Showing papers on "Transient (oscillation) published in 2011"

Efficient Modeling of Modular Multilevel HVDC Converters (MMC) on Electromagnetic Transient Simulation Programs

Abstract: The number of semiconductor switches in a modular multilevel converter (MMC) for HVDC transmission is typically two orders of magnitudes larger than that in a two or three level voltage-sourced converter (VSC). The large number of devices creates a computational challenge for electromagnetic transient simulation programs, as it can significantly increase the simulation time. The paper presents a method based on partitioning the system's admittance matrix and deriving an efficient time-varying Thevenin's equivalent for the converter part. The proposed method does not make use of approximate interfaced models, and mathematically, is exactly equivalent to modelling the entire network (converter and external system) as one large network. It is shown to drastically reduce the computational time without sacrificing any accuracy. The paper also presents control algorithms and other modelling aspects. The efficacy of the proposed method is demonstrated by simulating a point-to-point VSC-MMC-based HVDC transmission system.

Permanent Magnet Synchronous Generator-Based Standalone Wind Energy Supply System

Abstract: In this paper, a novel algorithm, based on dc link voltage, is proposed for effective energy management of a standalone permanent magnet synchronous generator (PMSG)-based variable speed wind energy conversion system consisting of battery, fuel cell, and dump load (i.e., electrolyzer). Moreover, by maintaining the dc link voltage at its reference value, the output ac voltage of the inverter can be kept constant irrespective of variations in the wind speed and load. An effective control technique for the inverter, based on the pulsewidth modulation (PWM) scheme, has been developed to make the line voltages at the point of common coupling (PCC) balanced when the load is unbalanced. Similarly, a proper control of battery current through dc-dc converter has been carried out to reduce the electrical torque pulsation of the PMSG under an unbalanced load scenario. Based on extensive simulation results using MATLAB/SIMULINK, it has been established that the performance of the controllers both in transient as well as in steady state is quite satisfactory and it can also maintain maximum power point tracking.

Direct Active and Reactive Power Regulation of Grid-Connected DC/AC Converters Using Sliding Mode Control Approach

Abstract: This paper proposes a new direct active and reactive power control (DPC) for the three-phase grid connected dc/ac converters. The proposed DPC strategy employs a nonlinear sliding mode control (SMC) scheme to directly calculate the required converter's control voltage so as to eliminate the instantaneous errors of active and reactive powers without involving any rotating coordinate transformations. Meanwhile, there are no extra current control loops involved, which simplifies the system design and enhances the transient performance. Constant converter switching frequency is achieved by using space vector modulation, which eases the design of the ac harmonic filter. Simulation and experimental results are provided and compared with those of the classic voltage-oriented vector control (VC) and conventional lookup table (LUT) DPC strategies. The proposed SMC-DPC is capable of providing enhanced transient performance similar to that of the LUT-DPC, and keeps the steady-state harmonic spectra at the same level as those of the VC scheme. The robustness of the proposed DPC to line inductance variations is also inspected during active and reactive power changes.

Dynamic Modeling and Performance Analysis of a Grid-Connected Current-Source Inverter-Based Photovoltaic System

Abstract: Voltage-source inverter (VSI) topology is widely used for grid interfacing of distributed generation (DG) systems. However, when employed as the power conditioning unit in photovoltaic (PV) systems, VSI normally requires another power electronic converter stage to step up the voltage, thus adding to the cost and complexity of the system. To make the proliferation of grid-connected PV systems a successful business option, the cost, performance, and life expectancy of the power electronic interface need to be improved. The current-source inverter (CSI) offers advantages over VSI in terms of inherent boosting and short-circuit protection capabilities, direct output current controllability, and ac-side simpler filter structure. Research on CSI-based DG is still in its infancy. This paper focuses on modeling, control, and steady-state and transient performances of a PV system based on CSI. It also performs a comparative performance evaluation of VSI-based and CSI-based PV systems under transient and fault conditions. Analytical expectations are verified using simulations in the Power System Computer Aided Design/Electromagnetic Transient Including DC (PSCAD/EMTDC) environment, based on a detailed system model.

Switching Frequency Reduction Using Model Predictive Direct Current Control for High-Power Voltage Source Inverters

Abstract: In this paper, a novel current control approach called model predictive direct current control (MPDCC) is presented. The controller takes into account the discrete states of the voltage source inverter (VSI), and the current errors are predicted for each sampling period. Voltage vectors are selected by a graph algorithm, whereby the most appropriate vector is chosen based on an optimization criterion. However, this depends on whether the state of the system is transient or steady. In the first case, the current error should be minimized as fast as possible in order to obtain fast dynamics. In the latter one, the VSI switching behavior is optimized since the switching losses account for a large amount of the total converter losses in high-power drive systems. MPDCC has been developed for a general neutral-point isolated resistive-inductive load with an internal voltage source. For demonstration, the presented control strategy has been implemented on a small-scale permanent-magnet synchronous machine drive system with a two-level VSI. This new approach has several advantages. The most important one is that the switching frequency is reduced up to 70% compared to linear control combined with pulsewidth modulation. Second, MPDCC obtains fast dynamic responses, which are already known from, e.g., direct torque control.

Demonstration and Validation of a 3D CFD Simulation Tool Predicting Pump Performance and Cavitation for Industrial Applications

Abstract: Due to complexities in geometry and physics, computational fluid dynamics (CFD) pump simulation has historically been very challenging and time consuming, especially for cases with cavitation. However, with the evolution and innovation of CFD technologies, pump cavitation simulation has improved significantly in recent years. In view of these developments, this paper will discuss a new generation CFD tool for pump cavitation simulation, using an axial flow water pump as a demonstration case. A novel CFD methodology and advanced cavitation model will be presented and discussed. Key components that are relevant to the improvement of accuracy and CFD simulation speed will be discussed in detail. An axial flow water pump is chosen as the test case to demonstrate and validate the capability and accuracy of the code discussed. Simulation results include pump head, hydraulic efficiency, and cavitation characteristic in terms of incipient net positive suction head for the whole pump flow passages using both multiple reference frame and transient approaches. Multiple operation conditions, from 70% to 120% of duty flow rate, have been evaluated and will be projected against experimental data. Furthermore, simulated cavitation patterns will be compared with video images recorded during the experiments.

Phase Angle Control of High-Frequency Resonant Currents in a Multiple Inverter System for Zone-Control Induction Heating

Abstract: This paper presents a phase angle control method of high-frequency resonant currents in a zone-control induction heating (ZCIH) system, which consists of split working coils and multiple inverters. The ZCIH system controls the amplitude of each coil current to make the temperature distribution on the workpiece uniform. The amplitude of the coil current can be controlled in a wide range when its phase angle is adjusted to be the same with other coil currents. This paper theoretically derives the phase-angle change of the coil current in transient states, and reveals that the phase-angle change can be considered as a first-order response. A phase-angle controller was designed and examined in experiments using a two-zone ZCIH system. It is clarified that the phase angle control makes it possible to adjust the current phase angle not only in steady states but also in transient states.

Intelligent Connection Agent for Three-Phase Grid-Connected Microgrids

Abstract: The high penetration of distributed generation power plants, based on renewable energy sources (RESs), is boosting the connection of power converters to the electrical network. This generation concept would permit to form local networks, microgrids, when the main grid falls due to any kind of contingency in the network. However, the connection and disconnection of these local networks may give rise to undesired transient overcurrents that should be avoided. In order to solve this drawback, this paper presents a method oriented to carry out a stable intentional disconnection/reconnection of local grids from the main electrical network under grid-fault conditions. This control method has been implemented in a grid-connected power converter that acts as an intelligent connection agent (ICA) and adapts its operation mode according to its connection state. The proposed control also manages the operation of a controlled switch, which is responsible of disconnecting/reconnecting the microgrid from the mains. In this paper, the behavior of the ICA under transient conditions will be discussed, and finally, its simulated and experimental performance will be shown.

Improved Direct Power Control of a Wind Turbine Driven Doubly Fed Induction Generator During Transient Grid Voltage Unbalance

Abstract: This paper proposes an improved direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind power generation system under unbalanced grid voltage dips. The fundamental and double grid frequency power pulsations, which are produced by the transient unbalanced grid faults, are mathematically analyzed and accurately regulated. Five selectable control targets, with proper power references given, are designed for different applications during network unbalance. In order to provide enhanced control performance, two resonant controllers, which are tuned to have large gain at the power pulsation frequencies, are applied together with the proportional-integral controller to achieve full control of the DFIG output power. The effectiveness of the proposed DPC strategy is verified by the experimental results of a 5-kW DFIG system under different unbalanced voltage dips, which are generated by a specially designed voltage dip generator.

Piezoelectric Energy Harvesting for Civil Infrastructure System Applications: Moving Loads and Surface Strain Fluctuations

Abstract: This article formulates the problem of vibration-based energy harvesting using piezoelectric transduction for civil infrastructure system applications with a focus on moving load excitations and surface strain fluctuations. Two approaches of piezoelectric power generation from moving loads are formulated. The first one is based on using a bimorph cantilever located at an arbitrary position on a simply supported slender bridge. The fundamental moving load problem is reviewed and the input to the cantilevered energy harvester is obtained to couple with the generalized electromechanical equations for transient excitation. The second approach considers using a thin piezoceramic patch covering a region on the bridge. The transient electrical response of the surface patch to moving load excitation is derived in the presence of a resistive electrical load. The local way of formulating piezoelectric energy harvesting from two-dimensional surface strain fluctuations of large structures is also discussed. For a thi...

A new measurement method for power signatures of non-intrusive demand monitoring and load identification

Abstract: Based upon the analysis of load signatures, this paper presents a Non-intrusive load monitoring (NILM) technique. With characterizing associated with the transient response of energy signature, a reliable and accurate recognition result can be obtained. In this study, artificial neural networks (ANN), in combination with turn-on transient energy analysis, are used to improve recognition accuracy and computational speed of NILM results. To minimize the distortion phenomenon in current measurements from the hysteresis of traditional current transformers (CTs) iron cores, coreless Hall effect current transformer is adopted to accurately detect non-sinusoidal waves to improve NILM accuracy. The experimental results indicate that the incorporation of turn-on transient energy algorithm into NILM significantly improve the recognition accuracy and computational speed.

Overcurrent relays coordination considering transient behaviour of fault current limiter and distributed generation in distribution power network

Abstract: The connecting of distributed generation (DG) to the distribution network has numerous advantages. However, in the presence of DG some problems in coordination of protection devices will occur, due to changes in short-circuit levels at different points. Fault current limiter (FCL) is applied to limit the fault current levels and the effect of DGs on coordination of overcurrent (O/C) relays during the faults. The use of DG and FCL in distribution networks causes to some transient currents during fault conditions. Steady-state coordination methods do not result in accurate settings in such networks. A new method is proposed for coordination of O/C relays by considering the transient behaviour of the network. This method is based on the genetic algorithm and uses the dynamic model of O/C relays instead of the fixed characteristic curves. For this purpose, transient behaviour of DG and FCL are simulated and the relay operating status is calculated for all primary and backup relays to achieve the optimal settings of relays in transient condition. Simulations are carried out on a sample network and the results demonstrate that the method gives feasible and effective solutions for optimal coordination in the practical power system networks in comparison to the previous steady-state methods.

Layout Technique for Single-Event Transient Mitigation via Pulse Quenching

Abstract: A layout technique that exploits single-event transient pulse quenching to mitigate transients in combinational logic is presented. TCAD simulations show as much as 60% reduction in sensitive area and 70% reduction in pulse width for some logic cells.

Onset of nondiffusive phonon transport in transient thermal grating decay

Abstract: The relaxation of a spatially sinusoidal temperature perturbation in a dielectric crystal at a temperature comparable to or higher than the Debye temperature is investigated theoretically. We assume that most phonons contributing to the specific heat have mean free path (MFP) much shorter than the thermal transport distance and can be described by the thermal diffusion model. Low-frequency phonons that may have MFP comparable to or longer than the grating period are described by the Boltzmann transport equation. These low-frequency phonons are assumed to interact with the thermal reservoir of high frequency phonons but not with each other. Within the single mode relaxation time approximation, an analytical expression for the thermal grating relaxation rate is obtained. We show that the contribution of "ballistic" phonons with long MFP to the effective thermal conductivity governing the grating decay is suppressed compared to their contribution to thermal transport at long distances. The reduction in the effective thermal conductivity in Si at room temperature is found to be significant at grating periods as large as 10 microns.

Hilbert-Transform-Based Transient/Intermittent Earth Fault Detection in Noneffectively Grounded Distribution Systems

Abstract: Conventional relaying algorithms are mostly based on phasor computation. Since it is difficult to obtain accurate phasor results in fast transient situations, most of the feeder relays fail to correctly respond to transient/intermittent earth faults, which are, however, frequent fault cases in distribution systems. This paper presents a novel approach for fault detection and direction determination for these transient/intermittent faults. The basic idea is to extract the fault direction using the instantaneous power's direction. The instantaneous power is obtained by using Hilbert transform. The proposed direction element inherently utilizes the high-frequency components of the transient fault signals at a conventional sampling rate. Together with an intermittence detection algorithm, a scheme for transient/intermittent earth fault detection has been proposed. It has been implemented as a prototype relay. Electromagnetic Transients Program tests as well as physical model tests have proved the effectiveness of the proposed scheme.

A piezoelectric device for impact energy harvesting

Abstract: This paper studies a piezoelectric impact energy harvesting device consisting of two piezoelectric beams and a seismic mass. The aim of this work is to find the influence of several mechanical design parameters on the output power of such a harvester so as to optimize its performance; the electrical design parameters were not studied. To account for the dynamics of the beams, a model including the mechanical and piezoelectric properties of the system is proposed. The impacts involved in the energy harvesting process are described through a Hertzian contact law that requires a time domain simulation to solve the nonlinear equations. A transient regime and a steady-state regime have been identified and the performance of the device is characterized by the steady-state mean electrical power and the transient electrical power. The time simulations have been used to study the influence of various mechanical design parameters (seismic mass, beam length, gap, gliding length, impact location) on the performance of the system. It has been shown that the impact location is an important parameter and may be optimized only through simulation. The models and the simulation technique used in this work are general and may be used to assess any other impact energy harvesting device.

Comparative Studies Between Classical and Mutually Coupled Switched Reluctance Motors Using Thermal-Electromagnetic Analysis for Driving Cycles

Abstract: This paper presents copper and iron loss models of a classical switched reluctance motor (SRM) and a mutually coupled switched reluctance motor (MCSRM). The iron losses in different parts of machines are then detailed. Based on the power losses model, a lumped parameter (LP) transient thermal model during driving cycles is performed, the analytical results are validated by the finite-element (FE) transient thermal model. Special attention has been paid to model the salient rotor and a method to transform the salient rotor into a nonsalient one has been proposed. A comparison between the maximum temperatures obtained by using different heat source (average power losses or instantaneous power losses during driving cycles) is given. The experimental tests are also realized to verify the analytical and numerical results.

Validation of Single- and Multiple-Machine Equivalents for Modeling Wind Power Plants

Abstract: This paper presents an exhaustive simulation study performed to validate the adequacy of the equivalencing method promoted by the National Renewable Energy Laboratory for modeling wind power plants by single- and multiple-machine equivalents. The main simulation results are presented for a number of steady state and transient wind turbine generator operating conditions following various faults and a typical low-voltage ride through. The impact of protection systems such as the crowbar is also taken into account. The aggregation technique has shown to be adequate for load flow, stability, and electromagnetic transient studies with limitations, as presented in the paper.

Application of SMES Unit in Improving the Performance of an AC/DC Power System

Abstract: This paper investigates the use of a superconducting magnetic energy storage (SMES) unit to improve the performance of an ac/dc power system. In this context, investigations have been conducted on a large turbine-generator unit connected to a high-voltage direct current (HVDC) system. The impact of HVDC converter station faults on the torsional torques induced in turbine-generator shafts with and without using an SMES unit is elaborated. Faults considered are fire-through, misfire, short circuit across the inverter station, flashover, and a three-phase short circuit in the ac system. These investigations are studied using an electromagnetic transient program power system simulation/electromagnetic transients including DC (PSCAD/EMTDC) and the results are presented in the form of typical time responses as well as harmonic analysis.

Field Test and Simulation of a 400-kV Cross-Bonded Cable System

Abstract: This paper discusses cable modeling for long high-voltage ac underground cables. In investigating the possibility of using long cables instead of overhead lines, the simulation results must be trustworthy. Therefore, model validation is of great importance. This paper gives a benchmark case for measurements on a 400-kV cable system with cross-bonded sheaths. This paper describes in detail the modeling procedure for the cable system and compares simulation results with the transient field test results. It is shown that although the main characteristics of the waveforms are well reproduced in the initial transient, there are significant deviations between the simulation and measurement results. An analysis indicates that the main cause for the deviation is inadequate representation of the current distribution on conductors since the modeling approach does not take proximity effects into account. The measurement results can be received by contacting the first author of this paper.

Detection of broken outer cage bars for double cage induction motors under the startup transient

Abstract: Unlike single cage rotor fault detection, FFT-based steady state spectrum analysis techniques can fail to detect outer cage faults in double cage induction motors due to the small outer cage current under running conditions. Double cage motors are typically employed in applications that require loaded starts. This makes the outer cage vulnerable to fatigue failure since it must withstand the high starting current and long startup time frequently. However, there are only a few publications that investigate detection techniques specifically for double cage motors. In this paper, considering that the influence of the faulty outer cage is strong at startup due to the large outer cage current, detection of outer cage faults under the startup transient is investigated. A Discrete Wavelet Transform-based method is proposed as a viable solution to detection of outer cage faults for double cage motors. An experimental study on fabricated copper double cage induction motors shows that the proposed method provides sensitive and reliable detection of double cage rotor faults compared to FFT.

Design of WAMS-Based Multiple HVDC Damping Control System

Abstract: High voltage direct current (HVDC) links are interties, characterized by good controllability, large adjustable capacity and fast responses, and have inherent advantage in damping interarea oscillations over power system stabilizers (PSS) and flexible alternating current transmission systems (FACTS). In this paper, a systematic and practical means to design the multiple HVDC damping control system, which aims to enhance the damping of certain interarea modes and minimize the interactions between different control loops, is provided. In the proposed procedure, the wide-area feedback signals that have the good observability of target modes are preselected by an index based on residue analysis firstly. Then the interactions between the candidate control loops, in which the setting power reference of the HVDC as the input and the preselected wide-area signals as the output, are investigated by relative gain array (RGA) analysis. According to the RGA results, a suitable input-output pairing for multiple HVDC damping controller is determined. The transfer function residues used in the feedback signal preselection and the RGA calculation are identified by Prony analysis in the transient stability program and electromagnetic transient program respectively. Finally, the damping controller parameters are optimized by time-domain simulations. The application in China Southern Power Grid (CSG) is illustrated as the case study.

Increasing photovoltaic penetration with local energy storage and soft normally-open points

Abstract: This paper considers the use of energy storage to mitigate the effects of power output transients associated with photovoltaic systems due to fast-moving cloud cover. In particular, the combination of energy storage with ‘soft’ normally-open points (SNOPs), referring to an AC/AC power electronic conversion device in place of switchgear, is considered. This paper will demonstrate the use of SNOPs with energy storage for providing this damping functionality simultaneous to its other functions. A formula to estimate the level of energy storage required to handle worst-case transient conditions, as well as two high-level control schemes for the SNOP are proposed then tested experimentally through simulation on typical distribution systems with measured historical insolation data.

Application of the Partial Element Equivalent Circuit Method to Analysis of Transient Potential Rises in Grounding Systems

Abstract: This paper presents calculations of lightning transient potential rises in grounding systems. A partial element equivalent circuit (PEEC) method, adopting a modified image method, is employed in this paper. The modified image method in this paper has two options either including or neglecting images of conduction currents along conductors for calculating series impedances. The effect of retardation in the PEEC method is also investigated. Comparisons of simulation results by the proposed method with those by the method of moments, the finite-difference time-domain method, and experimental results collected from the literature show that the PEEC method with the modified image method is quite effective in the evaluation of transient potential rise in a grounding system.