Showing papers on "Transient (oscillation) published in 2005"

Dynamic models and model validation for PEM fuel cells using electrical circuits

Abstract: This paper presents the development of dynamic models for proton exchange membrane (PEM) fuel cells using electrical circuits. The models have been implemented in MATLAB/SIMULINK and PSPICE environments. Both the double-layer charging effect and the thermodynamic characteristic inside the fuel cell are included in the models. The model responses obtained at steady-state and transient conditions are validated by experimental data measured from an Avista Labs SR-12 500-W PEM fuel-cell stack. The models could be used in PEM fuel-cell control related studies.

Doubly fed induction generator model for transient stability analysis

Abstract: This paper proposes a model of the doubly fed induction generator (DFIG) suitable for transient stability studies. The main assumption adopted in the model is that the current control loops, which are much faster than the electromechanic transients under study, do not have a significant influence on the transient stability of the power system and may be considered instantaneous. The proposed DFIG model is a set of algebraic equations which are solved using an iterative procedure. A method is also proposed to calculate the DFIG initial conditions. A detailed variable-speed windmill model has been developed using the proposed DFIG model. This windmill model has been integrated in a transient stability simulation program in order to demonstrate its feasibility. Several simulations have been performed using a base case which includes a small grid, a wind farm represented by a single windmill, and different operation points. The evolution of several electric variables during the simulations is shown and discussed.

Experimental study of non-linear energy pumping occurring at a single fast frequency

Abstract: Experimental verification of passive non-linear energy pumping in a two-degree-of-freedom system comprising a damped linear oscillator coupled to an essentially non-linear attachment is carried out. In the experiments presented the non-linear attachment interacts with a single linear mode and, hence, energy pumping occurs at a single ‘fast’ frequency in the neighborhood of the eigenfrequency of the linear mode. Good agreement between simulated and experimental results was observed, in spite of the strongly (essentially) non-linear and transient nature of the dynamics of the system considered. The experiments bear out earlier predictions that a significant fraction of the energy introduced directly to a linear structure by an external impulsive (broadband) load can be transferred (pumped) to an essentially non-linear attachment, and dissipated there locally without spreading back to the system. In addition, the reported experimental results confirm that (a) non-linear energy pumping in systems of coupled oscillators can occur only above a certain threshold of the input energy, and (b) there is an optimal value of the energy input at which a maximum portion of the energy is absorbed and dissipated at the NES.

Efficient Generation of Initial- and Inflow-Conditions for Transient Turbulent Flows in Arbitrary Geometries

Abstract: A method is presented to artificially generate initial conditions and transient inflow-conditions for DNS and LES. It creates velocity fields that satisfy a given Reynolds-stress-tensor and length-scale. Compared to existing approaches, the new method features greater flexibility, efficiency and applicability. It is well suited for the complex geometries and for the arbitrary grids that occur in technical applications. This is demonstrated in connection with the generation of initial data for an internal combustion engine. To assess the accuracy and efficiency of the new approach, it is applied to the test-case of a non-premixed jet-flame, which is known to be sensitive to transient inflow-data.

Broken rotor bar detection in induction machines with transient operating speeds

Abstract: Previous work on condition monitoring of induction machines has focused on steady-state speed operation. Here, a new concept is introduced based on an analysis of transient machine currents. The technique centers around the extraction and removal of the fundamental component of the current and analyzing the residual current using wavelets. Test results of induction machines operating both as a motor and a generator shows the ability of the algorithm to detect broken rotor bars.

Digital current control of a voltage source converter with active damping of LCL resonance

Abstract: LCL filters installed at converter outputs offer higher harmonic attenuation than L-filters but careful design is required to damp LCL resonance, which can cause poorly damped oscillations and even instability. A new topology is presented for a discrete-time current controller which damps this resonance, combining deadbeat current control with optimal state-feedback pole assignment. By separating the state feedback gains into deadbeat and damping feedback loops, transient overcurrent protection is realizable while preserving the desired pole locations. Moreover, the controller is shown to be robust to parameter uncertainty in the grid conductance. Experimental tests verify that fast well-damped transient response and overcurrent protection is possible at low switching frequencies relative to the resonant frequency.

An engineering model for transient analysis of grounding system under lightning strikes: nonuniform transmission-line approach

Abstract: A nonuniform transmission line approach is adopted in this paper for modeling the transient behavior of different types of grounding systems under lightning strikes in time domain by solving Telegrapher's equations based on finite-difference time-domain (FDTD) technique. Electromagnetic couplings between different parts of the grounding wires are included using effective per-unit length parameters (l, c, and g), which are space and time dependent. The present model can predict both the effective length and the transient voltage of grounding electrodes accurately, while, an uniform transmission line approach with electrode length dependent per-unit length parameters fails to predict the same. Unlike the circuit theory approach , the present model is capable of predicting accurately the surge propagation delay in the large grounding system. The simulation results for buried horizontal wires and grounding grids based on the present model are in good agreement with that of the circuit and electromagnetic field approaches , . From an engineering point of view, the model presented in this paper is sufficiently accurate, time efficient, and easy to apply.

A New Digital Control Algorithm to Achieve Optimal Dynamic Performance in DC-to-DC Converters

Abstract: In this paper, a new optimal control algorithm is proposed to achieve the best possible dynamic performance for DC-to-DC converters under load changes and input voltage changes. Using the concept of capacitor charge balance, the proposed algorithm predicts the optimal transient response for a DC-to-DC converter during the large signal load current change, or input voltage change. The equations used to calculate the optimized transient time and the optimized duty cycle series are presented. By using the proposed algorithm, the best possible transient performance, including the smallest output voltage overshoot/undershoot and the shortest recovery time, is achieved. In addition, since the large signal dynamic response of power converters is successfully predicted, the large signal stability is guaranteed. Experimental results show that the proposed method produces much better dynamic performance than a conventional current mode PID controller

Gas Distribution System for Improved Transient Phase Deposition

Abstract: Embodiments of the present invention are directed to a gas distribution system which distributes the gas more uniformly into a process chamber. In one embodiment, a gas distribution system comprises a gas ring including an outer surface and an inner surface, and a gas inlet disposed at the outer surface of the gas ring. The gas inlet is fluidicly coupled with a first channel which is disposed between the outer surface and the inner surface of the gas ring. A plurality of gas outlets are distributed over the inner surface of the gas ring, and are fluidicly coupled with a second channel which is disposed between the outer surface and the inner surface of the gas ring. A plurality of orifices are fluidicly coupled between the first channel and the second channel. The plurality of orifices are spaced from the gas inlet by a plurality of distances, and have sizes which vary with the distances from the gas inlet as measured along the first channel, such that the size of the orifice increases with an increase in the distance between the orifice and the gas inlet as measured along the first channel.

Improving of transient stability of power systems using UPFC

Abstract: The objective of this paper is to answer to the following question: How the unified power flow controller (UPFC) parameters should be controlled in order to achieve the maximal desired effect in solving first swing stability problem. This problem appears for bulky power systems with long transmission lines. Various methods of reference identification of the series part, in order to improve the transient stability of the system based on: optimal parameters, state variables and also injection models were studied. Finally, a new identification method based on state variables and using the local measurement was proposed.

Transient analysis of self-excited induction Generator with electronic load controller (ELC) supplying static and dynamic loads

Abstract: This paper presents a transient analysis of a self-excited induction generator (SEIG) with electronic load controller (ELC) used in stand-alone micro-hydro power generation employing uncontrolled turbines. In view of the need to feed both dynamic [three-phase induction motor (IM)] and static loads from such systems, the transient behavior due to switching in of such loads is of interest and is carried out here. A composite mathematical model of the total system has been developed by combining the modeling of prime mover, SEIG, ELC, and load. Simulated results are compared with the experimental ones, obtained on a developed prototype of an SEIG-ELC system for the starting of an IM and switching in a resistive load. For the starting of an IM, a star/delta starter is used to avoid inrush current. Harmonic analysis is carried out to find total harmonic distortion of the terminal voltage and current to assess its power quality.

Parameter determination for modeling system transients-Part I: overhead lines

Abstract: Overhead line parameters are calculated, taking into account the length of the line and the frequency range of the transient to be simulated. Depending on the frequency range and the origin of the transient, several parts of the line (phase conductors, shield wires, towers or poles, footing impedances, insulators) have to be included in the model. In all cases, the most important part is phase conductors. This paper describes an input requirement for obtaining electric overhead line parameters and investigates the sensitivity of these parameters with respect to some input data. The aim is to conclude on the accuracy with which input values must be specified. An illustrative example is included to support the main conclusions.

Numerical methods for modeling transient flow in distribution systems

Abstract: The authors compared two numerical methods-one Eulerian-based, the other Lagrangian-based-for modeling hydraulic transients in water distribution systems. The method of characteristics (MOC), considered to be the most accurate of the Eulerian methods in its representation of the governing equations, requires a number of steps or calculations to solve a typical transient pipe flow problem. The wave characteristic method (WCM), a Lagrangian approach, tracks movement and transformation of pressure waves and computes new conditions either at fixed intervals or only at times When a change actually occurs. The WCM requires orders of magnitude fewer pressure and flow calculations, allowing large systems to be solved in an expeditious manner. Furthermore, because the method is continuous in both time and space, it is less sensitive to the structure of the network and to the length of the simulation process, resulting in improved computational efficiency. Results indicated that both the MOC and WCM provide fast and accurate results for small pipe systems of short duration transients. However, WCM was superior for transient analysis of large distribution systems because it yields accurate results in a fraction of the computational time required for MOC. Pressure transients can adversely affect the quality of treated water because of potential intrusion by pathogens associated with transient events. Accurate modeling studies can help utility managers determine adequate surge protection, strengthen the integrity of their systems, and minimize costly disruptions to service.

Direct measurement of transient pulses induced by laser and heavy ion irradiation in deca-nanometer devices

Abstract: This paper investigates the transient response of 50-nm gate length fully and partially depleted SOI and bulk devices to pulsed laser and heavy ion microbeam irradiations. The measured transient signals on 50-nm fully depleted devices are very short, and the collected charge is small compared to older 0.25-/spl mu/m generation SOI and bulk devices. We analyze in detail the influence of the SOI architecture (fully or partially depleted) on the pulse duration and the amount of bipolar amplification. For bulk devices, the doping engineering is shown to have large effects on the duration of the transient signals and on the charge collection efficiency.

On the shielding effectiveness of enclosures

Abstract: This paper deals with new definitions of shielding effectiveness, in particular for high-frequency and transient electromagnetic fields. They are practicable and supposed to better characterize the shielding ability than the commonly used definitions. From the ratio of the time-averaged input power of the unshielded load to that one of the shielded load, in the limiting case of a vanishing load the electromagnetic shielding effectiveness is derived. This is a simple combination of the commonly used and easily measurable electric and magnetic shielding effectiveness. A similar procedure is then employed for the transient case, where in the limiting case of a vanishing load the ratio of the absorbed energies turn into the transient shielding effectiveness. Numerical results are shown for closed as well as for nonclosed cylindrical shields.

Voltage stability in weak connection wind farms

Abstract: The total operating wind power capacity in the world increases fast, and these types of generating units also bring new opportunities and problems to the utilities and customers. It becomes necessary and important to evaluate their impact on the electrical network voltage stability, especially for weak connected systems. This paper uses the detailed wind energy conversion system and the full order induction generator model to analyze the voltage stability in a weak connection wind farm. The possibility of network voltage drop and instability are investigated by the detailed electromagnetic transient simulation program. Some techniques to improve the transient response of voltage are also discussed.

Fault analysis of a PM brushless DC Motor using finite element method

Abstract: Three-phase trapezoidal back-EMF permanent magnet (PM) machines are used in many applications where the reliability and fault tolerance are important requirements. Knowledge of the machine transient processes under various fault conditions is the key issue in evaluating the impact of machine fault on the entire electromechanical system. The machine electrical and mechanical quantities whose transient behaviors are of importance under fault conditions include the voltages and currents of the coils and phases, the electromagnetic torque, and the rotor speed. Experimental test based on true machines for such a purpose is impractical for its high cost and difficulty to make. Computer simulation based on the finite element method has shown its effectiveness in fault study in this paper. Before the finite element model was used to perform simulations under fault conditions, it was validated by test data under normal conditions. Three types of fault conditions-single-phase open circuit fault, phase-to-phase terminal short-circuit, and internal turn-to-turn short-circuit have been studied.

Physics-based simulation of buffer-trapping effects on slow current transients and current collapse in GaN field effect transistors

Abstract: Two-dimensional transient analyses of GaN metal-semiconductor field effect transistors (MESFETs) are performed in which a three level compensation model is adopted for a semi-insulating buffer layer, where a shallow donor, a deep donor, and a deep acceptor are included. Quasipulsed current-voltage (I‐V) curves are derived from the transient characteristics and are compared with steady-state I‐V curves. It is shown that when the drain voltage VD is raised abruptly, the drain current ID overshoots the steady-state value, and when VD is lowered abruptly, ID remains at a low value for some periods, showing drain-lag behavior. These are explained by the deep donor’s electron capturing and electron emission processes quantitatively. The drain lag could be a major cause of current collapse, although some gate lag is also seen due to the buffer layer. The current collapse is shown to be more pronounced when the deep-acceptor density in the buffer layer is higher and when an off-state drain voltage is higher, beca...

Efficient optimization of transient dynamic problems in MEMS devices using model order reduction

Abstract: One of the main obstacles to including transient dynamic effects into the performance functions of a structural optimization for microelectromechanical systems (MEMS) is the high computational cost of each time-dependent response simulation. This paper focuses on the application of model order reduction techniques to optimal design so as to reduce the transient analysis time for the optimization process. To do this, our open-source software mor4ansys performs model order reductions via the block Arnoldi algorithm directly to ANSYS finite element models. We adopt a micro accelerometer as an example to demonstrate the advantages of this approach. The harmonic and transient results of a reduced-order model of the accelerometer yield very good agreement with that from the original high-dimensional ANSYS model. The use of reduced-order models within the optimization iterations produces almost the same results as those without model order reduction, and speeds up the total computation by at least an order of magnitude.

Failure Monitoring and Asset Condition Assessment in Water Supply Systems

Abstract: In this thesis, different aspects of failure management in urban water supply systems are discussed. As assets are getting older, the number of pipe failures is increasing and an efficient failure management strategy becomes important. Two types of failure management strategies can be applied: proactive asset condition assessment and reactive failure detection and location. Currently available condition assessment techniques cannot be extensively applied in water supply systems due to high cost and slow speed of inspection. Existing failure detection and location approaches do not allow for quick reaction to failures. Automatic failure monitoring and systematic asset condition assessment methods are presented in this thesis. Due to the different topology and hydraulic characteristics of the transmission (pipelines) and distribution (network) components of a water supply system, separate failure detection and location techniques are proposed. For both pipeline and network cases, two types of failures are considered - sudden pipe ruptures and breaks that develop over a longer period of time. For the pipeline case, a periodical leak diagnosis system based on transient response difference monitoring, is presented together with a burst monitoring, detection and location system, which is designed for rapid reaction to sudden pipeline ruptures. A single continuous pressure monitoring station is sufficient to detect and locate a failure along the whole length of a pipeline. For the network case, two continuous failure monitoring approaches are developed, which are based on steady-state (first method) and unsteady-state (second method) analysis. Continuous monitoring of the pressure is performed at a number of locations within the network. The optimal placement of pressure monitoring stations, limits of burst sizes that will be detected, uncertainty of the results and implementation aspects are discussed for both approaches. A transient-analysis-based low-cost, long-range nondestructive pipe condition assessment technique is presented that can be used as a proactive failure management tool and for rehabilitation planning in water transmission pipelines. A comparative evaluation of different pipeline sections can be made and critical sections with a high degree of deterioration can be identified. Finally, a transient-based methodology is presented, which is designed to test the seal quality of inline valves that are used to isolate pipe failures. The techniques presented in this thesis contribute to different points in the pipe asset management cycle and can improve reliability, availability, safety and efficiency of the urban water supply. (Less)

Control of an Electrostatic Microelectromechanical System Using Static and Dynamic Output Feedback

Abstract: This paper examines control strategies for electrostatically actuated microelectromechanical systems (MEMS), with the goals of using feasible measurements to eliminate the pull-in bifurcation, robustly stabilize any desired operating point in the capacitive gap, decrease settling time, and reduce overshoot. We show that input-output linearization, passivity-based design, and the theory of port-controlled Hamiltonian systems lead naturally to static output feedback of device charge. This formalizes and extends previously reported results from the MEMS literature. Further analysis suggests that significantly improving transient behavior in lightly damped MEMS requires dynamic estimation of electrode velocity. We implement output-feedback control using a reduced-order nonlinear observer. Simulations predict greatly improved transient behavior, and large reductions in control voltage.