has emerged as one of the most promising topologies for distributed maximum power point tracking applications. Pressure to reduce cost and volume of photovoltaic module integrated converters often compels bootstrap capacitor type gate drivers. The bootstrap technique is a simple and economical solution to create a floating power supply for high-side gate drivers. However, it comes with a major challenge, as the low-side switch must be periodically turned on , to recharge the high-side bootstrap capacitor. In the buck and the boost mode, one of the switching legs of the converter must be held with the high-side switch on , which eventually discharges the high-side capacitor. When the non-inverting buck-boost input voltage is regulated by means of a single voltage loop, recharging the bootstrap capacitor causes an undesired ripple in the PV module voltage, degrading the regulation quality. In order to mitigate this problem, this paper proposes a cascaded control technique, entailing regulation of the PV module voltage and inductor current. The cascaded controller dramatically improves the rejection of the disturbance induced by the periodic recharge of the bootstrap capacitor, thanks to the fast action of the current loop, regulating the average value of the inductor current. Converter transfer functions and controller design are derived and validated. Experimental results clearly show that the cascaded control scheme outperforms the traditional voltage control scheme, resulting in a better regulation of the PV module voltage. This main advantage comes at the cost of an additional measurement of the average inductor current.

The problem of boundary stabilization of thermoacoustic oscillations in the Rijke tube is investigated using the backstepping method; as a first step, this work only considers the full-state design. This system consists of a vertical tube open at both ends and a heater placed in the lower half of the tube. To study this problem we consider that the mathematical model takes the form of 2 × 2 linear first-order hyperbolic partial differential equations (PDEs) with a point source term (induced by the Dirac delta distribution) on the right hand side, plus the coupling of an ordinary differential equation (ODE), and control input at one boundary condition. The presence of the Dirac delta distribution implies that the system solution has a discontinuity on a point of the domain, but is continuous everywhere else. We use a coordinate transformation to rewrite the equations into a system of four transport PDEs convecting in opposite directions and to translate the discontinuity to the boundary conditions. Then, a full state feedback backstepping controller is designed to exponentially stabilize the origin. However, the model is non-strict-feedback making unfeasible the use of standard backstepping designs. This issue is tackled by formulating a well-posed and invertible integral transformation with Volterra and Fredholm terms that maps the Rijke system into a target system with desirable stability properties. An exact piecewise-differentiable expression for the kernels of this transformation is found, allowing us in turn to derive an explicit feedback control law. Simulation results are presented to illustrate the effectiveness of the proposed control design.

An important parameter in the study of multiphase flows is the volume fraction, which is defined by the ratio between the volume occupied by a specific phase and the total volume of the mixture. This is a fundamental parameter that appears in most of the flow models and frequently is applied to assist flow pattern identification routines as well as to estimate two-phase flow rates. This paper presents a water volume fraction estimation method based on electrical capacitance tomography (ECT) called electrical capacitance tomometry (ECTM). An electrode combining strategy using rotating electrodes has been implemented to transform a 12-electrode sensor into a 2-electrode sensor, simplifying the measurement hardware circuit. This strategy increases the signal-to-noise ratio and consequently improves the measurement quality. Moreover, the proposed ECTM method needs just 12 capacitance measurements and three iteration steps to converge, providing confidence in real-time applications. Simulations using COMSOL Multiphysics software and experimental results allow us to evaluate the performance of the proposed water volume fraction measurement method which presents smaller absolute errors than the conventional ECT method.

The noninverting buck–boost converter has attracted significant attention in the photovoltaic (PV) module integrated application, as it offers high efficiency while maintaining a low cost and a simple topology. When this converter is employed, special care must be taken at the transition between buck and boost operating modes, as a dead-zone in the voltage transfer function can cause abrupt perturbations in the controlled voltage, decreasing the regulation quality and ultimately lowering the power conversion efficiency. In this paper, a new dead-zone compensation technique is proposed with the scope of smoothing the transition between operating modes, eliminating the voltage ripple and improving the regulation performance, while maintaining high efficiency. The converter under analysis is studied together with its gate driving circuit, which is based on a bootstrap capacitor power supply for the high-side switches. The proposed dead-zone compensation technique is deduced by using the principle of maintaining the ideal voltage gain function across the converter operating range. The technique is analyzed, implemented and tested on a purposely built PV module integrated noninverting buck–boost converter. The experiments reveal a sensible improvement of the voltage regulation during mode transition, confirming the effectiveness of the proposed technique and its fitness for the PV application.

In many industrial activities, the so-called multiphase flow is commonly found, which denotes the simultaneous stream of two or more distinct substances. A typical example is oil-gas and/or oil-water mixtures flowing through production pipelines in the petroleum industry. Flow behavior often determines the efficiency and safety of plants and equipment where multiphase flow is present. Therefore, monitoring of such flows and derived parameters is highly desired in many applications. In the past, a number of measurement techniques and instruments have been developed and applied [1]. Multiphase flow instrumentation relies on a number of physical principles, including common instrumentation and measurements that are based on electrical, sonic or radiation methods. This paper shows three different sensors currently being developed in Brazil, which are based on electrical measurements. They range from a simple phase concentration measurement up to multiphase flow imaging. Each instrument offers a tailored solution with potential either for field applications or for detailed multiphase flow investigation in pilot plant studies.

Currently, high-performance power conversion requirements are of increasing interest in microgrid applications. In fact, isolated bidirectional dc-dc converters are widely used in modern dc distribution systems. The dual active bridge (DAB) dc-dc converter is identified as one of the most promising converter topology for the mentioned applications, due to its benefits of high power density, electrical isolation, bidirectional power flow, zero-voltage switching, and symmetrical structure. This study presents a power management control scheme in order to ensure the power balance of a dc microgrid in stand-alone operation, where the renewable energy source (RES) and the battery energy storage (BES) unit are interfaced by DAB converters. The power management algorithm, as introduced in this work, selects the proper operation of the RES system and BES system, based on load/generation power and state-of-charge of the battery conditions. Moreover, a nonlinear robust control strategy is proposed when the DAB converters are in voltage-mode-control in order to enhance the dynamic performance and robustness of the common dc-bus voltage, in addition to overcoming the instability problems that are caused by constant power loads and the dynamic interactions of power electronic converters. The simulation platform is developed in MATLAB/Simulink, where a photovoltaic system and battery system are selected as the typical RES and BES, respectively. Assessments on the performance of the proposed control scheme are conducted. Comparisons with the other control method are also provided.

This paper addresses the development of a hierarchical control strategy for microalgal production in a tubular photobioreactor. The proposed control scheme is composed of two layers. On one hand, a lower layer is in charge of tracking a pH set-point by a PI controller plus feedforward compensator, which was used due to its recognized efficiency in industrial control processes. On the other hand, the upper layer calculates optimal pH set-points based on an economic model predictive control approach. The main aim of this control scheme is to maximize profits, computed as the difference between the incomes obtained from the final production sale and the associated production costs (including the environmental impact of the exhausted CO2 losses). For this purpose, two types of models have been used depending on the requirements of each layer. Simulated and experimental results of the proposed hierarchical control architecture are presented, as well as a comparison with a single-layer architecture with constant reference implemented by the controller used in the lower layer of the hierarchical structure.

This paper describes the development of a distributed sliding mode control controller to regulate the pH of a tubular photobioreactor using the total inorganic carbon as the controlled variable of the process. The main purpose of the control system is to maintain the pH around a desired reference value and, at the same time, attenuate the undesirable transients caused by the disturbances. To study this problem, it is considered that this system is written as a set of hyperbolic partial differential equations. The distributed parameter model of the photobioreactor is transformed into a finite dimensional system by the method of characteristics. Then, the control design is performed in the finite dimensional system instead of the original distributed parameter model. The proposal allows to characterize the sliding regimes and their fundamental properties by a geometric approach. Through simulation and experimental results, the method is shown to be effective in controlling a tubular photobioreactor.

Solar plants have nonlinear dynamics which must be taken into account when a control system is applied to them. The main purpose of the control systems is to maintain the outlet temperature in a desired reference value and, at the same time, attenuate the undesirable transients caused by the disturbances. Linear controllers, like PID ones, are not able to obtain good performance over the whole operation range of these kind of plants. To overcome these limitations two nonlinear controllers, a nonlinear model-based predictive controller and a distributed sliding mode controller, are applied to a solar plant in this work. The performance of these controllers is tested through experimental and simulation results, which show the tracking and disturbance rejection capabilities of the proposed controllers.

The stability of dc microgrids (MG s ) depends on the control strategy adopted for each mode of operation. In an islanded operation mode, droop control is the basic method for bus voltage stabilization when there is no communication among the sources. In this paper, it is shown the consequences of droop implementation on the voltage stability of dc power systems, whose loads are active and nonlinear, e.g., constant power loads. The set of parallel sources and their corresponding transmission lines are modeled by an ideal voltage source in series with an equivalent resistance and inductance. This approximate model allows performing a nonlinear stability analysis to predict the system qualitative behavior due to the reduced number of differential equations. Additionally, nonlinear analysis provides analytical stability conditions as a function of the model parameters and it leads to a design guideline to build reliable (MG s ) based on safe operating regions.

In multiphase flow measurement, one of the most challenging issues is to define an adequate technology for a specific scenario, taking into account the measurement accuracy, implementation feasibility and costs. The electromagnetic technology based on resonant cavities is often employed in water-cut meters to measure two-phase flows such as water/oil and water/gas mixtures. The main disadvantage of this technology is the electromagnetic signal attenuation that occurs as the water content decreases. This undesirable behavior is amplified due to the impedance mismatch between the sensor ports and the transmitter/receiver modules. This paper presents a study to implement an impedance matching network in order to improve the instrument performance. Impedance matching networks were built, taking into account the matching for a 100%, 50% and, also, for the worst case of 0% of water fraction where there is a significant signal attenuation. The implemented networks improved the signal amplitude ratio between the first resonant mode and the other modes, increasing the identification accuracy of the first resonance peak.

The theory of sliding mode control is applied to a solar power plant described by a system of hyperbolic partial differential equations. The method of characteristics is used to extend the sliding mode concepts for application to partial differential equations. The main purpose of the controller is to manipulate the oil flow rate to maintain the field outlet temperature around the desired reference value, and, at the same time, to attenuate the disturbances effects. Through simulations the method is shown to be effective in controlling a solar power plant under real disturbances data.

The aim of the present study is to contribute some knowledge of phase separation phenomena of gas–liquid turbulent flow in curved pipe and provide a basis for the development of an in-line gas–liquid separator and flow pattern conditioning device. A systematic study of gas–liquid three-phase flow in 3D helical pipes was numerically performed. Gas phase distribution in the flow through the helical pipe was analyzed for various flow parameters conditions and different pipe geometries. Experimental qualitative results show that a helical pipe can, successfully, be used to condition a bubble flow into a stratified (stratified wavy flow). The main idea is to put it just upstream of sensors that are flow regime dependent; ensuring that the sensor, once calibrated to work in this flow pattern, suffers no reduction in its performance and, consequently, avoid additional sources of error.

Modular and cascaded multilevel converters provide a functional solution for the integration of energy storage systems (ESSs). This paper develops a hybrid multilevel converter based on the modular multilevel converter (MMC) that can be functionally extended with partial embedded ESS as a fraction of the overall converter power rating. The configuration, which can operate as a typical DC-AC converter, enables multi-directional power flow between the DC- and AC-side of the converter, as well as the embedded energy storage elements. The use of a three-phase flying-capacitor submodule eliminates the second-order harmonic oscillations present in modular cascaded multilevel converters. Current, voltage and power control are discussed in the paper while simulation results illustrate the operation of the hybrid MMC as a DC-AC converter in a typical inverter application and the additional functions and control of the embedded ESS. Published version

In gas-lifted oil fields, high pressure gas is injected at the bottom of the production tubing of the wells to artificially lift oil to the surface. Lift-gas should enter each well at a certain mass flow and pressure, giving rise to the problem of deciding which compressors (facilities) should be installed and how they supply the demands of the wells (clients). This compressor scheduling is a mixed-integer, nonconvex, nonlinear programming problem that generalizes the standard facility location problem. By piecewise-linearizing the performance curve of each compressor—a function relating output mass flow and discharge pressure, the problem is recast as a mixed-integer linear program. This paper presents this linear reformulation, proposes families of valid inequalities, and reports on results from the application of these inequalities to solve representative instances of the compressor scheduling problem.

O presente trabalho apresenta uma nova técnica de controle por modo deslizante para a geração de oscilações autônomas em um inversor monofásico. A técnica de controle proposta utiliza a abordagem de modelagem de energia e nenhum sinal de referência senoidal externo é necessário. São apresentados resultados de simulação e experimentais obtidos considerando cargas resistiva e não linear.

This paper addresses the problem of determining robust stability regions for a class of nonlinear systems with time-invariant uncertainties subject to actuator saturation. The unforced nonlinear system is represented by differential-algebraic equations where the system matrices are allowed to be rational functions of the state and uncertain parameters, and the saturation nonlinearity is modelled by a sector bound condition. For this class of systems, local stability conditions in terms of linear matrix inequalities are derived based on polynomial Lyapunov functions in which the Lyapunov matrix is a quadratic function of the state and uncertain parameters. To estimate a robust stability region is considered the largest level surface of the Lyapunov function belonging to a given polytopic region of state. A numerical example is used to demonstrate the approach. Este artigo trata do problema de determinar regiões de estabilidade robustas para uma classe de sistemas não lineares com incertezas invariantes no tempo e sujeitos à saturação no sinal de controle. O sistema não linear é representado por uma equação algébrico- diferencial onde as matrizes do sistema são funções racionais dos estados e incertezas e a saturação no controle é representada como uma condição de setor. As condições de estabilidade local propostas são expressas por LMIs e estão baseadas numa função de Lyapunov que é polinomial (ordem 4) nos estados e quadrática nos parâmetros incertos. Para estimar a região de estabilidade robusta propõe-se um problema de maximização da maior curva de nível da função de Lyapunov dentro de um politopo dado representando as condições iniciais. Os resultados são ilustrados através de um exemplo numérico.

Variable structure systems are nonlinear because of the discontinuities introduced by switching. This brief analyzes local bifurcations directly related to the presence of a discontinuity surface and to the variation of its parameters. All types of bifurcations described in this brief are illustrated by an example from power electronics.

In this paper the nonlinear dynamics of interconnected power converters in an islanded direct current (DC) microgrid is analyzed. By using a simplified scheme based on two cascaded converters we analyze the dynamical behavior that can arise from the interconnection of these devices on a common DC bus. Furthermore, in order to address the bus voltage control problem, we propose a Sliding Mode Controller for a DC-DC bidirectional power converter to control the DC bus voltage under instantaneous constant power loads. This class of loads introduces a destabilizing nonlinear effect on the converter through an inverse voltage term that can lead to significant oscillations in the DC bus voltage. Simulation results are shown to illustrate the nonlinear analysis.