Showing papers on "Transient (oscillation) published in 1993"

Power system voltage stability

Abstract: Voltage stability is a major concern in the planning and operation of electric power systems. This book provides a clear, in-depth explanation of voltage stability, covering both transient and longer-term phenomena and presenting proven solutions to instability problems. The book describes equipment characteristics for transmission, generation, and distribution/load subsystems of a power system, together with methods for the modelling of equipment. Readers will find static and dynamic computer simulation examples for small equivalent power systems and for a very large power system, plus an account of voltage stability associated with HVDC links. They will also get helpful planning and operating guidelines, computer methods for power flow and dynamic simulation, and descriptions of actual voltage instability incidents.

Fluid Transients in Systems

Abstract: Fluid transient flow concepts basic differential flow concepts solution by characteristics method the complete equations other methods of solution complex systems with multi-pipe and non-pipe elements transients caused by turbomachines two-component and single-component two-phase transient flows valve stoking methods for controlling transients multi-element systems and transmission lines concepts of oscillatory flow and resonance analysis of oscillatory flow in systems natural gas pipeline transients open-channel transient flow special topics.

Transient stability enhancement and voltage regulation of power systems

Abstract: Improvement of the transient stability and voltage regulation of a single-machine-infinite-bus power system under the effects of a symmetrical three-phase short-circuit fault is detailed. The dynamical model of the system is described. A design strategy for nonlinear controllers is considered, and the design of a nonlinear variable-structure excitation controller is described. Simulation results obtained using the nonlinear excitation controller are given, and a new nonlinear coordinated controller is proposed. Simulation results obtained by using the nonlinear coordinated controller are presented. >

Highly transient elastodynamic crack growth in a bimaterial interface : higher order asymptotic analysis and optical experiments

Abstract: A higher Order asymptotic analysis of the transient deformation field surrounding the tip of a crack running dynamically along a bimaterial interface is presented. An asymptotic methodology is used to reduce the problem to one of the Riemann-Hilbert type. Its solution furnishes displacement potentials which are used to evaluate explicitly the near-tip transient stress field. Crack-tip fields corresponding to crack speeds up to the lower of the two shear wave speeds are investigated. An experimental study of dynamic crack growth in PMMA steel interfaces using the optical method of CGS and high speed photography, is also described. Transonic terminal speeds (up to 1.4cPMMAS) and initial accelerations ( $ 108 ms2) are reported and discussed. Transient effects are found to be severe and more important than in homogeneous dynamic fracture. For subsonic crack growth, these experiments arc used to demonstrate the necessity of employing a fully transient expression in the analysis of optical data to predict accurately the complex dynamic stress intensity factor history.

Limits to neural stimulation in echo‐planar imaging

Abstract: Simple electrical circuits are used to model neural stimulation. The Stimulation current is evaluated for a variety of magnetically induced waveforms and for sinusoidal stimulation as a function of frequency. Experimental results obtained using a small scale magnetic stimulator show that the transient response rather than the steady-state behavior determines the stimulation characteristics thus suggesting a nonlinear model. We show that a nonlinear circuit changes a steady-state current solution into a repeated transient solution and thereby better explains our experimental results. Our model also shows that for very short times and high frequencies, contrary to the current widely held view, neural stimulation is independent of the magnetic field switching rate dBldt, but depends on the final magnetic field value, Bm. At lower frequencies and rise-times, there may be a small advantage in using square wave over sine wave gradient modulation for whole body echo-planar imaging (EPI). It is found that the peak stimulation fields, Bm, for both sinusoidal and trapezoidal waveforms are equal for equal risetimes. This means that for a given image array size and imaging time, trapezoidal modulation EPI ultimately can be made to operate below the neural triggering threshold while the equivalent sine modulation EPI version transcends the threshold.

Transient thermal response of power semiconductors to short power pulses

Abstract: Thermal response curves used to calculate the peak junction temperature of power semiconductors are normally derived by experimental identification of the parameters of a known model. Unfortunately the model, developed many years ago, is inappropriate for large surges of short time duration, as they are encountered in present day power conditioning systems. An alternative model is derived, the limits of its accuracy are estimated, and a correction factor is described. A verification of the accuracy of the two methods is also presented. For pulse widths shorter than the thermal transit time, which is in the order of 300 mu s, the peak junction temperature can be more accurately calculated with an expression derived in the present work, which takes into consideration the active volume in which the heat is generated, than with the transient thermal response curve. A correction factor, a function of the width of the pulse, inserted in this equation, further improves its accuracy. >

Improved approximate method for simulating frequency-dependent friction in transient laminar flow

Abstract: A new approximation to the weighting function in Zielke's equation is used in an improved implementation of Trikha's method for including frequency dependent friction in transient laminar flow calculations. The new, five-term approximation was fitted to the weighting function using a nonlinear least squares approach. Transient results obtained using the new approximation function are nearly indistinguishable from results obtained using the exact expression for the weighting function.

Explanation of reverse short channel effect by defect gradients

Abstract: Reverse short channel effect (RSCE), the paradoxical increase in threshold voltage (V/sub t/) of short channel MOSFETs, has previously been explained by diffusion broadening of a buried channel profile. We report here on RSCE in transistors which have very shallow or flat channel profiles, where such broadening cannot be the mechanism. It is shown that for several different dopings and process sequences, both RSCE and anomalous body effects can be traced back to transient enhanced diffusion (TED) of the channel profile induced by source/drain implantation. A new mechanism for RSCE is proposed, in which the surface recombination of interstitials under the gate gives rise to an impurity flux to the surface, which raises the threshold. A coupled defect/impurity diffusion model allows all short channel effects on threshold to be accurately predicted. >

Stiffness of ODEs

Abstract: It is argued that even for a linear system of ODEs with constant coefficients, stiffness cannot properly be characterized in terms of the eigenvalues of the Jacobian, because stiffness is a transient phenomenon whereas the significance of eigenvalues is asymptotic. Recent theory from the numerical solution of PDEs is adapted to show that a more appropriate characterization can be based upon pseudospectra instead of spectra. Numerical experiments with an adaptive ODE solver illustrate these findings.

Thermal-stress relaxation behavior in thin-films under transient laser-pulse heating

Abstract: Transient laser pulse has been used to generate nonsynchronous change of temperature rise and thermal expansion in thin films. The transient process of thermal expansion is recorded by a photoelectric high‐speed acquisition system. By comparison of the calculated temperature rise and thermal expansion, thermal stress relaxation processes in Al films of 20–50 μm thickness is obtained. This result shows that, for transient heating, thermal stress exists even in the case of uniform temperature distribution and free expansion. And also there is a transient high stress in thin films under high‐speed heating.

Transmission level instrument transformers and transient event recorders characterization for harmonic measurements

Abstract: A technique for laboratory characterization of instrument transformers designed for transmission level voltage and current measurements is presented. The technique is also extended to transient event recorders (TERs). The objective of the method is to determine the suitability of existing substation instrument transformers for harmonic measurements, particularly in the frequency range of 60 to 1500 Hz covering the first 25 harmonics. Specifically, the following characteristics are of interest: transfer function magnitude and phase, linearity, and sensitivity of the frequency response to burdens. The measurement technique is based on exciting the instrument transformer primary with an impulsive waveform. Both input and output waveforms are recorded using laboratory grade probes and digitizers. Digital signal processing techniques are used to compute the instrument transformer frequency response. Several voltage transformers and current transformers in the 230 kV-765 kV voltage range were tested. The results are described. >

Double and triple charge pump for power IC: dynamic models which take parasitic effects into account

Abstract: An optimized design methodology for the double and triple charge pump is proposed. The circuits discussed given an output voltage greater than the supply voltage and are commonly used to power 1C or memory to allow the switching on of an MOS device. Theoretical models of charge pumps in the transient region are given. The models take parasitic capacitances and current leakage into account. They allow better knowledge of the circuit dynamics to be obtained and an optimized design to be achieved. >

Transient Wave Propagation Methods for Determining the Viscoelastic Properties of Solids

Abstract: The following two solutions are proposed for deducing the viscoelastic properties of a solid from the change in the shape of a one dimensional transient mechanical wave as it propagates through the medium: the general solution (the phase velocity and the attenuation coefficient are expressed in terms of the Fourier transfors of the pulse after two distances of travel), and a filter method. This method fills a gap between the existing vibratory and ultrasonic methods

A calculation method and some features of transient field under polarity reversal voltage in HVDC insulation

Abstract: Mathematical equations are derived for the transient electric field in insulation media with permittivity and resistivity during boundary excitation due to changing from one steady state to another, such as a polarity reversal voltage imposed on the insulation of HVDC apparatus. The equations are solved by the finite element method, and a program for calculating the transient field is developed. Calculated results of some illustrative examples, including a typical bushing barrier system of power converter transformers, verify the method. Some distinguishing features of the transient field are discussed. >

Numerical simulation of heavy ion charge generation and collection dynamics

Abstract: Describes a complete simulation approach to investigating the physics of heavy-ion charge generation and collection during a single event transient in a p-n diode. The simulations explore the effects of different ion track models, applied biases, background dopings and LET (linear energy transfer) on the transient responses of a p-n diode. The simulation results show that ion track structure and charge collection via diffusion-dominated processes play important roles in determining device transient responses. The simulations show no evidence of rapid charge collection in excess of that deposited in the device depletion region in typical funneling time frames. Further, the simulations clearly show that the device transient responses are not simple functions of the ion's incident LET. The simulation results imply that future studies should consider the effects of ion track structure and extend transient charge collection times to insure that reported charge collection efficiencies include diffusion-dominated collection processes. >

Dynamic simulation of voltage collapses

Abstract: The authors show some examples of dynamic simulations of voltage phenomena using a new general purpose stability program (EUROSTAG) covering in a continuous way the classical fields of transient, mid-term and long-term stability, and also the quasi steady state conditions of a power system. The three examples show the ability of dynamic simulations to study in depth all the phenomena taking part in the voltage collapses. The first example considers the case of an infinite bus feeding a reactive load through a purely reactive line. With this simple example, it is possible to show the basics of voltage instability. The second example illustrates the operation of a system on the lower part of the V, P curve. A two bus system is considered. The third example shows transient phenomena merged with mid-term voltage dynamics. >

Precharge for non-disruptive bus live insertion

Abstract: The invention is related to hot plugging of an electrical circuit (1) into a separate non-quiesced signal net (6) in an active system (7), such as a digital or analog bus. The inventive solution proposes the addition of a preconditioning network (4) to precondition the electrical circuit (1) to be hot plugged by partially precharging the parasitic input capacitances (Cc, Ce, Cm) of the electrical circuit (1) before hot plugging. The precharging of the parasitic input capacitances serves to minimize transient effects on the active system (7).

Calculation and measurement of rail impedances applicable to remote short-circuit fault currents

Abstract: The authors present a detailed analysis of appropriate electrical impedance models for steel railway rails suitable for power supply circuit transient calculations. They extend existing models to fully describe the skin effect, so that power circuit transient behaviour can be taken into account with little error in prediction of response. Practical measurements of rail impedance are compared with calculated values of circuit impedance and demonstrate an accuracy of within 9% for resistance and 18% for inductance. An associated paper (see ibid., vol.139, no.4, p.289-94, 1992) shows how this model can be used to predict remote short-circuit-fault substation current profiles.< >

Finite difference time-domain calculation of transients in antennas with nonlinear loads

Abstract: Transient currents for an antenna with a nonlinear load are calculated using finite-difference-time-domain (FDTD) methods. The FDTD electric field across the nonlinear load (which determines the voltage across the load) is calculated by solving a nonlinear equation at each time step. This allows the time step to be at the Courant limit, and is more efficient than reducing the time step size in all the FDTD cells to maintain stability in just the cell containing the nonlinear load. The stability of this approach relative to a simpler approach of approximating the diode as a variable resistor is demonstrated. As a validation of the method the transient current in a long dipole antenna with a nonlinear load excited by a pulsed plane wave is computed and compared with calculated results obtained by another method. The approach given here extends the applicability of the FDTD method to problems involving radiation and scattering from antennas including nonlinear loads. >

A fast and accurate gate-level transient fault simulation environment

Abstract: Mixed analog and digital mode simulators have been available for accurate transient fault simulation. However, they are not fast enough to simulate a large number of transient faults on a relatively large circuit in a reasonable amount of time. The authors describe a gate-level transient fault simulation environment which has been developed based on realistic fault models. The simulation environment uses a timing fault simulator as well as a zero-delay parallel fault simulator. The timing fault simulator uses high level models of the actual transient fault phenomenon and latch operation to accurately propagate the fault effects to the latch outputs, after which point the zero-delay parallel fault simulator is used to speed up the simulation without any loss in accuracy. The simulation environment is demonstrated on ISCAS-89 sequential benchmark circuits.

EMTP transient modeling of a distance relay and a comparison with EMTP laboratory testing

Abstract: A method to test the transient performance of a sampled-data relay is given. A model of one measuring unit of a digital relay is created on a personal computer. The simulations are compared with laboratory results obtained by applying digitally generated waveforms to the actual relay. Computer modeling of the digital relay is done within the models version of the Transient Analysis of Control System (TACS) subsection of the ElectroMagnetics Transient Program (EMTP). The input filter, analog-to-digital converter, Fourier fundamental frequency detector, and the relay measuring principle of one digital microprocessor-based relay are modeled. An actual 115 kV 150 mile long transmission line is simulated using the EMTP. >

Control and operation of PM synchronous motor drives in the field-weakening region

Abstract: Due to the upper limit on the available DC link voltage and current rating of a given inverter, the motor input voltage and current ratings are limited. The voltage and current limits impact the maximum speed upto which constant rated torque is available and the maximum torque producing capability of the motor drive system, respectively. It is required and desirable to produce the rated power with the highest attainable speed for many applications such as electric vehicles, golf carts, people carriers in airport lobbies, forklifts, machine tool spindle drives etc. This paper reviews the operation of the permanent magnet synchronous motor drives (PMSM drives) when they are constrained to be within the permissible envelope of maximum inverter voltage and current to produce the rated power and to provide this with the highest attainable rotor speed. The rated power is for steady state operation and multiple times that is preferred for fast accelerations and decelerations during transient operation. Effective current control during the field weakening operation with transient capability is preferable compared to the saturation of the current loop resulting in higher peak currents and hence higher copper losses. From these considerations, this paper addresses the steady state operation, transient operation, and control techniques for the PMSM drives in the field-weakening region. >