Unstable Operation Research Articles

Reduced-order discrete modeling method and nonlinear analysis of a discontinuous conduction mode buck converter with a constant power load

Discontinuous conduction mode (DCM) DC–DC converters with constant power loads (CPLs) are the widely used power electronics cascade topologies of distributed renewable energy power systems. There exists rich nonlinear dynamics of these cascade topologies, which tend to induce unstable operation of the distributed renewable energy systems. However, the difficulties of output regulation and discrete modeling for DCM DC–DC converters with CPLs decelerate the progress of the study of nonlinear dynamics in these systems. To study the mechanism of these nonlinear dynamics for DCM DC–DC converters with CPLs, this paper proposes a reduced-order discrete modeling method for the system by considering a current-controlled discontinuous conduction mode (CC-DCM) Buck converter with a CPL as example. The nonlinear dynamics of CC-DCM Buck converter with a CPL are studied by using the proposed reduced-order discrete model. The theoretical results display complex nonlinear behaviors of this system, such as multicycle orbits, states jumping between different periodic orbits or between periodic orbits and chaos, chaotic states, and orbits interleaving with the variation of the power of CPL. Both simulation and experimental results illustrate the effectiveness of the proposed reduced-order discrete modeling method and the correctness of nonlinear characteristic analysis of the system. Based on the proposed reduced-order discrete modeling method, the problems of discrete modeling DCM DC–DC converters with CPLs in DCM states using the stroboscopic mapping method are resolved. This study is expected to provide reference for the further study and application of these power electronic cascaded systems.

Fixed-Frequency PWM Control and Stability of Series-Stacked Buffer for 2ω-Power Decoupling

Active second-harmonic ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\omega $ </tex-math></inline-formula> ) filters have become an integral technology for single-phase power converters in high-power density designs. The series-stacked buffer (SSB) has emerged as an attractive topology among the existing solutions due to its low power, high efficiency, and compact design. However, one of the challenges in adopting SSB lies in its control, where hysteresis current control has been adopted conventionally. This results in a wide variation in the switching frequency, making the digital control implementation and filter design complex. On the other hand, fixed-frequency pulsewidth modulation (PWM) control necessitates developing a model for the systematic controller design to ensure stability and desired filtering performance. The assumption of SSB modeled as a second-harmonic current source, independent of the dc bus circuit components, fails to capture the dc bus loading effect on the SSB. In this work, a dynamic model of SSB is developed where the main dc bus and its passive elements are considered. The derived model enables a systematic approach for the controller design while ensuring the desired <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\omega $ </tex-math></inline-formula> filtering. The stability limits and parameter sensitivities of the model are studied analytically and in simulations. The analytical model is validated, and the proposed controller performance and stability limit are verified experimentally on a hardware prototype. A video demonstrating the transition from stable to unstable operation is provided.

Design and Mechanical Analysis of a Cam-Linked Planetary Gear System Seedling Picking Mechanism

This study designed a cam-linked planetary gear system automatic seedling picking mechanism to address unstable operation and issues of high damage to the substrate caused by the picking mechanism of the dryland vegetable seedling transplanter. Through an analysis of the kinematic principle of the structure and the establishment of the kinematic model, computer-aided analysis software was developed using Visual Basic 6.0. Consequently, a set of structural parameter values satisfying the target trajectory was derived employing the human–computer interaction method, and the 3D model was designed. The model was imported into Adams for kinematic simulation, and the seedling picking mechanism’s trajectory during the operation was obtained through simulation. Modal analysis of the model was performed using Ansys, and the first six-order modal vibration patterns and modal frequencies of the seedling picking mechanism were obtained under the simulated working environment. The results confirmed that no resonance occurred during the operation. Comparisons of the seedling picking needle trajectory with the idle test revealed that the theoretical, simulated, and test trajectories were approximately identical. This proved the reliability of the theoretical design of the seedling picking mechanism, the machining of the parts, and the test bench construction. The success rates of seedling picking were 97.66, 96.09, 93.75, and 90.63% at 90, 100, 110, and 120 plants/min, respectively, with rates of substrate damage of 4.43%, 6.73%, 9.57%, and 14.37%, respectively. Thus, the experimental results confirmed that the operating parameters of the cam-linked planetary gear system seedling picking mechanism satisfied the design requirements.

Construction of bubbles enrichment sites and gas-diffusion-microchannel to assist Cr(VI) electrochemical reduction in a flow-through electrochemical system

Due to characteristics of the forced convection and enhanced mass transfer, graphite felt cathode based flow-through electrochemical system presented high electroreduction efficiency for aqueous Cr(VI). However, disadvantage is that bubbles generated by water electrolysis cannot pass through the liquid filled microchannel in time, which is easy to cause gas accumulation and an unstable electrochemical operation. Herein, we demonstrated graphite felt flow-through cathode designed with hydrophobic polytetrafluoroethylene (PTFE) as bubbles enrichment sites and construction of gas-diffusion-microchannel to assist in Cr(VI) electrochemical reduction. Cr(VI) reduction kinetics constant by graphite felt flow-through cathode increased 4.6 times after loading PTFE and was 2.6 ∼ 5.7 times higher than parallel-plate electrodes system. Typical electrochemical analysis suggested that PTFE can enrich and accelerate the dissolved oxygen mass transfer rate at PTFE-electrode-solution micro-interface, realizing oxygen digestion via two-electron reduction reaction. The unconsumed bubbles were transported out of electrochemical cell along gas-diffusion-microchannel to avoid gas accumulation and maintained a stable operation. Batch experiments indicated the ∼ 99.9% Cr(VI) reduction efficiency when the initial pH 2 and 100 mg/L Cr(VI) feed was treated under a single-pass pattern with flow rate of 1.5 mL/min and current density of 12.74 mA/cm2. Mechanism study revealed that faraday reaction was the main driving force for Cr(VI) reduction, while Cr(VI)-induced activation of in-situ electro-generated H2O2 contributed to formations of •OH and intermediate chromium species. This work also shows the feasibility of remediation for Cr(VI)-organic coexistence environment by a catalyst-free flow-through electrochemical system.

Adaptive Complex Systems Applied in Electrical Submersible Pumps Control

Summary The oil and gas industry uses very expensive electrical submersible pumps (ESPs) to lift oil to the surface. It is common for the ESP to operate with a biphasic gas-liquid flow. However, large amounts of gas inside the pump generate instabilities, such as surges and gas locks. If operated for a long time under such instabilities, the performance of the ESP can decrease significantly, causing premature deterioration, which can lead to heavy losses and even a complete stoppage of production. To reduce the occurrence of this unstable operation, the research presented here proposes an adaptive controller based on the monitoring of the biphase condition. A nonlinear ESP model represents the pump behavior for a variable percentage of the two-phase flow, used by the controller learning complex adaptive technique, which is based on a genetic algorithm to generate a set of rules to keep the ESP in safe operating conditions. At the beginning of the training, the adaptive complex system generated erratic behavior; however, over time, it came to represent more precise rules, bringing performance to stable conditions. Furthermore, if the ESP eventually reaches a harmful condition, the adaptive controller takes action to return it to a safe condition. The result of the simulated tests shows that the complex adaptive controller can acquire enough knowledge after training to keep an ESP working in stable conditions, increasing the life of the production equipment and avoiding sudden stops during oil lifting.

Leading Edge Bumps for Flow Control in Air-Cooled Condensers

Air-cooled condensers (ACCs) are commonly found in power plants working with concentrated solar power or in steam power plants operated in regions with limited water availability. In ACCs, the flow of air is driven toward the heat exchangers by axial fans that are characterized by large diameters and operate at very high mass flow rates with a near-zero static pressure rise. Given the overall requirements in steam plants, these fans are subjected to inflow distortions, unstable operations, and are characterized by high noise emissions. Previous studies show that leading edge bumps in the tip region of axial fans can effectively reduce the sound pressure levels without affecting the static efficiency. Nevertheless, the effects of this treatment in terms of flow patterns and heat exchange in the whole ACC system were not investigated. In this work, the effect of leading edge bumps on the flow patterns is analyzed. Two RANS simulations were carried out using OpenFOAM on a simplified model of the air-cooled condenser. The fans are simulated using a frozen rotor approach. Turbulence modeling relies on the RNG k-epsilon model. The fan is characterized by a diameter of 7.3 m and a 333 m3/s volumetric flow rate at the design point. The presence of the heat exchanger is modeled using a porous medium. The comparison between the flow fields clearly exerts that the modified blade is responsible for the redistribution of radial velocities in the rotor region. This drastically reduces the losses related to the installation of the fan in a real configuration.

Research on afterburning control of more electric engine with a nonlinear fuel supply system

More electric engine (MEE) with electric pump metering fuel has the problem of unstable engine control due to the dead-zone characteristics of afterburner fuel actuator. On the basis of conventional μ modified adaptive control method, a compound modified adaptive control (CMAC) system based on neural network inverse model is proposed in this study. At first, the afterburner fuel actuator model of MEE is established through Simulink/AMESim co-simulation to study its fuel supply characteristics and reveal the reasons for dead-zone characteristics of the actuator. Subsequently, an integrated model of afterburner fuel actuator/MEE is established to investigate the effect of dead-zone characteristics on engine performance. Finally, the CMAC system for afterburner fuel flow closed-loop control based on neural network inverse model and improved μ modified adaptive control method is designed. The simulation results show that the dead-zone characteristics of afterburner fuel actuator can cause engine shaking and unstable operation. The engine thrust has a continuous fluctuation with amplitude of about 0.2%. The proposed CMAC system can effectively avoid the dead-zone interval of actuator and achieve stable transition of the engine state in the entire operating range of the afterburner fuel actuator. Thus, the stability of the control system can be significantly improved. Meanwhile, the improved μ modified adaptive control algorithm in the CMAC system has better dynamic characteristics when compared with the conventional μ modified adaptive control algorithm.

CFD-Based Prediction of Combustion Dynamics and Nonlinear Flame Transfer Functions for a Swirl-Stabilized High-Pressure Combustor

Thermoacoustic instabilities in gasturbine combustor systems can be predicted in the design phase with a thermoacoustic network model. In this model, the coupling between acoustic pressure fluctuations and the combustion rate is described by the Flame Transfer Function. The present paper introduces a new, efficient, and robust method for deriving the FTF from CFD predictions by means of a discrete multi-frequency sinusoidal fuel flow excitation method. The CFD-based FTF result compares well with experimental data for the time delay, but for the gain, only up to 400 Hz. Above 400 Hz, the CFD result reveals a smooth low-amplitude gain, which is not found in the measured data. A novel, accurate continuous correlation function for the FTF gain is computed based on the results for discrete frequencies. When this is implemented into a 1D acoustic network model, the stability map shows, below 600 Hz, two eigenfrequencies, by both the experiment and CFD-based FTF, that are identical. The CFD-based FTF correctly predicts marginal activity at the highest eigenfrequency, while the experimentally based FTF suggests an unstable operation. The unstable operation is not observed in the experiments. This suggests that the CFD-based FTF is also correct for high frequencies.

ZnO-MEMS sensor-cell prepared by inkjet printing for low concentration acetone detection

In this study, inkjet printing tactics is employed to deposit sensitive materials, which is a nest-like array vertically assembled by petal-shaped structure composed of nanoparticles. It overcomes the disadvantages of cumbersome and unstable operation of traditional deposition technology for gas sensor sensitive materials based on MEMS. The mesoporous ZnO-sensor shows high gas-sensing response to low concentration acetone. The excellent performance of mesoporous ZnO-sensors lies in the uniform small nanoparticles, their multi-dimensional assembly structures and rich oxygen vacancy defects.

Analysis of the Core Competitiveness of Chinese Banks Based on Financial Statements

China adapts to market trends and improves its financial system, unified large-scale commercial banks, joint-equity commercial banks, agricultural banks, and other commercial banking systems to create a competitive market. Increasing competition in the banking industry has driven all commercial banks to focus on improving their key skills. This paper analyses Bank of China's fundamental skills through financial statements and makes development-related insights. Core competences help organizations maximise earnings and sustain a competitive advantage, according to this paper. The paper analyses Bank of China's background and external environment, then its balance sheet, income statement, and cash flow statement. Industrial and Commercial Bank of China, Agricultural Bank of China, and China Merchants Bank are used as comparative objects to analyse Bank of China's operation from multiple perspectives. The study says Chinese banks are extremely liquid but less profitable than competitors. Bank of China's strengths include its liability management and worldwide diversification, but it also has a poor resource allocation, a single capital structure, unstable operating cash flows, and considerable external impact.

Digitalization. Weekdays of accredited conformity assessment body

The last four years in the activities of accredited conformity assessment bodies have been marked by global modernization of the FSIS (Federal State Information System) of the Federal Service for Accreditation and e-services of the national accreditation system. The accredited conformity assessment bodies (ICRM among them) encountered a number of problems which have arisen in this regard, i.e., the impossibility of complying the requirements of normative legal acts being put into effect, the unstable operation of the functionality and, as a result, the disruption of the rated deadlines for providing information about the activities of an accredited body, ineffectual appeals to technical support, impossibility to realize the requirements of accreditation experts when working with the accreditation scope configurator. Thus, the digitalization which has been realized by the Federal Service for Accreditation failed to bring the desired results and save time of the accredited bodies.

Stability Analysis for DFIG-Based Wind Farm Grid-Connected System Under All Wind Speed Conditions

The stable operation capability of the doubly-fed induction generation-based wind farm grid-connected system (DFIG-WFGS) is closely related to wind speed, and the windward velocity of wind turbine unit at different locations may be discrepant due to wake effect. However, the existing mathematical models of DFIG is limited to some specific wind speed points, which may not be directly applied to stability analysis in all wind speed operating conditions. The wind speed is used to characterize the steady state operating point in this paper, meanwhile considering the wake effect and single-input and dual-output (SIDO) characteristics, the two-variable admittance models of wind farm based on DFIG are proposed. Further, the improved analysis method based on two-variable open-loop transfer function is proposed to investigate the stable wind speed interval with different series compensation level for a 50MW DFIG-WFGS in Qinghai, China. In addition, a design guideline of wind farm for oscillation suppression is studied to enhance the performance of DFIG-WFGS and avoid unstable operations. Theoretical analysis and simulation results show that the wake effect may discontinuous the stable wind speed interval, and the damping variation is not positively correlated with the wind speed, which provides basis for accurate analysis and effective improvement of the stability for wind power generation system.

Study of vortex rope for the flow field pulsation law

When a hydro turbine deviates from the best efficiency conditions and carries the partial load, the helical vortex rope appears in the center of the draft tube. It will cause the unstable operation. In order to study the characteristics of the vortex rope more clearly, this study monitored the pressure pulsation of the draft tube through large eddy simulation. The temporal-spatial features of spiral vortex rope are well analyzed and visualized by pulsation tracking network (PTN). The results show that the draft tube vortices can be divided into spiral vortex, fragmented vortex and reunited vortex. The frequency of 1.9 Hz is the dominant frequency, which is located in the center of the spiral vortex rope. The area proportion of 1.9 Hz is reduced to 3.63% at the outlet. There are three types of phase difference characteristics. Type-1 is through the center and the whole road section. Compared with linearity, S-shape represents the radial hysteresis of circumferential pressure wave propagation. Type-2 is similar to a circle. Type-3 is similar to the combination of Type-1 and Type-2. This study is of great significance for the design and operation stability of hydro turbine.

Chemical aging of sealing gasket of proton exchange membrane fuel cell

AbstractAs the key sealing part of proton exchange membrane fuel cell (PEMFC), the gasket is easily damaged in acidic solution and wet air for a long time in PEMFC, resulting in unsafe and unstable operation of PEMFC. In this paper, the regular solution (RS) and accelerated durability test (ADT) solution in PEMFC were prepared to study the aging damage process of silicone rubber gasket, and the attenuated total reflection Fourier transform infrared spectrometer (ATR‐FTIR) was used to analyze the surface changes of gaskets soaked in solutions. The results showed that the weight loss of samples increased with the soaking time in both ADT solution and RS solution, and the damage of samples in ADT solution was faster and more serious than that in RS solution. Meanwhile, the microstructure diagram also indicated that the damage of the samples in ADT solution was more serious than that in the RS solution. The ATR‐FTIR analysis results showed that the intensity of characteristic wave peaks of samples at 706, 787, 867, 1006, 1071, 1260, and 2961cm−1 in ADT solution decreased faster than that in RS solution, which was consistent with the results of weight analysis and microstructure analysis.

The Principle of Organization of Motivational Behavior and Automatic Goal Setting of Autonomous Intelligent Mobile Systems

The expediency of organizing the motivational behavior of autonomous intelligent mobile systems focused on solving various complex problems in unstable a priori undescribed problematic environments is substantiated. That need is due to the fact that this type of goal-seeking behavior allows intelligent systems of various purposes to ensure safe and efficient activity in unstable operating conditions. A model of knowledge representation of autonomous intelligent mobile systems is proposed without regard to a specific subject area and built on the basis of active fuzzy semantic networks. In such networks, vertices are labeled with slots that have many characteristics, which enables in the process of goal-seeking activity to label active vertices with objects and events occurring in the problematic environment that meet their requirements. Edges in such networks are labeled with generalized fuzzy values of corresponding conditions that must be met in a problematic environment between an autonomous intelligent mobile system, various objects, and occurring events. This model of a formalized description of various situations and subsituations of the problematic environment allows intelligent systems to adapt to a priori undescribed operating conditions and, on this basis, automatically plan goal-seeking activities under conditions of uncertainty. To control the motivational behavior and self-organization of autonomous intelligent mobile systems, tools have been developed intended to identify threats to productive activities that arise in a problematic environment. A generalized production-based model of knowledge representation has been built without regard to a specific subject area, which allows intelligent systems to quickly respond to various types of security threats that arise in a problematic environment and maintain operability in the process of performing complex tasks. In conclusion, one of the effective ways of further development of the proposed principle of organizing the safe and efficient operation of autonomous intelligent mobile systems is outlined, which is related to the management of their collective activities in the process of performing a complex task with changes occurring spontaneously in a problem environment, accompanied by the emergence of various types of threats in it that prevent their effective operation.

Box-Behnken Response Surface Method to Optimize Amine Liquid Purification Process Based on Ion Exchange

In the process of recycling MDEA (N-methyldiethanolamine) solution in a terminal decarburization system of CNOOC, the quality of MDEA solution was deteriorated due to the generation and accumulation of impurities such as solid suspended solids and heat stable salts, which led to blockage and unstable operation of pipeline equipment in the decarburization system, thus affecting the CO₂ removal effect of the solution, the Box-Behnken effect surface method was used to design the three main factors of the ion exchange-based amine purification process: the continuous adsorption time of the amine solution, the resin regeneration temperature and the resin regeneration time. Taking the conductivity of the MDEA solution after purification as the response function, a corresponding mathematical model was established to optimize the test method, and the correlation coefficient R² (R²=0.9418) of the regression equation obtained was greater than 0.9, indicating that the predicted value had a high correlation with the experimental value, and the test method was optimized accordingly. The best test conditions obtained were: the continuous adsorption time of amine solution was 50 min, the resin regeneration temperature was 40°C, and the resin regeneration time was 30 min. Under this condition, the conductivity of the purified MDEA solution was 1.56 ms/cm, and the salt content was 0.8%. The experiment was repeated 3 times according to this method, and the salt content of the MDEA solution after purification was ≯0.8%, which met the technological requirements. The Box-Behnken experimental design method was feasible for amine solution purification process optimization screening based on ion exchange, and the predicted value of the mathematical model was consistent with the experimental observation value.

Advanced Control Algorithm for Three-Phase Shunt Active Power Filter Using Sliding DFT

This paper describes n digital control algorithm for a three-phase shunt active power filter (APF) using switching–sliding discrete Fourier transform (SSDFT). Traditionally, APF control algorithms use first harmonic detector circuits, which can be implemented using the SDFT algorithm. The classical SDFT algorithm is characterized by long-term stability issues, resulting from numerical errors. Many complex modifications to improve the stability of the SDFT algorithm have been proposed in the scientific literature. This article proposes a solution to the SDFT stability problem using the basic structure of the algorithm. The authors propose the use of the switching SDFT algorithm, where two original SDFT algorithms are implemented in parallel. Both algorithms are turned on alternately after a set period of time and reset to zero in order to reset numerical errors causing their unstable operation. Compared to the classical three-phase SDFT algorithm, the three-phase SSDFT requires only about 25% more arithmetic operations. The proposed approach has been validated using experimental tests and dramatically reduces the numerical errors. The control algorithm has been implemented using the TMS320F28379D microcontroller. The sampling method using only the internal A/D converter of a microcontroller is also presented. The article includes the experimental test results of the complete APF.

Performance of a low power Hall effect thruster with several gaseous propellants

There is a growing need to consider alternative propellants for operation in Hall thrusters, away from the conventional legacy of using xenon. This is particularly the case for small Hall thrusters; which are growing exponentially in their use, whilst needing to be operational on inexpensive propellants. Alternative propellants include condensable propellants (i.e., zinc and iodine), or other gaseous propellants (krypton, argon, and nitrogen (N2)) - which are the focus here. A low power (nominally 100 W) miniature Hall thruster has been designed and manufactured based upon standard scaling laws, with specific considerations for operation with the use of alternative gaseous propellants, and with an attempted novel field topology. The thruster, designated the HEKT-100, was designed to be a low power magnetically shielded thruster to operate on krypton, however the level of shielding present is unknown or tested. Here the thruster has subsequently been operated on xenon, krypton, argon, and diatomic nitrogen successfully, with an unstable operation with neon. The thruster is operated at discharge powers of between 30–810 W. Using a pendulum thrust balance, the thruster performance was measured across a wide range of flow rates, magnetic field strengths, and anode discharge voltages. The performance measured for xenon, krypton, argon, and diatomic nitrogen respectively was, peak thrusts of 12.6 mN, 6.9 mN, 6.6 mN and 5.7 mN, anode efficiencies up to 26.3%, 15.2%, 9.6% and 5.4%, and specific impulses up to 2160 s, 1730 s, 1390 s, and 1000 s. This dataset and the following analysis provides one of the few times a comprehensive selection of alternative gaseous propellants have been tested within the same thruster and compared directly with unchanged geometry or set-up.

Residential electricity package model considering load difference penalty in China

The increasing load difference between peak and off-peak seasons has led to the unstable operation of China’s power systems in recent years. In addition, the current electricity package structure system of China, which is gradually liberalizing its electricity market, remains imperfect. This paper establishes a model of residential electricity packages that considers load difference penalty during peak and off-peak seasons. First, consumers’ utility of different packages is established and thus the optimal is determined by maximizing the utility function. Then, a mathematical model focusing on the minimization of load difference of retail electricity packages during peak and off-peak seasons is established. We thus design a particle-swarm based algorithm to solve the problem. Through simulation results of several numerical examples, we demonstrate that the profit of retailer is increased by 37.35% and Load difference during peak and off-peak seasons is reduced by 78.01%, improving the power demand curve of users and the operation efficiency of power systems.

Performance optimization of centrifugal pump based on particle swarm optimization and entropy generation theory

Centrifugal pumps are frequently utilized in general machinery, but they have issues with low efficiency, unstable operation, and high-power consumption in actual engineering, making performance improvements necessary. In this paper, the particle swarm algorithm is utilized to find the optimal solution among the Pareto front solutions and obtain the most suitable impeller for the centrifugal pump with the lowest entropy generation and highest efficiency as the optimization objectives. Based on the entropy generation theory, the energy losses of the original model and the optimized model are compared and analyzed. The results show that the centrifugal pump impeller and volute are the two areas with the largest entropy generation ratio, the optimized centrifugal pump entropy generation decreases by 5.41%, and the efficiency increases by 3.89%. Turbulent entropy generation and wall entropy generation are the important causes of hydraulic losses. Most of the losses within the impeller occur mainly at leading edge and trailing edge of blade. As the flow rate increases, the closer the distribution of the high entropy generation rate at the front cover of the impeller, the more serious the flow losses. This paper can provide a theoretical reference for the performance optimization of centrifugal pumps.