Showing papers on "Transient (oscillation) published in 1997"

Transient power quality problems analyzed using wavelets

Abstract: In the literature, wavelet techniques have been proposed for the identification of power system transient signals (e.g., lightning impulse, and capacitor switching transients). In this paper, the wavelet technique is proposed for the analysis of the propagation of transients in power systems. The advantages and disadvantages of the method are discussed and the way in which these analysis methods complement previously reported identification methods is described. An example based on the discretized solution of a differential equation is given.

Qualitative and quantitative analysis of volatile organic compounds using transient and steady-state responses of a thick-film tin oxide gas sensor array

Abstract: Quantitative analysis of gases, by means of semiconductor sensor arrays and pattern-recognition techniques such as artificial neural networks, has been the goal of a great deal of work over the last few years. However, the lack of selectivity, repeatability and drifts of the sensors, have limited the applications of these systems to qualitative or semi-quantitative gas analysis. While the steady-state response of the sensors is usually the signal to be processed in such analysis systems, our method consists of processing both, transient and steady-state information. The sensor transient behaviour is characterised through the measure of its conductance rise time (Tr), when there is a step change in the gas concentration. Tr is characteristic of each gas/sensor pair, concentration-independent and shows higher repeatability than the steady state measurements. An array of four thick-film tin oxide gas sensors and pattern-recognition techniques are used to discriminate and quantify among ethanol, toluene and o-xylene [concentration range: 25, 50 and 100 ppm]. A principal component analysis is carried out to show qualitatively that selectivity improves when the sensor behaviour is dynamically characterised. The steady-state and transient conductance of the array components are processed with artificial neural networks. In a first stage, a feed-forward back-propagation-trained ANN discriminates among the studied compounds. Afterwards, three separate ANN (one for each vapour) are used to quantify the previously identified compound. Processing data from the dynamic characterisation of the sensor array, considerably improves its identification performance, rising the discrimination success rate from a 66% when only steady-state signals are used up to 100%.

Attenuation of single event induced pulses in CMOS combinational logic

Abstract: Results are presented of a study of SEU generated transient pulse attenuation in combinational logic structures built using common digital CMOS design practices. SPICE circuit analysis, heavy ion tests, and pulsed, focused laser simulations were used to examine the response characteristics of transient pulse behavior in long logic strings. Results show that while there is an observable effect, it cannot be generally assumed that attenuation will significantly reduce observed circuit bit error rates.

Frequency Dependent and Transient Characteristics of Substation Grounding Systems

Abstract: In spite of the existence of a number of analytical models aimed for transient analysis of large grounding systems, more detailed analysis of the influence of different parameters on the transient performance of large ground grids subjected to lightning current impulse is not available. This paper presents analysis of the influence of soil conductivity, location of feed point, grid size, depth, conductor separation, ground rods, and shape of the lightning current impulse, on the transient performance of ground grids with sizes ranging from 10/spl times/10 m/sup 2/ to 120/spl times/120 m/sup 2/ and with 4 to 124 meshes. Maximal transient ground potential rise and frequency dependent impedance are analyzed in time and frequency domain, respectively. Computations are made with computer model based on the electromagnetic field theory approach, taking accurately into account frequency dependent characteristics of large ground grids. Instead of usual simple approximations of the lightning current impulse, recorded channel base currents from triggered lightning are used for the time domain analysis.

Transient triggering of near and distant earthquakes

Abstract: We demonstrate qualitatively that frictional instability theory provides a context for understanding how earthquakes may be triggered by transient loads associated with seismic waves from near and distance earthquakes. We assume that earthquake triggering is a stick-slip process and test two hypotheses about the effect of transients on the timing of instabilities using a simple spring-slider model and a rate- and state-dependent friction constitutive law. A critical triggering threshold is implicit in such a model formulation. Our first hypothesis is that transient loads lead to clock advances; i.e., transients hasten the time of earthquakes that would have happened eventually due to constant background loading alone. Modeling results demonstrate that transient loads do lead to clock advances and that the triggered instabilities may occur after the transient has ceased (i.e., triggering may be delayed). These simple “clock-advance” models predict complex relationships between the triggering delay, the clock advance, and the transient characteristics. The triggering delay and the degree of clock advance both depend nonlinearly on when in the earthquake cycle the transient load is applied. This implies that the stress required to bring about failure does not depend linearly on loading time, even when the fault is loaded at a constant rate. The timing of instability also depends nonlinearly on the transient loading rate, faster rates more rapidly hastening instability. This implies that higher-frequency and/or longer-duration seismic waves should increase the amount of clock advance. These modeling results and simple calculations suggest that near (tens of kilometers) small/moderate earthquakes and remote (thousands of kilometers) earthquakes with magnitudes 2 to 3 units larger may be equally effective at triggering seismicity. Our second hypothesis is that some triggered seismicity represents earthquakes that would not have happened without the transient load (i.e., accumulated strain energy would have been relieved via other mechanisms). We test this using two “new-seismicity” models that (1) are inherently unstable but slide at steady-state conditions under the background load and (2) are conditionally stable such that instability occurs only for sufficiently large perturbations. For the new-seismicity models, very small-amplitude transients trigger instability relative to the clock-advance models. The unstable steady-state models predict that the triggering delay depends inversely and nonlinearly on the transient amplitude (as in the clock-advance models). We were unable to generate delayed triggering with conditionally stable models. For both new-seismicity models, the potential for triggering is independent of when the transient load is applied or, equivalently, of the prestress (unlike in the clock-advance models). In these models, a critical triggering threshold appears to be inversely proportional to frequency. Further advancement of our understanding will require more sophisticated, quantitative models and observations that distinguish between our qualitative, yet distinctly different, model predictions.

Effect of surge voltage risetime on the insulation of low voltage machines fed by PWM converters

Abstract: This paper investigates the repetitive surge-voltage withstand of low-voltage mush-wound machines operated on adjustable-speed drives (ASDs) using insulated gate bipolar transistor (IGBT) semiconductor technology. Historical work on surge testing of motor insulation has focused on one or more of the following aspects: (1) large horsepower motors; (2) medium-voltage form-wound motors; (3) single-shot impulse-type transients; or (4) low-voltage machines with surge risetimes >200 ns. IGBT drives can have risetimes of 50-200 ns. Thus, a new study on electrical stress of insulation systems due to the nonlinear voltage distribution of mush-wound motors when subjected to repetitive steep dV/dt square-pulse waveforms (rather than impulse wave testing) is presented. Magnitude and risetime of the repetitive ASD surge-voltage transient induced on the machine terminals is reviewed first. Next, surge propagation into the winding was investigated to identify maximum voltage stress points on the conductor insulation. Potential failure mechanisms observed at these points are then discussed. The significance of decreasing surge risetime and increasing cable lengths on internal nonlinear voltage distribution is studied with experimental results from a 7.5-hp motor with a tapped stator winding.

Computation of energy dissipation in transient flow

Abstract: A new model for the computation of unsteady friction losses in transient flow is developed and verified in this study. The energy dissipation in transient flow is estimated from the instantaneous velocity profiles. The ratio of the energy dissipation at any instant and the energy dissipation obtained by assuming quasi-steady conditions defines the energy dissipation factor. This is a nondimensional, time-varying parameter that modifies the friction term in the transient flow governing equations. The model was verified for laminar and turbulent flows and the comparison of measured and computed pressure heads shows excellent agreement. This model can be adapted to an existing transient program that uses the well-known method of characteristics for the solution of the continuity and momentum equations.

Permafrost zonation and climate change in the northern hemisphere: results from transient general circulation models

Abstract: Numerous studies have demonstrated that both global patterns and local details of permafrost distribution are highly responsive to climatic fluctuations, at several temporal and spatial scales Permafrost currently underlies about one fourth of the land area of the northern hemisphere, and many qualitative predictions have been made for a severe reduction of this area in response to global warming A map of permafrost distribution compiled using the ‘frost index’, a dimensionless number that can be related to the zonal arrangement of permafrost, shows very good correspondence with a recently published empirical map The frost index was used in conjunction with three transient general circulation models to compile maps of permafrost zonation for conditions in the mid-21st century Although regional patterns and local details differ substantially between the three scenarios, all result in reductions in the area occupied by each permafrost zone Localized expansions of the area underlain by permafrost are apparent from two of the scenarios Reductions in the areal extent of equilibrium permafrost predicted from two of the three transient models are much less than those indicated by runs using 2 × CO models

Validation of a fast voltage stability analysis method on the Hydro-Quebec system

Abstract: This paper reports on tests validating a fast voltage stability analysis method, envisaged for operational planning and real-time applications. The simulation method relies on quasi-steady-state approximation, replacing the transient dynamics with equilibrium equations and focusing on the long-term dynamics. Various comparisons have been performed with full time simulation, on the 514-bus Hydro-Quebec system. The accuracy is equivalent while the new method is up to 3 orders of magnitude faster.

Analysis of very fast transient overvoltage in transformer winding

Abstract: Owing to the operation of disconnector switches, very fast transient overvoltages can generate a voltage oscillation inside power transformers connected to them. A practical method to analyse such oscillations is developed based on multiconductor transmission-line theory. The analysis is conducted in two steps in the frequency domain: first the whole transformer winding is simulated using a single transmission-line model; and secondly, the phenomena in the first coil are analysed using a multiconductor transmission-line model. The numerical results for an actual 525 kV power transformer conform with experiments. The interturn voltage shows a resonance determined by the travelling waves in the coil. The propagation of travelling waves is visualised using time domain analysis.

Low-dropout voltage regulator incorporating a current efficient transient response boost circuit

Abstract: An improved low-dropout ("LDO") voltage regulator incorporates a transient response boost circuit which is added to the slew-rate limited node at the control terminal of the LDO voltage regulator output transistor and provides improved transient response performance to the application of various load current step stimuli while requiring no standby or quiescent current during zero output current load conditions. The transient boost circuit supplies current to the slew-rate limited node only upon demand and may be constructed as either a localized positive feedback loop or a number of switching devices which conduct current only during slew-rate conditions.

The effect of rotor design on sensorless speed estimation using rotor slot harmonics identified by adaptive digital filtering using the maximum likelihood approach

Abstract: Adaptive digital filtering has been demonstrated as an effective technique for extracting a real-time, sensorless, speed signal from rotor slot harmonics (RSHs) embedded in the line current waveform of induction motor drives. It is known that sensorless speed estimation techniques using RSHs may exhibit poor performance with certain motor designs. This paper examines the reasons for that poor performance which reflects differences in the magnitude of the slot harmonic signals consequent upon rotor design. Experimental results for a 30 kW motor with 6 different rotors are presented. The significant parameters are the number of rotor slots, skew and the accuracy of construction. It is clearly shown how inferior performance can arise. Conversely, improvements in reliability of speed estimation and transient response can be obtained by recognising those aspects which provide an enhanced signal, by minimising the background noise of the inverter/machine, or by enhancing the adaptive filter. The recursive maximum likelihood technique is presented as an improved algorithm for tuning the digital filter which aids transient response and reliability of speed estimation. Real time, experimental transient performance is demonstrated for the different rotors used in this paper and the performance failure of a particular slot combination demonstrated.

An Integral Model for the Transient Pyrolysis of Solid Materials

Abstract: The modelling of the spread of fire and its extinguishment still represents a significant challenge. As part of a combined experimental and computational study of fires we have developed a general Computational Fluid Dynamics (CFD) model of fire spread and extinguishment. The primary objective was to produce a flexible computational tool which can be used by engineers and scientists for design or research purposes. The present paper deals with the description and validation of a solid pyrolysis model which has been applied, as a sub-model, in this general computer fire code. The pyrolysis model has been formulated using the heat-balance integral method. The model can be applied to slabs of char forming solids, such as wood, as well as non-charring thermoplastic materials, such as PMMA. Results are compared with analytical solutions, numerical simulations and experimental data. In all cases the integral model performs well. ( 1997 by John Wiley & Sons, Ltd.

Grounding System Analysis in Transients Programs Applying Electromagnetic Field Approach

Abstract: Lightning protection studies of substations and power systems require knowledge of the dynamic behavior of large grounding grids during electromagnetic transients. This paper presents strategies which allow incorporation of complex grounding structures computed using a rigorous electromagnetic model in a transients programs. A novel technique for rational function representation of frequency-dependent grounding system impedances in the EMTP is described. An arbitrary number of feeding points can be modeled as mutual coupling is taken into account. Overvoltages throughout electrical power systems and the transient ground potential rise in the surroundings of grounding structures can be computed.

Two channel switched reluctance starter/generator results

Abstract: Measured data and simulation results for a two channel experimental switched reluctance motor (SRM) based aircraft engine starter/generator system is presented The two channels of the system provide power to two separate and independent loads The results presented here document the performance capability of this system from load step response to output ripple voltage Both measured and predicted system performance is presented and compared In addition, operating characteristics that are unique to this SRM-based system are described Of particular interest is the interaction between the two separate channels when one channel experiences a load transient and the other does not The source of channel interactions is described and measured data are provided The issue of clearing load faults is also addressed Design results for adding this capability to the experimental system are described

A New Formulation of Flossy Ground Return Parameters for Transient Analysis of Multiconductor Dissipative Lines

Abstract: A new prediction model of the ground distributed parameters for the transient analysis of multiconductor lines is proposed, which applies in a wide frequency range and/or high ground resistivity. The obtained formulation, basing on a rigorous procedure, is simple to handle for implementation in a computer code. The comparison between the developed approach and the Carson model is carried out in order to define analytically the differences concerning the expressions of the line parameters. Both the models are applied to the computation of the transient voltages of a power line excited by a current source or an EMP-plane wave.

Coupled Thermal Analysis Method with Application to Metallic Thermal Protection Panels

Abstract: Metallic thermal protection systems are being considered for next-generation reusable launch vehicles. Experiments and numerical simulations show that heated metallic panels expand from the structure to create a quilt-like flow surface that alters the expected surface heating distribution. To model the thermal response of a metallic panel, this paper presents a two-dimensional boundary element procedure that is loosely coupled to a hypersonic computational fluid dynamics algorithm to solve coupled steady-state and transient heat conduction. First, the flowfield and internal temperature distribution of ceramic wing leading edge in a Mach 15 freestream is computed. Steady-state results are consistent with previous finite difference and finite element calculations. Transient computations show the peak temperature to occur at steady-state conditions. Second, the flowfield and resulting transient heat transfer is computed for convex and concave metallic panels. It is demonstrated that a transient conduction solution for a deformed metallic panel can be approximated by imposing the heat fluxes generated from a deformed surface flow solution onto an undeformed panel.

A new method for power system transient instability detection

Abstract: Reliability and computational simplicity are of primary importance in developing on-line instability detection methods. With regard to these requirements, this paper has proposed a new method for instability detection based on generator angles, angular velocities, and their rates of change. According to the developed method, system instability or out-of-step condition is detected by identifying the characteristics-concave or convex-of a surface on which the post-fault system trajectory lies. The variation mode and probabilistic property of the proposed index are investigated by using the 10 machine 39 bus IEEE test system.

Predicting the transient effects of PWM voltage waveform on the stator windings of random wound induction motors

Abstract: In this paper, the effect of PWM voltage waveform on the stator windings of random wound cage induction motors is studied. A method of estimating the distributed circuit parameters of the motor using finite element analysis is described. From these parameters, an equivalent circuit is formed with distributed capacitances and inductances. This circuit is then used for simulation using SABER in order to predict the voltage distribution among the turns and coils during a PWM wavefront. The effect of switching time of the inverter on the voltage distribution is also simulated. In order to validate the method adopted, the simulated results are compared with experimental results.

New current control concept-minimum time current control in the three-phase PWM converter

Abstract: In this paper, a new current controller that guarantees the fastest transient response is proposed. The basic concept is to find the optimal control voltage for tracking the reference current with minimum time under the voltage limit constraint. Though this minimum time control concept is also applicable to all the machine drive systems, this paper focuses on the current regulation in the three-phase pulse width modulation (PWM) converter. In the simulation and experimental results, it is observed that the proposed controller has much less transient time than the conventional synchronous PI regulator and the performance of the DC link voltage control is also greatly improved with the proposed current controller.

Voltage Escalation and Reignition Behavior of Vacuum Generator Circuit Breakers During Load Shedding

Abstract: This paper focuses on multiple reignitions and voltage escalation that may occur during load shedding when vacuum circuit-breakers are used as generator circuit-breakers. The probability of multiple reignitions and voltage escalation is proportional to the arc angle and is very small. However, repeated reignitions and voltage escalation may be observed in vacuum switching devices after opening operations due to their ability to interrupt HF currents. The transformer side capacitance and the magnitude of the load shedding play a significant role in developing repeated reignitions and voltage escalation when vacuum circuit breakers are used as generator circuit breakers. Of particular concern is the case when a protective capacitor is connected to the system side of the circuit breaker. Although this capacitor reduces the magnitude of the of transient recovery voltage, it decreases the transient frequencies and increases the reignition current that flows through the vacuum circuit breaker after a voltage breakdown. In all the cases under study, the reignition/escalation problems are more severe with the protective capacitor connected to the power system for relative low load shedding at short arc angles. This paper reports on many simulated conditions analyzed by using the Electromagnetic Transient Program (EMTP). Ten generators in the range of 50 through 300 MVA have been considered.

Nonlinear dynamic modeling of micromachined microwave switches

Abstract: Nonlinear dynamic lumped models of micromachined microwave switches have been formulated and successfully applied to analyses of transient characteristics and geometrical scaling. Parameter extraction through electrical measurements is summarized. The results are compared to transient quasi-2D simulations.

Transient Radiation Transport in Participating Media Within a Rectangular Enclosure

Abstract: This paper outlines the formulation of the two-dimensional transient radiation transport through a scattering‐ absorbing medium. The P1 approximation in a Cartesian coordinate system is invoked to simplify the transient radiative transfer equation because it is very complicated to solve in its general form. A boundary-driven radiative problem is considered in which the radiation intensities at some areas on a surface are modeled as boundary conditions and maintained at constant values in all angular directions.