Showing papers on "Transient (oscillation) published in 1991"

A fundamental study of inter-area oscillations in power systems

Abstract: A fundamental study of the nature of inter-area oscillations in power systems is presented. The effects of the system structure, generator modeling, excitation type, and system loads are discussed in detail. Both small signal and transient stability analyses are used to determine the characteristics of the system. >

Vector analysis and control of advanced static VAr compensators

Abstract: The advanced static VAr compensator (ASVC) is based on the principle that a self-commutating static inverter can be connected between three-phase AC power lines and an energy-storage device, such as an inductor or capacitor, and controlled to draw mainly reactive current from the lines. This capability is analogous to that of the rotating synchronous condenser and it can be used in a similar way for the dynamic compensation of power transmission systems, providing voltage support, increased transient stability, and improved damping. The authors present a simplified mathematical model of the ASVC that has made it possible to derive the transfer functions needed for control system synthesis. The resulting control system designs are briefly outlined and further analysis is presented to show the behaviour of the ASVC when the line voltage is unbalanced or distorted. The analysis is based on a vectorial transformation of variables, first described by R.H. Park (1928) for AC machine analysis, and later, using complex numbers, by W.V. Lyon (1954) in the theory of instantaneous symmetrical components.

Direct torque control of induction machines using space vector modulation

Abstract: A direct induction machine torque control method based on predictive, deadbeat control of the torque and flux is presented. By estimating the synchronous speed and the voltage behind the transient reactance, the change in torque and flux over the switching period is calculated. The stator voltage required to cause the torque and flux to be equal to their respective reference values is calculated. Space vector PWM is used to define the inverter switching state. An alternative approach to deadbeat control for use in the transient or pulse-dropping mode is also presented. An alternative modulation scheme is presented in which transient performance is improved by specifying the inverter switching states and then calculating the required switched instants to maintain deadbeat control of the flux while reducing the torque error during the entire switching interval. A similar approach is used for a transient in the flux. The implementation of the control scheme using DSP-based hardware is described, with complete experimental results given. >

Analysis of a concentrated winding induction machine for adjustable speed drive applications. I. Motor analysis

Abstract: The performance of multiphase machines designed for operation with static power converters is investigated. The winding distributions are intentionally rectangular to better accommodate the rectangular waveforms of solid-state inverters. Equations which define the transient as well as steady-state behavior, including the computation of all machine inductances, are derived. In deriving these equations the space harmonics are specifically included. Equations for calculation of terminal voltages and electromagnetic torque are modified to account for nonsinusoidal air-gap-flux distributions. A conventional three-phase induction motor including the effect of space harmonics is simulated. >

Turbomachine blade vibration

Abstract: Single Blade Vibrations Discrete Analysis of Blades Small Aspect Ratio Blades Blade Group Frequencies and Mode Shapes Excitation Damping Forced Vibrations Transient Vibrations Coupled Blade-Disc Vibrations Some Thoughts on Fatigue Life Estimation Some Reflections Appendices: Heat Transfer and Thermal Stress of Turbine Blades SI Units Nomenclature Index.

An estimation of load characteristics of an ultrasonic motor by measuring transient responses

Abstract: To measure the characteristics of ultrasonic motors, such as the maximum torque, torque-speed relationship and the frictional coefficient at the contact surface, a method in which the torque is calculated from the transient responses is proposed. The rise curve that is the transitional change in the rotor speed soon after turning on the motor gives the load characteristics, while the fall curve that is the decay of the rotor speed after turning off the motor yields the frictional coefficient of the contact surface. This method requires only a short time (the transient time of the motor) to complete the measurement. The relations between the transient responses, the load characteristics and the frictional force are analyzed, and the method is applied to a hybrid transducer type rotary motor and a traveling wave type linear motor. >

Simulation of arbitrary transmission line networks with nonlinear terminations

Abstract: The transient response of a high-speed digital circuit is determined by the interaction of a lossy distributed transmission line network and lumped nonlinear circuits. A robust and accurate method for the analysis of such a system is reported, and its implementation in an analog circuit simulator is discussed. The method uses a convolution technique and a time-domain impulse response to perform a transient simulation of a transmission line network. The time-domain impulse response is derived using a Fourier transform of modified frequency-domain scattering parameters. Implementation of the technique is verified by comparison with measured results. >

Transient state analysis of a doubly-fed induction generator under three phase short circuit

Abstract: The doubly fed induction generator (DFG) is a variable-speed constant-frequency generator operating in either subsynchronous or supersynchronous mode. The transient behavior of a symmetrically loaded DFG after a three-phase short circuit is presented. Both speed and rotor excitation voltage and frequency remain unchanged during short circuit. The complete mathematical model of the transient state and experimental results are given, along with the transient state equivalent circuit. >

Waveform relaxation analysis of nonuniform lossy transmission lines characterized with frequency-dependent parameters

Abstract: An efficient method for the transient simulation of nonuniform frequency-dependent transmission lines is presented. The method consists of iterative waveform relaxation analyses of asymmetric disjoint two-port networks constructed with FFT waveform generators and characteristic impedances synthesized by applying the Gauss-Marquardt optimization technique. The method can also be adapted for discrete-time analysis by replacing the FFT waveform generators with ideal transmission lines connected with waveshaping networks. Transient responses of uniform and nonuniform transmission lines with and without skin-effect parameters and terminated with linear and nonlinear loads are simulated for illustrations. The accuracy and efficiency of the relaxation technique are substantiated with exact analytical solutions and experimental data. >

Transient currents in nematic liquid crystals.

Abstract: Transient currents induced by the polarity reversal of a dc voltage applied to a liquid-crystal cell have been experimentally studied. For voltages less than a characteristic value, the time occurrence of the peak current decreases with decreasing applied voltage, which is contrary to behavior expected for a normal carrier-transport process. To explain the experiments, a simple model based on the time variation of an applied voltage and the internal electric field induced by the adsorbed electric charges on the cell substrate has been proposed. The results from the model agree reasonably well with the experimental observations.

Finite element modeling of the RF heating process

Abstract: The heating of heterogeneous materials by radio frequency excitation is modeled by sequential electromagnetic and thermal analysis using the finite-element method. The electromagnetic analysis comprises electrostatic modeling using temperature-dependent complex dielectric permittivity to account for the energy dissipating losses in the material. The thermal analysis consists of a transient finite-element procedure with temperature-dependent thermal and material properties. The results show good agreement with transient thermal measurements. >

Switching transient fields in substations

Abstract: Measured electric and magnetic fields of switching transients are characterized in terms of their macroburst (series of transients) and micropulse (individual transient) properties and are classified according to substation voltage and type. Transient fields typical for 115-500 kV air-insulated substations (AISs) and gas-insulated substations (GISs) are reported. Macrobursts of up to 10000 individual transients with varying amplitudes and pulse repetition frequencies of 40 kHz or more can be produced during a single switch. The highest amplitude micropulse field transients are shown to scale with substation voltage. Peak vertical electric fields of 16 kV/m and horizontal magnetic fields of 212 A/m have been measured on the ground under the bus in 500 kV AISs during disconnect switching. Peak field component amplitudes are shown to double at several meters above ground and to scale approximately as 1/r with distance from the bus. Electric field enhancement above a grounded structure to many times the normal free-space value was measured. The dominant frequencies of switching transients are between 0.5 MHz and 120 MHz, vary inversely with substation voltage, and are highest in GISs. >

Modelling of periodically switching networks

Abstract: Two time-domain analysis methods for periodically switched systems, like converters, are presented. The first method analyzes the steady-state periodic behavior of the system, while the second describes the transient behavior. In contrast with state-space averaging, the methods do not use approximations, so exact results are obtained. The methods give more insight into the dynamical behavior of the system. This knowledge is useful in designing controllers. >

Zero-voltage switched FM-PWM converter

Abstract: A variable-frequency pulse-width-modulated D-C to D-C converter allows operation at very high frequencies with minimal power transistor switch transient turn-on losses. A frequency control scheme is utilized wherein the controller of the converter causes switching to occur when the voltage across the power transistor switch is near zero volts. In addition to reducing transient turn-on losses, zero-voltage switching reduces noise generation in the circuit and simplifies the power transistor drive requirements. Other characteristics of opimal power transistor switch utilization are met in addition to turning the switch on under a zero potential. Maximum peak voltage across the switch is limited to approximately the ouput or input voltage level depending on the configuration used. In addition, peak and average currents through the switch are proportional to output power requirements and turn off transient dissipation is held to a minimum.

Boundary element approach for inverse heat conduction problems; Application to a bidimensional transient numerical experiment

Abstract: A boundary element approach is used to solve inverse heat conduction problems in multidimensional and nonlinear situations. In stationary and transient cases, discretized boundary integral equations are expressed and, to obtain satisfying results, are associated with regularization procedures over space and time (transient case). The efficiency of the method, which depends on the accuracy of the measurements, is explored through an example of transient bidimensional application. A comparison with the superposition method (linear problems) is presented.

Finite element analysis of heat transfer and flow problems using adaptive remeshing including application to solidification problems

Abstract: An error estimation technique for adaptive finite element analysis of heat conduction problems is described. The performance of this technique is demonstrated using a two-dimensional steady-state linear benchmark problem with a known analytical solution. A transient non-linear heat conduction problem involving solidification is then solved as a typical practical application. Some preliminary investigations for error indicators in flow problems are presented.

Low trigger voltage SCR protection device and structure

Abstract: A device for protecting an integrated circuit from transient energy is disclosed. This device provides an SCR having a reduced "snap-back" trigger voltage.

Means and method for testing and monitoring a circuit breaker panel assembly

Abstract: The operation and functioning of various elements in a circuit breaker panel assembly are monitored and tested using a permanently installed or a portable test module that can be plugged into a plug-in jack on the housing of the circuit breaker panel assembly. The test module includes readout elements, data recording elements as well as telemetry networks for relaying data to a central station where such data can be reduced, recorded and analyzed by experts. The circuit breaker panel can include a voltage surge and transient protection network. A method of monitoring and testing is also disclosed.

Some transient aspects of tidal propagation

Abstract: The effects of transient forcing on the structure of some tidal modes are investigated using a time-dependent numerical model. After presentation of the numerical scheme, some of the main features of tidal field variations are presented. The simulated tidal structures are interpreted as consisting of beats between tidal modes and transient waves near tidal periods. From these simulations, several types of information are obtained. Vertical group velocities are calculated and compared with those computed using the analytical expressions of Richmond (1975). Several possible definitions of the setup time are given, and the results from the model calculations are compared with those obtained by Bernard (1981). The authors found that the (2,2) setup time is longer (several weeks) than the (1,1) setup time, contrary to the Bernard conclusions. From these setup time estimates, some conclusions regarding the interpretation of observations and comparisons with steady state model calculations are drawn. Finally, similar results obtained for a two-dimensional gravity wave are analyzed to clarify further basic mechanisms acting on tidal structure variations induced by transient forcing.

Correction circuit and method for improving the transient behavior of a two-way loudspeaker system

Abstract: A circuit for improving the transient behavior of a two-way loudspeaker system includes a crossover circuit with high selectivity, amplitude and phase correction circuitry for separately correcting the amplitude and phase responses of the high and low frequency drivers in their mounting environment, and correction circuitry for correcting the composite amplitude and phase response of the overall loudspeaker system after insertion of the crossover. A further phase offset technique and circuit provides for introducing frequency dependent phase shift in the loudspeaker system's high or low frequency channels for offsetting the phase responses of the high and low frequency drivers within the crossover frequency range. According to the phase offset technique of the invention, phase shift is added, preferably in the high frequency channel, until composite amplitude response curves observed on-axis and at different vertical angles off-axis are forced to be consistent. After consistency is achieved the deterioration of the amplitude response resulting from the phase offset is corrected to a flat response by means of a forced series amplitude correction circuit inserted before the crossover. The result is improved transient response off-axis as well as on-axis.