Showing papers on "Transient (oscillation) published in 2008"

Nonintrusive Load Monitoring and Diagnostics in Power Systems

Abstract: This paper describes a transient event classification scheme, system identification techniques, and implementation for use in nonintrusive load monitoring. Together, these techniques form a system that can determine the operating schedule and find parameters of physical models of loads that are connected to an AC or DC power distribution system. The monitoring system requires only off-the-shelf hardware and recognizes individual transients by disaggregating the signal from a minimal number of sensors that are installed at a central location in the distribution system. Implementation details and field tests for AC and DC systems are presented.

The Use of the Wavelet Approximation Signal as a Tool for the Diagnosis of Rotor Bar Failures

Abstract: The aim of this paper is to present a new approach for rotor bar failure diagnosis in induction machines. The method focuses on the study of an approximation signal resulting from the wavelet decomposition of the startup stator current. The presence of the left sideband harmonic is used as evidence of the rotor failure in most diagnosis methods based on the analysis of the stator current. Thus, a detailed description of the evolution of the left sideband harmonic during the startup transient is given in this paper; for this purpose, a method for calculating the evolution of the left sideband during the startup is developed, and its results are physically explained. This paper also shows that the approximation signal of a particular level, which is obtained from the discrete wavelet transform of the startup stator current, practically reproduces the time evolution of the left sideband harmonic during the startup. The diagnosis method proposed here consists of checking if the selected approximation signal fits well the characteristic shape of the left sideband harmonic evolution described in this paper. The method is validated through laboratory tests. The results prove that it can constitute a useful tool for the diagnosis of rotor bar breakages.

The dynamic model and hybrid phase-shift control of a dual-active-bridge converter

Abstract: This paper presents an averaged model and a small-signal model of a bidirectional dual active bridge (DAB) DC-DC converter Based on the proposed model, the stability and transient power flow in the system can be analyzed A novel hybrid controller is proposed for the phase-shift based feed-back and feed-forward control of a 600V/10kW prototype The hybrid controller can ease the difficulties of the parameter selection of a traditional PI The effectiveness of the models and proposed hybrid controller are validated by simulations and experiments in both transient and steady state operations

High frequency resonant SEPIC converter with wide input and output voltage ranges

Abstract: This document presents a resonant SEPIC converter and control method suitable for high frequency (HF) and very high frequency (VHF) dc-dc power conversion. The proposed design features high efficiency over a wide input and output voltage range, up-and-down voltage conversion, small size, and excellent transient performance. In addition, a resonant gate drive scheme is presented which provides rapid startup and low-loss at HF and VHF frequencies. The converter regulates the output using an on-off control scheme modulating at a fixed frequency. This control method enables fast transient response and efficient light load operation while providing controlled spectral characteristics of the input and output waveforms. An experimental prototype has been built and evaluated. The prototype converter, built with two commercial vertical MOSFETs, operates at a fixed switching frequency of 20 MHz, with an input voltage range of 3.6 V to 7.2 V, an output voltage range of 3 V to 9 V and an output power rating of up to 3 W. The converter achieves higher than 80% efficiency across the entire input voltage range at nominal output voltage, and maintains good efficiency across the whole operating range.

Transient Stability Augmentation of Power System Including Wind Farms by Using ECS

Abstract: This paper presents a new method to enhance the transient stability of multimachine power system including wind farms, when a severe network disturbance occurs in the power system. For this purpose, the energy capacitor system (ECS) composed of power electronic devices and electric double layer capacitor (EDLC) is proposed. The control scheme of ECS is based on a sinusoidal PWM voltage source converter (VSC) and fuzzy logic controlled dc-dc buck/boost converter using insulated gate bipolar transistors (IGBT). Two wind farms are considered to be connected to the power system. Two-mass drive train model of wind turbine generator system (WTGS) is used in the analyses as the drive train modeling has great influence on the dynamic characteristics of WTGS. Real wind speed data are used in the analyses to obtain realistic responses. Different types of symmetrical and unsymmetrical faults are considered as the network disturbance. Simulation results clearly show that the proposed ECS can enhance the transient stability of wind generators in multimachine power system as well as their low voltage ride through (LVRT) capability.

Time-Domain Spectral Element Method for Built-In Piezoelectric-Actuator-Induced Lamb Wave Propagation Analysis

Abstract: An investigation was performed to develop a computational model based on a spectral element method to simulate piezoelectric-actuator-induced acousto-ultrasonic wave propagation in a metallic structure. The model solves the coupled electromechanical field equations simultaneously in both three-dimensional and two-dimensional plane strain conditions, and so it can accept any arbitrary waveform of electrical voltage as input to any piezoelectric transducer and produce piezoelectric sensor output in voltage as a result of the excitation generated by the transducer. Basically, the model inputs electrical voltage to actuators and outputs electrical signals of sensors. To visualize the transient dynamic wave motions in the structure generated by the transducer, the code is integrated with commercial pre/postprocessing software to provide graphical outputs of the dynamic deformations of the structure. The code was verified by comparison with experimental results. Performance of the model was examined in terms of solution convergence compared with the finite element method.

PEM Fuel Cells Modeling and Analysis Through Current and Voltage Transient Behaviors

Abstract: In the last 15 years, polymer electrolyte membrane (PEM) fuel cells have received much attention, mostly through experimental and empirical studies in scientific and industrial research. In most of the works, attention has been given to the steady state analysis of the PEM fuel cells. However, considerable efforts still need to be done to explain different transient behaviors of PEM fuel cells. This paper presents an analysis of the double layer charging effect and reactant diffusion through the cathode gas diffusion layer on voltage transients after sudden current variations. These transient phenomena have typical time durations of less than 5 s. The double layer charging dynamic explains the main voltage transient behaviors when the cathode inlet pressure is constant. In this case, a bicriteria optimization procedure is proposed for numerical characterization of the double-layer charging capacitance. When the air pressure is variable, a pseudo 2-D modeling of oxygen diffusion through the cathode gas diffusion layers, based on the Stephan-Maxwell multicomponent diffusion equations, is used to explain its contribution to the voltage transient overshoots/undershoots.

A Minimal Harmonic Controller for a STATCOM

Abstract: In this paper, a novel controller with fixed modulation index (MI) and variable dc capacitor voltage reference to minimize voltage and current harmonics is presented for a distribution static synchronous compensator (STATCOM). The STATCOM with the proposed controller consists of a three-phase voltage-sourced inverter and a dc capacitor and is used to provide reactive power compensation and regulate ac system bus voltage with minimum harmonics. A systematic design procedure based on pole-zero cancellation, root locus method, and pole assignment method has been developed to determine proper parameters for the current regulator, the dc voltage controller, and the ac voltage controller of the STATCOM. With the proposed STATCOM controller, harmonic distortions in the inverter output current and voltage can be reduced since the MI is held constant at unity in steady state. In addition, a fast adjustment in the STATCOM output reactive power is achieved to regulate the ac bus voltage through the adjustment of the dc voltage reference during the transient period. Simulation and experimental results for the steady-state operating condition and transient operating conditions for the system subjected to a reactive current reference step change, a three-phase line to neutral fault, and a step load change are presented to demonstrate the effectiveness of the proposed controller.

Transient behavior of serial production lines with Bernoulli machines

Abstract: The steady-state performance of production systems with unreliable machines has been analyzed extensively during the last 50 years. In contrast, the transient behavior of these systems remains practically unexplored. Transient characteristics, however, may have significant manufacturing implications. Indeed, if, for example, transients are sluggish and the steady state is reached only after a relatively long settling time, the production system may lose some of its throughput, thus leading to a lower efficiency. This paper is devoted to analytical and numerical investigation of the transient behavior of serial production lines with machines having the Bernoulli reliability model. The transients of the states (i.e., the probabilities of buffer occupancy) are described by the Second Largest Eigenvalue (SLE) of the transition matrix of the associated Markov chain. The transients of the outputs (i.e., production rate, PR, and work-in-process, WIP) are characterized by both the SLE and Pre-Exponential Factors ...

Closed-Loop Control of Medium-Voltage Drives Operated With Synchronous Optimal Pulsewidth Modulation

Abstract: Inverters for medium voltage drives operate at reduced switching frequency so as to restrain the dynamic losses of the power semiconductor devices. The resulting current harmonics can be reduced by synchronous optimal pulsewidth modulation (PWM), provided that steady-state conditions prevail. Transient conditions, however, interfere adversely with the optimal modulation patterns. Such conditions necessarily occur when the modulator forms part of a conventional closed-loop control scheme. Trajectory tracking control is employed to achieve high dynamic control in conjunction with synchronous optimal PWM. An optimal trajectory of the stator flux linkage vector is derived from the pulse pattern in actual use. The stator flux linkage vector is forced to follow this target trajectory. Modifying the target trajectory in transient conditions enables closed-loop torque control in a deadbeat fashion while conserving optimal modulation. Experimental results obtained from a 30-kW prototype drive operated at only 200 Hz switching frequency demonstrate the effectiveness of the approach.

Single-Event Transient Pulse Propagation in Digital CMOS

Abstract: Dynamic circuit equations are used to analyze the response of CMOS inverter chains to stimuli of various forms. Using a normalized description of the CMOS inverter, the conditions under which a single-event transient pulse will propagate are derived in terms of basic technology and circuit parameters. The inverter characteristic waveform, derived in a technology-independent way, is shown to be key to unattenuated pulse propagation. The pulse broadening mechanism is shown to be predictable from CMOS device hysteretic effects, and technology-normalized values of pulse broadening are quantified.

Transient analysis of coaxial magnetic gears using finite element comodeling

Abstract: In this paper, the transient performance of coaxial magnetic gears is firstly analyzed and discussed in literature. The key is to employ finite element comodeling in such a way that both the finite element model of the driving motor and the finite element model of the magnetic gear are coupled together to perform time-stepping finite element analysis. Hence, the dynamic interactions between the motor rotor and the gear inner rotor as well as the gear outer rotor and the external load can be taken into account. The proposed transient analysis verifies that the magnetic gear takes the definite advantage of overload self-protection, whereas reveals that it suffers from speed oscillations with significant time delays between the two rotors which are unacceptable for servomechanism.

Methodology for Cable Modeling and Simulation for High-Frequency Phenomena Studies in PWM Motor Drives

Abstract: The analysis of the transient overvoltages in a pulsewidth modulation (PWM) motor drive system comprises a wide frequency range, which starts with the low values corresponding to the motor speed, includes the switching harmonics, which can reach up to few hundreds of kHz, and also the cable resonance frequency, which value can be in the MHz range, depending on the cable length. In this context, this work presents a time domain methodology for cable modeling able to represent the cable parameters variation due to skin effect in this broad range of frequencies. The proposed technique reproduces accurately the wave propagation and reflection phenomena, thus showing to be very appropriate to transient overvoltage studies in PWM motor drives. A new alternative to represent the frequency-dependent cable earth-return path is also included, allowing the computation of the zero-sequence currents generated by the common-mode voltage produced by the inverter. Simulations using the proposed methodology are conducted and the obtained results are compared with measurements, showing good agreement.

Finite Element Simulation of Heat Transfer

Abstract: Part 1: Steady conduction1. Mathematical formulation.2. The finite element method.3. Isoparametric elements.Part 2: Transient conduction, non-linearities, convective heat transfer4. Transient conduction.5. Non-linearites.6. Convective heat transfer.Part 3: Coupled problems7. Radiative heat transfer in cavities.8. Fluid-structure interaction in a pipe.9. Metallurgical phase change.10. Thermal and electrical phenomena.

Transient Measurement of the Junction-To-Case Thermal Resistance Using Structure Functions: Chances and Limits

Abstract: The accurate and reproducible measurement of the junction-to-case thermal resistance Rth-JC of power semiconductor devices is far from trivial. In the recent time several new approaches to measure the Rth-JC have been suggested, among them transient measurements with different interface materials between the package and a heat-sink which allow identifying the Rth-IC in the structure function of the heat flow path. This paper shows that numerical effects during the calculation of the structure function as well as 3D heat spreading have a big influence on the structure function which makes it often difficult to determine the Rth-IC. Finite element simulations can provide a clue to identify this value in the structure function. The theoretical findings are applied to and demonstrated for actual measurements and the new approach is compared to the traditional method (involving a thermo-couple measurement of the case temperature) with respect to accuracy and reproducibility. Finally an alternative approach to determining the Rth-IC from transient dual- interface measurements, which is not based on structure functions, is presented.

L 1 adaptive controller for a class of systems with unknown nonlinearities: Part I

Abstract: This paper presents an extension of the L 1 adaptive controller to a class of general nonlinear uncertain systems, nonaffine in control The control signal interacts with system states and time-varying uncertainties in an unknown nonlinear way The adaptive controller ensures uniformly bounded transient for system's both input and output signals simultaneously The performance bounds can be systematically improved by increasing the adaptation rate Simulation results verify the theoretical findings

A Coupled Dynamical Model of Redox Flow Battery Based on Chemical Reaction, Fluid Flow, and Electrical Circuit

Abstract: The redox (Reduction-Oxidation) flow battery is one of the most promising rechargeable batteries due to its ability to average loads and output of power sources. The transient characteristics are well known as the remarkable feature of the battery. Then it can also compensate for a sudden voltage drop. The dynamics are governed by the chemical reactions, fluid flow, and electrical circuit of its structure. This causes the difficulty of the analysis at transient state. This paper discusses the transient behavior of the redox flow battery based on chemical reactions. The concentration change of vanadium ions depends on the chemical reactions and the flow of electrolysis solution. The chemical reaction rate is restricted by the attached external electric circuit. In this paper, a model of the transient behavior is introduced. The validity of the derived model is examined based on experiments for a tested micro-redox flow battery system.

Time optimal, parameters-insensitive digital controller for DC-DC buck converters

Abstract: In this paper a digital control approach is investigated for time-optimal load step response of DC-DC synchronous buck converters intended for point-of-load applications employing low-ESR ceramic output capacitors. Unlike previously reported approaches, the proposed technique is insensitive to the power stage parameters, as its operation does not rely on the knowledge of the output filter inductance or capacitance. The time-optimal response is achieved through a single on/off switching action undertaken as soon as a load transient is detected. An asynchronous A/D converter has been employed, realized in a standard 0.35 mum CMOS process. The A/D converter quantizes the output voltage and triggers a nonlinear, event-based digital controller whenever a quantization level transition is detected. Time-optimal response is based solely on output voltage measurements and on the knowledge of the steady-state duty cycle, a number easily available within the digital controller. Effectiveness and properties of the proposed robust time-optimal approach are validated through both computer simulations and experimental tests on a synchronous buck converter prototype and a VHDL implementation of the control algorithm on an FPGA device.

Transmitter system, method of inducing a transient electromagnetic field in an earth formation, method of obtaining a transient electromagnetic response signal, and method of producing a hydrocarbon fluid

Abstract: A transmitter system for inducing a transient electromagnetic field in an earth formation comprises an inductive element to generate an electromagnetic field in response to a flow of electric current through the inductive element. Furthermore, switching means arranged to interrupt the flow of electric current through the inductive element, which switching means comprises a primary switch and an auxiliary switch arranged in series connection with each other. The auxiliary switch has a lower breakdown voltage than the primary switch. This induces a transient electromagnetic field in the earth formation. Delay circuitry may impose a time delay between switching of the auxiliary switch relative to switching of the primary switch. A transient electromagnetic response signal may be recorded, and used in a method of producing a mineral hydrocarbon fluid.

Compensation technique for DC and transient instability of thin film transistor circuits for large-area devices

Abstract: A reliable driving scheme that can compensate for the inherent instability of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) is essential for implementation of large-area devices including displays and sensor arrays for bio-imaging applications. In particular, for high precision and high-resolution devices, the technique should be accurate and fast. A new driving scheme is presented that enables control of the DC and transient shift in the threshold voltage (V T ) and gate voltage of drive/amplifier TFT, while fulfilling the timing requirements for the different applications. The transient shift in the gate voltage has been known to contribute as much as 10% error in controlling the DC shift in the V T whereas it is less than 0.5% for the driving scheme presented here.

Single Event Transients in Logic Circuits—Load and Propagation Induced Pulse Broadening

Abstract: The generation and propagation of single event transients (SET) in logic gate chains is studied and modeled. Regarding SET generation, we investigate the dependence of the generated SET pulse width on the struck node capacitance. Rising node capacitance may lead to amplified pulse width, indicating that increasing load capacitance alone is not an option for radiation hardening. SET propagation in logic chains is also studied, and it is shown that significant broadening or attenuation of the propagated transient pulse width may be observed. It is shown that the chain design (propagation delay of high to low and low to high transitions) has a major impact on broadening or attenuation of the propagated transient pulse. For the first time a suitable model for SET broadening is provided.

On the Choice Between Transmission Line Equations and Full-Wave Maxwell's Equations for Transient Analysis of Buried Wires

Abstract: In this paper, we evaluate the validity of transmission line (TL) solutions in the study of interaction of lightning transients with buried wires. The considered transients have frequencies between a few kilohertz to a few megahertz with risetimes 0.1-10 mus. Comparative simulations using TL equations and full- wave Maxwell's equations are carried out in the paper, and the solutions to both the equations are based on the finite-difference time-domain method. It is found that TL solutions are sufficiently accurate for lightning transient analysis of buried wires. It is also claimed that the TL approach remains valid for all transients having frequencies lower than those of lightning. TL solutions are computationally efficient, particularly when dealing with distributed power and railway systems. The TL approach is valid as long as the transverse electromagnetic mode (TEM) is dominant. However, other modes of propagation, classified as antenna modes, might be present depending upon the type of excitation source, its location, frequency, and the associated media. A possible approximate formula for the frequency above which the validity of TL solutions for buried systems is questionable is proposed based on the concept of penetration depth of fields into the soil. Discussions presented in the paper could motivate the application of TL solutions for electromagnetic transient analyses of the buried conductors of power, railway, and telecommunication systems.

Mixing in bubble column reactors: Role of unsteady flow structures

Abstract: Gas-liquid flows in bubble column reactors are inherently unsteady. In the present work, we have characterized dynamics of gas-liquid flow in rectangular bubble columns and studied the role of unsteady flow structures in liquid phase mixing using experiments and CFD simulations. The need for considering the unsteady nature of gas-liquid flows was demonstrated by using mixing simulations carried out with transient and time-averaged flow. A new methodology of using a computationally amenable multiple snapshots approach, which can adequately account for dynamics of underlying flow, is proposed. CFD simulations were carried out to study the influence of superficial gas velocity and height-to-width ratio on mixing time. The predictions were compared with mixing time measured using conductivity probes. The present work gives useful insights into the mixing process in inherently unsteady flows.

Analytical Model for the Electron-Injection Statistics During Programming of Nanoscale nand Flash Memories

Abstract: We present a detailed analytical modeling for the constant-current Fowler-Nordheim program operation of NAND flash memories, able to describe both the average transient of the cell threshold voltage and its statistical spread due to the granular nature of the electron current flowing through the cell tunnel oxide. We analytically investigate the electron-injection process, highlighting that the steepness of the tunneling current versus floating-gate voltage characteristics and the control-gate to floating-gate capacitance give the field feedback factor, determining the average number of injected electrons at which the injection process becomes sub-Poissonian. Finally, we show that cell scaling will reduce the achievable accuracy of the program algorithm, due to the reduction in the number of electrons controlling cell state.