Showing papers on "Transient (oscillation) published in 2004"

A wireless controller to enhance dynamic performance of parallel inverters in distributed generation systems

Abstract: This paper presents a novel control strategy for parallel inverters of distributed generation units in an AC distribution system. The proposed control technique, based on the droop control method, uses only locally measurable feedback signals. This method is usually applied to achieve good active and reactive power sharing when communication between the inverters is difficult due to its physical location. However, the conventional voltage and frequency droop methods of achieving load sharing have a slow and oscillating transient response. Moreover, there is no possibility to modify the transient response without the loss of power sharing precision or output-voltage and frequency accuracy. In this work, a great improvement in transient response is achieved by introducing power derivative-integral terms into a conventional droop scheme. Hence, better controllability of the system is obtained and, consequently, correct transient performance can be achieved. In addition, an instantaneous current control loop is also included in the novel controller to ensure correct sharing of harmonic components when supplying nonlinear loads. Simulation and experimental results are presented to prove the validity of this approach, which shows excellent performance as opposed to the conventional one.

Control and testing of a dynamic voltage restorer (DVR) at medium voltage level

Abstract: The dynamic voltage restorer (DVR) has become popular as a cost effective solution for the protection of sensitive loads from voltage sags. Implementations of the DVR have been proposed at both a low voltage (LV) level, as well as a medium voltage (MV) level; and give an opportunity to protect high power sensitive loads from voltage sags. This paper reports practical test results obtained on a medium voltage (10 kV) level using a DVR at a distribution test facility in Kyndby, Denmark. The DVR was designed to protect a 400-kVA load from a 0.5-p.u. maximum voltage sag. The reported DVR verifies the use of a combined feed-forward and feed-back technique of the controller and it obtains both good transient and steady-state responses. The effect of the DVR on the system is experimentally investigated under both faulted and nonfaulted system states, for a variety of linear and nonlinear loads. Variable duration voltage sags were created using a controllable LV breaker fed by a 630 kVA distribution transformer placed upstream of the sensitive load. The fault currents in excess of 12 kA were designed and created to obtain the required voltage sags. It is concluded the DVR works well in all operating conditions.

The effects of energy management on reliability in real-time embedded systems

Abstract: The slack time in real-time systems can be used by recovery schemes to increase system reliability as well as by frequency and voltage scaling techniques to save energy. Moreover, the rate of transient faults (i.e., soft errors caused, for example, by cosmic ray radiations) also depends on system operating frequency and supply voltage. Thus, there is an interesting trade-off between system reliability and energy consumption. This work first investigates the effects of frequency and voltage scaling on the fault rate and proposes two fault rate models based on previously published data. Then, the effects of energy management on reliability are studied. Our analysis results show that, energy management through frequency and voltage scaling could dramatically reduce system reliability, and ignoring the effects of energy management on the fault rate is too optimistic and may lead to unsatisfied system reliability.

Dynamic and transient analysis of power distribution systems with fuel Cells-part I: fuel-cell dynamic model

Abstract: The first part of this two-part-paper develops a comprehensive nonlinear dynamic model of a solid-oxide fuel cell (SOFC) that can be used for dynamic and transient stability studies. The model based on electrochemical and thermal equations, accounts for temperature dynamics and output voltage losses. The output voltage response of a stand-alone fuel-cell plant to a step load change, a fuel flow step change, and fast load variations are simulated to illustrate the dynamic behavior of SOFC for fast and slow perturbations. A method for interfacing the proposed fuel-cell models to a power system stability package is developed.

Influence of the variable-speed wind generators in transient stability margin of the conventional generators integrated in electrical grids

Abstract: This work demonstrates that the integration of variable-speed wind systems with doubly fed induction generators (DFIG) and a four-quadrant AC-to-AC converter connected to the rotor windings increases the transient stability margin of the electrical grids, when compared with the case where the fixed speed wind systems with cage generators are used. It is due to the influence of the two dedicated rotor current regulators of the DFIG on the dynamic behavior of the other generators in the system. Besides, adequate models to represent the behavior of the DFIG in transient stability studies are presented. From the simulation results, some important conclusions can be extracted to guide the integration of the wind farms on weak or strong grids.

The dynamic effect of pipe-wall viscoelasticity in hydraulic transients. Part I—experimental analysis and creep characterization

Abstract: The mechanical behaviour of the pipe material determines the pressure response of a fluid system during the occurrence of transient events. in viscoelastic pipes, typically made of polyethylene (pe), maximum or minimum transient pressures are rapidly attenuated and the overall pressure wave is delayed in time. this is a result of the retarded deformation of the pipe-wall. this effect has been observed in transient data collected in a high-density pe pipe-rig, at imperial college (london, uk). several transient tests were carried out to collect pressure and circumferential strain data. the pipe material presented a typical viscoelastic mechanical behaviour with a sudden pressure drop immediately after the fast valve closure, a major dissipation and dispersion of the pressure wave, and transient mechanical hysteresis. the creep-function of the pipe material was experimentally determined by creep tests, and, its order-of-magnitude was estimated based on pressure-strain data collected from the pipe-rig. a goo...

Modified smoothed particle hydrodynamics method and its application to transient problems

Abstract: A modification to the smoothed particle hydrodynamics method is proposed that improves the accuracy of the approximation especially at points near the boundary of the domain. The modified method is used to study one-dimensional wave propagation and two-dimensional transient heat conduction problems.

Using coupled inductors to enhance transient performance of multi-phase buck converters

Abstract: Motivated by modern microprocessors' demand for high-current, low-voltage power with large transient load steps, a scalable multi-phase coupled inductor buck circuit is presented which delivers 80 A at 1.2 V from a 12 V input. 4-phase coupling and 1 MHz per-phase switching frequency enable the use of 50 nil per-phase leakage inductance so that the system is capable of supporting 15%-to-full-to-15% load transient steps with a 72 mV output voltage window using only a small bank of MLCC output capacitors.

Single event transient pulse widths in digital microcircuits

Abstract: The radiation effects community has long known that single event transients in digital microcircuits will have an increasing importance on error rates as device sizes shrink. However separating these errors from static errors in latch cells has often proved difficult. Thus determining both the significance and the nature of these transient errors has not been easy. In this study, by utilizing a latch that is radiation hard at static clock frequencies the errors due to transients could be separated. By separating the transient error rate from the static upset error rate, the pulse structure of the propagating transients was studied using SPICE. The implications of these pulsewidths will also be discussed.

Control system for doubly fed induction generator

Abstract: A controller (28) for a doubly fed induction generator (12, 20) adjusts control signals to a rotor side converter (24) and line side converter (22) to adjust rotor current when a voltage transient on a utility grid (10) occurs, so that the doubly fed induction generator can ride through the transient. The controller can also turn off the transistors of the rotor side converter (24) to reduce rotor current and/or activate a crowbar (42) to reduce the voltage of the DC link (26) connecting the converters (22, 24) when significant voltage transients occur on the grid (10). This permits continued operation of the DFIG system without disconnecting from the grid.

Surge damping analysis in pipe systems: modelling and experiments

Abstract: The current study focuses on the analysis of pressure surge damping in single pipeline systems generated by a fast change of flow conditions. A dimensionless form of pressurised transient flow equations was developed, presenting the main advantage of being independent of the system characteristics. In lack of flow velocity profiles, the unsteady friction in turbulent regimes is analysed based on two new empirical corrective-coefficients associated with local and convective acceleration terms. A new surge damping approach is also presented taking into account the pressure peak time variation. The observed attenuation effect in the pressure wave for high deformable pipe materials can be described by a combination of the non-elastic behaviour of the pipe-wall with steady and unsteady friction effects. Several simulations and experimental tests have been carried out, in order to analyse the dynamic response of single pipelines with different characteristics, such as pipe materials, diameters, thickness, lengt...

Modeling of solid conductors in two-dimensional transient finite-element analysis and its application to electric machines

Abstract: We present an approach for directly coupling transient magnetic fields and electric circuits. The circuit can contain arbitrarily connected solid conductors located in the magnetic field region. A systematic procedure suitable for both nodal method and loop method is used to couple fields and circuits. The structures of the system equations of the two methods are analogous. The formulations allow the equations in stranded windings and solid conductors to be unified and the coefficient matrix of the system equations to be symmetrical. In order to reduce the solution domain, the periodic boundary conditions are still applicable when the solid conductors are involved. Our approach has been applied to the simulation of electric machines. We give four examples: 1) calculation of the input phase current and output torque when a single-phase induction motor with shaded rings is in locked-rotor operation; 2) simulation of the sudden short circuit of a synchronous generator with starting cage; 3) study of the phase current waveform of an induction motor when the rotor bars are broken; and 4) investigation of the parasitic capacitive impact of the surge voltage on a winding due to drive switching and cable ring.

Optical phase modulators for MHz and GHz modulation in silicon-on-insulator (SOI)

Abstract: This paper reports the simulation of the direct current (dc), transient, and optical characteristics of low-loss single-mode optical phase modulators based on silicon-on-insulator (SOI) material. The devices operate by injecting free carriers to change the refractive index in the guiding region and have been modeled using the two-dimensional (2-D) device simulation package SILVACO and the optical simulator BeamPROP to determine their electrical and optical performance, respectively. These simulators have been employed to optimize the overlap between the injected free carriers in the intrinsic region and the propagating optical mode. Attention has been paid to both the steady state and transient properties of the device. In order to produce quantitative results, a particular p-i-n device geometry has been employed in the study, but the trends in the results are sufficiently general to be of help in the design of many modulator geometries. The specific example devices used are designed to support a single optical guided mode and are of approximately 1 mm in cross-sectional dimensions. The modeling results predict that the transient performance of the device is affected significantly by the contact width and the rib doping depth. Results presented encompass Gaussian and constant doping profiles in the n/sup +/ regions. The doping profile of the contacts has a tremendous effect on both the dc and transient performances. Phase modulators with drive currents as low as 0.5 mA and transient rise times of 0.3 ns and fall times of 0.12 ns are predicted. Following from these results, a realistic doping profile is proposed that surpasses the electrical results of the Gaussian and most of the constant doping profiles. The improvements in electrical device characteristics are at the expense of a slightly increased optical absorption loss. An alternative switching technique is also presented that could further improve the device speed.

New method for generators' angles and angular velocities prediction for transient stability assessment of multimachine power systems using recurrent artificial neural network

Abstract: Recurrent radial basis function (RBF) and multilayer perceptron (MLP) artificial neural network (ANN) schemes are proposed for dynamic system modeling, and generators' angles and angular velocities prediction for transient stability assessment. The method is presented for multimachine power systems. In this scheme, transient stability is assessed based on monitoring generators' angles and angular velocities with time, and checking whether they exceed the specified limits for system stability or not. Data generation schemes have been proposed. The proposed recurrent ANN scheme is not sensitive to fault locations. It is only dependent on the postfault system configuration.

Nonlinear control strategies for cascaded multilevel STATCOMs

Abstract: Two internal nonlinear control strategies based on the feedback linearization technique for cascaded multilevel static compensators (STATCOMs) are presented. The strategies depend on the control capability of the converter output voltage and are suitable for line frequency-switched converters. The first strategy considers a STATCOM where the voltage is set independently of the dc link voltage. Fast reactive power control within subcycle time response is achieved. The second strategy is constrained to a voltage whose amplitude remains proportional to the dc link voltage. Despite this limitation, the proposed strategy allows full stabilization of the STATCOM dynamics and relatively fast control of the reactive current (within one cycle). This may be adequate for most STATCOM applications. Simulation results, using power system computer-aided design/electromagnetic transient program (PSCAD/EMTP), presented for both strategies confirm the effectiveness of the control schemes to impose linear STATCOM dynamics. Preliminary experimental results from a five-level prototype are presented for a converter using fixed control angles.

Transient performance of voltage source converter under unbalanced voltage dips

Abstract: Robust operation of voltage source converters under unbalanced voltage dips can be obtained by controlling the positive and negative-sequence currents separately with a dual vector current controller (DVCC). In this paper, the performance of the DVCC is investigated under all voltage dips that can affect the converter. Two different methods for taking into account the oscillating powers dissipated in the filter are presented, tested and compared. The effect of the phase-angle jump of the dip is also studied.

Impacts of distributed generation penetration levels on power systems transient stability

Abstract: Concerns on environmental and economical issues drive the increasing developments that support small scale generators to be connected close to distribution networks, i.e. distributed generation (DG). When connected in small amounts, the impact of DG on the power system transient stability will be negligible, however, when the penetration of DG increases, its impact is no longer restricted to the distribution network but starts to influence the whole system, including the transmission system transient stability. In this paper, the transmission system transient stability is investigated when a fault is applied in all possible branches (regarding the N-1 security analysis). In this studie the penetration level of DG implementation is raised in two ways: (1) a load increase is covered by DG implementation (with a constant centralized generation) or increased CG output, and (2) a reduction of centralized generation is covered by DG (with a constant load).

Method for measuring transient electromagnetic components to perform deep geosteering while drilling

Abstract: A transverse induction transmitter on an instrument induces currents in an earth formation when it is pulsed. Transient measurements made at transverse and axial receivers are used for determination of a distance to a bed boundary. This may be used to control the drilling direction. Alternatively, a transmitter on an instrument having a conductive body induces currents in the earth formation. Transient signals are measured and the effect of the conductive body is removed by using a reference signal measured in a homogenous space.

System and method for enhanced thermophotovoltaic generation

Abstract: A system and method for lower cost, high efficiency, thermophotovoltaic distributed generation includes: an emitter, a photovoltaic cell, and transient electrical energy storage.

Three-dimensional transient radiative transfer modeling using the finite-volume method

Abstract: This article presents a finite-volume method to calculate transient radiative transfer in a three-dimensional enclosure. The fully implicit scheme is used to discretize the transient term. The step and CLAM spatial differencing schemes are used in this study. The procedure is validated using available published results. The ability of the present formulation in modeling an absorbing, emitting and isotropically scattering medium is examined using heat fluxes and incident radiation. Ray effect and false scattering are discussed.

Controlling a pressure transient in a well

Abstract: A system for controlling a dynamic pressure transient in a well during operations to provide fluid communication between the wellbore and adjacent formation. A method includes determining the characteristics of an adjacent formation, selecting a perforating tool for increasing fluid communication, determining a dynamic pressure transient to enhance the quality of the fluid communication and prevent damage, selecting a wellbore fluid to achieve the desired pressure transient, and performing the fluid communication process based on the selected parameters.

Decay of Pressure and Energy Dissipation in Laminar Transient Flow

Abstract: In the present paper, some peculiar characteristics of transient laminar flow are dis-cussed. After presenting a review of the existing literature, attention is focused on tran-sient energy dissipation phenomena. Specifically, results of both laboratory and numericalexperiments are reported, the latter by considering one-dimensional (1D) along withtwo-dimensional (2D) models. The need of modifying a criterion for simulating unsteadyfriction proposed some years ago by one of the writers, and extensively used for water-hammer calculations, is pointed out. Differences between accelerating and deceleratingflows as well as between transients in metallic and plastic pipes are also highlighted.@DOI: 10.1115/1.1839926#Keywords: Transient, Laminar, Energy, Dissipation, Viscoelasticity

A fast hybrid field-circuit simulator for transient analysis of microwave circuits

Abstract: A plane-wave-time-domain accelerated time-domain integral-equation solver is coupled to a SPICE-like transient circuit simulator to analyze electromagnetic platform-circuit interactions. The hybrid field-circuit simulator simultaneously solves surface-wire-volume time-domain integral equations that model electromagnetic interactions with the platform and modified nodal analysis equations that govern the behavior of the potentially nonlinear lumped circuits. A shielded nonlinear microwave amplifier is analyzed using the proposed scheme, and its immunity to electromagnetic interference is assessed.

Methods and apparatuses for transient analyses of circuits

Abstract: Methods and apparatuses for transient analyses of a circuit using a hierarchical approach. In one embodiment, the cells are grouped locally on the power supply network according to average power dissipation. A time varying current of each cell group is estimated using a probabilistic approach to represent the cell group so that the probability of a more severe waveform for the current of the cell group is under a certain level. For example, the cells in a group are partitioned as switching cells and non-switching cells using cell toggle rates for the determination of the time varying current. The circuit model of the power supply network includes the current sources according to the estimated time varying currents for the cell groups, the power supply wire resistance, the power supply to ground wire capacitance, well capacitance and the de-coupling capacitance from non-switching cells.

VSC-HVDC station with SSSC characteristics

Abstract: Commercial installations of voltage-source converter-high voltage direct current (VSC-HVDC) are now in service as asynchronous link and as underwater/underground dc transmission. The research reported in this paper is directed to providing VSC-HVDC with synchronizing power and damping power so that after a transient, the steady-state is reached in minimal time. Damping power is achieved by injecting incremental power which is proportional to the frequency deviation measured by a phase lock loop. Synchronizing power is achieved by: 1) introducing a delay in the power measurement and 2) by incorporating a static synchronous series compensation in the VSC-station. The effectiveness of both the synchronizing and damping power is demonstrated through digital simulations of a turbine-generator system which is connected to a VSC-HVDC Station. It is shown that with the synchronizing power and damping power, recovery from the fault disturbance is very much faster.

The Effect of Various Dynamic, Thermodynamic and Design Parameters on the Performance of a Turbocharged Diesel Engine Operating under Transient Load Conditions

Abstract: Thermodynamic, dynamic and design parameters have a significant and often conflicting impact on the transient response of a compression ignition engine. Knowing the contribution of each parameter on transient operation could direct the designer to the appropriate measures for better engine performance. To this aim an explicit simulation program developed is used to study the performance of a turbocharged diesel engine operating under transient load conditions. The simulation developed, based on the filling and emptying approach, provides various innovations as follows: Detailed analysis of thermodynamic and dynamic differential equations, on a degree crank angle basis, accounting for the continuously changing nature of transient operation, analysis of transient mechanical friction, and also a detailed mathematical simulation of the fuel pump. Each equation in the model is solved separately for every cylinder of the 6-cylinder diesel engine considered. The model is validated against experimental data for various load changes. The effect of several dynamic, thermodynamic and design parameters is studied, i.e. load schedule (type, and duration of load applied), turbocharger mass moment of inertia, exhaust manifold volume and configuration, cylinder wall temperature, aftercooler effectiveness as well as an interesting case of a malfunctioning fuel pump. Explicit diagrams are given to show how, after an increase in load, each parameter examined affects the engine speed response, as well as other properties of the engine and turbocharger such as fuel pump rack position, boost pressure and turbocharger speed. It is shown that certain parameters, such as the type of connected loading, the turbocharger inertia, a damaged fuel pump and the exhaust manifold volume, can have a significant effect on the engine and turbocharger transient performance. However others, such as the cylinder wall temperature, the aftercooler effectiveness and the exhaust manifold configuration have a less important effect as regards transient response and final equilibrium conditions.

Design of a prototype D-STATCOM for voltage sag mitigation

Abstract: This paper presents the design of a prototype distribution static compensator (D-STATCOM) for voltage sag mitigation in an unbalanced distribution system. The D-STATCOM is intended to replace the widely used static Var compensator (SVC). For fast response requirement, the feedforward compensation scheme is employed to enable the D-STATCOM to mitigate voltage sag and at the same time correct the power factor, thus acting as a load compensator. The compensation scheme of the D-STATCOM is derived using the symmetrical component method. In this work, the 12-pulse D-STATCOM configuration with IGBT has been designed and the graphic based models of the D-STATCOM have been developed using the PSCAD/EMTDC electromagnetic transient simulation program. Accordingly, simulations are first carried out to illustrate the use of D-STATCOM in mitigating voltage sag in a distribution system. Simulation results prove that the D-STATCOM is capable of mitigating voltage sag as well as improving power quality of a system.