Showing papers on "Transient (oscillation) published in 2013"

Impact of increased penetration of photovoltaic generation on power systems

Abstract: Present renewable portfolio standards are changing power systems by replacing conventional generation with alternate energy resources such as photovoltaic (PV) systems. With the increase in penetration of PV resources, power systems are expected to experience a change in dynamic and operational characteristics. This paper studies the impact of increased penetration of PV systems on static performance as well as transient stability of a large power system, in particular the transmission system. Utility scale and residential rooftop PVs are added to the aforementioned system to replace a portion of conventional generation resources. While steady state voltages are observed under various PV penetration levels, the impact of reduced inertia on transient stability performance is also examined. The studied system is a large test system representing a portion of the Western U.S. interconnection. The simulation results obtained effectively identify both detrimental and beneficial impacts of increased PV penetration both for steady state stability and transient stability performance.

An energy-based method for location of power system oscillation source

Abstract: An energy-based method to locate oscillation sources in power systems is proposed. The energy is identical to the transient energy. The amount of energy is consistent with the oscillation amplitude, and then the component producing energy has negative contribution to the damping and is considered as the oscillation source. The consistency of energy dissipation with damping torque of a generator is proved. A method to compute energy flow in the network based on wide area measurement system data and independent of energy functions is proposed. The energy dissipation or production of a component in the system can be obtained from the net energy flow. The components producing energy are oscillation sources and emergency control actions should be taken on them. The method is validated by simulations and actual oscillation incidents analyses.

Three-Phase Cascaded Delayed Signal Cancellation PLL for Fast Selective Harmonic Detection

Abstract: Fast and accurate selective harmonic detection has crucial value for many power system harmonic compensation systems. The existing detection methods are known to have a few drawbacks, such as long delay time, sensitivity to grid frequency variation, and difficulty to achieve zero steady-state error. In order to overcome these drawbacks, this paper proposes a selective harmonic detection system based on the three-phase cascaded delayed signal cancellation phase-locked loop. The system can be flexibly configured to detect any individual harmonic from the source with various background harmonics. It also features very short transient and excellent adaptability under small and considerable frequency variations, as verified by comprehensive experimental results. Finally, this paper provides guidance on how to tailor the detection system for different applications and a solution to address the practical implementation issues.

Low-budget transient imaging using photonic mixer devices

Abstract: Transient imaging is an exciting a new imaging modality that can be used to understand light propagation in complex environments, and to capture and analyze scene properties such as the shape of hidden objects or the reflectance properties of surfaces.Unfortunately, research in transient imaging has so far been hindered by the high cost of the required instrumentation, as well as the fragility and difficulty to operate and calibrate devices such as femtosecond lasers and streak cameras.In this paper, we explore the use of photonic mixer devices (PMD), commonly used in inexpensive time-of-flight cameras, as alternative instrumentation for transient imaging. We obtain a sequence of differently modulated images with a PMD sensor, impose a model for local light/object interaction, and use an optimization procedure to infer transient images given the measurements and model. The resulting method produces transient images at a cost several orders of magnitude below existing methods, while simultaneously simplifying and speeding up the capture process.

Assessment and Optimization of the Transient Response of Proportional-Resonant Current Controllers for Distributed Power Generation Systems

Abstract: The increasing number of distributed power generation systems (DPGSs) is changing the traditional organization of the electrical network. An important part of these DPGSs is based on renewable energy sources. In order to guarantee an efficient integration of renewable-based generation units, grid codes must be fulfilled. Their most demanding requirements, such as low-voltage ride-through and grid support, need a really fast transient response of the power electronics devices. In this manner, the current controller speed is a key point. This paper proposes a methodology to assess and optimize the transient response of proportional-resonant current controllers. The proposed methodology is based on the study of the error signal transfer function roots by means of pole-zero plots. Optimal gains are set to achieve fast and nonoscillating transient responses, i.e., to optimize the settling time. It is proved that optimal gain selection results from a tradeoff between transients caused by reference changes and transients caused by changes at the point of common coupling. Experimental results obtained by means of a three-phase voltage source converter prototype validate the approach. Short transient times are achieved even when tests emulate very demanding realistic conditions: a +90° phase-angle jump in the current reference and a “type C” voltage sag at the point of common coupling.

Transient natural convection in a conducting enclosure heated from above

Abstract: Graphical Abstract

Accelerated Model of Modular Multilevel Converters in PSCAD/EMTDC

Abstract: Modular multilevel converters (MMC) are an effective option for the continuously growing demands of voltage-sourced converter-based high-voltage direct-current (VSC-HVDC) transmission. However, accurate modeling of MMC with a high level in PSCAD/EMTDC is extremely time-consuming and requires hardware. Based on the Kirchhoff's Law, an equivalent accelerated model for MMC is proposed. The essence of the new model is the partition of one large-scale admittance matrix into substantial small-scale matrices, which is mathematically demonstrated with the nodal analysis method. Finally, the detailed electromagnetic transient simulations are implemented for the comparisons of steady state and transient performances, and the results validate the proposed model.

Enhanced Load Step Response for a Bidirectional DC–DC Converter

Abstract: An essential requirement for a high-performance dual active bridge (DAB) dc-dc converter is to rapidly and accurately maintain its DC output voltage under all operating conditions This paper uses a novel harmonic modeling strategy to create a linearized dynamic model of a DAB that accurately identifies its transient response to both a reference voltage change and an output load-current change Using this model, a feedforward compensation strategy is presented that significantly improves the DAB's transient response to an output load change The transient performance is then further enhanced by analytically compensating for the nonlinear dead-time distortion that is caused by the converter switching processes The resultant control system achieves rapid and precise output voltage regulation for both reference voltage and output load changes The theoretical analysis is confirmed by both matching simulation and experimental investigations

Load Detection Model of Voltage-Fed Inductive Power Transfer System

Abstract: Detecting load parameters in the inductive power transfer (IPT) system is essential to establishing a stable and efficient wireless power supply of good quality for kitchen appliances. This paper presents an effective load detection approach, namely transient load detection model, to detect load conditions by utilizing the energy injection mode and free resonant mode. To realize the proposed model, the differential equation of the primary resonant current under the free resonant mode was used. Besides, real-time sampled data, including the operating frequency in the free resonant mode and peak value of the primary resonant current were collected. Imitating the wireless power supply for kitchen appliances, simulation and experimental results with the full-bridge SS-type voltage-fed IPT system have shown that this transient load detection model is accurate and reliable.

Hybrid Voltage and Current Control Approach for DG-Grid Interfacing Converters With LCL filters

Abstract: This paper presents a hybrid voltage and current control method to improve the performance of interfacing converters in distributed generation (DG) units. In general, current-controlled methods have been widely adopted in grid-connected converters nowadays. Nevertheless, in an islanded system, the voltage control of DG units is desired to provide direct voltage support to the loads. Due to the absence of closed-loop line current controller, the voltage control scheme can hardly regulate the DG unit's line current harmonics. Furthermore, if not addressed properly, the transfer between the grid-connected operation and autonomous islanding operation will introduce nontrivial transient currents. To overcome the drawbacks of voltage- and current-controlled DG units, this paper develops a hybrid voltage and current control method (HCM). The proposed method allows the coordinated closed-loop control of the DG unit fundamental voltage and line harmonic currents. With the HCM, local harmonic loads of the DG unit can even be compensated without using harmonic current extraction. In addition, the HCM guarantees smooth transition during the grid-connected/islanding operation mode transfer. Simulated and experimental results are provided to verify the feasibility of the proposed approach.

Composite Nonlinear Feedback Control and Stability Analysis of a Grid-Connected Voltage Source Inverter With $LCL$ Filter

Abstract: In this paper, the stability analysis of a grid-connected voltage source inverter with an LCL filter is presented The Poincare map is used to analyze the stability of the system from a geometric point of view and provides a better understanding of the system performance In addition, a composite nonlinear feedback (CNF) control is proposed in order to improve the transient and steady-state performance of the control system Simulation and experimental analysis verify the validity of the analysis and also show the superiority of the CNF-based controller compared to the conventional proportional-resonant controller during transients and steady-state operation

Fault Ride-Through of a DFIG Wind Turbine Using a Dynamic Voltage Restorer during Symmetrical and Asymmetrical Grid Faults

Abstract: This paper introduces a new solution for doubly fed induction generators to stay connected to the grid during Faults. The main idea is to increase the stator voltage to a level that creates the required flux to keep the rotor side converter current below its transient rating. To accomplish this goal, a series compensator is added to inject voltage in series to the stator side line. The Dynamic Voltage Restorer can compensate the faulty line voltage while the DFIG wind turbine can continue its nominal operation as demanded in actual grid codes. Fuzzy Logic controller is used as a controller in order to control the dc link voltages and to reduce the harmonics. Simulation results for a 2 MW wind turbine are presented, especially for asymmetrical grid faults. They show the effectiveness of the DVR in comparison to the low- voltage ride-through of the DFIG using a crowbar that does not allow continuous reactive power production.

Wavelet-based Burst Event Detection and Localization in Water Distribution Systems

Abstract: In this paper we present techniques for detecting and locating transient pipe burst events in water distribution systems. The proposed method uses multiscale wavelet analysis of high rate pressure data recorded to detect transient events. Both wavelet coefficients and Lipschitz exponents provide additional information about the nature of the signal feature detected and can be used for feature classification. A local search method is proposed to estimate accurately the arrival time of the pressure transient associated with a pipe burst event. We also propose a graph-based localization algorithm which uses the arrival times of the pressure transient at different measurement points within the water distribution system to determine the actual location (or source) of the pipe burst. The detection and localization performance of these algorithms is validated through leak-off experiments performed on the [email protected] wireless sensor network test bed, deployed on the drinking water distribution system in Singapore. Based on these experiments, the average localization error is 37.5 m. We also present a systematic analysis of the sources of localization error and show that even with significant errors in wave speed estimation and time synchronization the localization error is around 56 m.

Feedforward Transient Compensation Control for DFIG Wind Turbines During Both Balanced and Unbalanced Grid Disturbances

Abstract: A feedforward transient compensation (FFTC) control scheme with proportional-integral-resonant current regulators is proposed to enhance the low-voltage ride through (LVRT) capability of doubly fed induction generators (DFIGs) during both balanced and unbalanced grid faults. Compensation for the DFIG stator transient voltage is feedforward injected into both the inner current control loop and the outer power control loop. The FFTC current controller improves the transient rotor-current control capability and minimizes the DFIG control interruptions during both balanced and unbalanced grid faults. Without the need of sequence-component decomposition, the torque ripple is reduced by injecting 60- and 120-Hz rotor-current components during unbalanced stator voltage conditions. The proposed FFTC control introduces minimal additional complexity to a regular DFIG vector-control scheme and shows promising enhancements in the LVRT capability of DFIGs. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed FFTC control scheme.

Tolerance-band modulation methods for modular multilevel converters

Abstract: Modular multilevel converters (M2C) are increasingly used in high voltage direct current (HVDC) systems. The efficiency of M2Cs is highly related to the modulation method which determines the switching frequency and capacitor voltage ripple in the converter station. A new approach to modulation of M2C is presented in this paper. Tolerance-band methods are employed to obtain switching instants, and also cell selection. The proposed methods overcome the modulation problem for converters with few numbers of cells and also reduce the sorting efforts for cell balancing purposes while maintaining the cell-capacitor voltage limits. The evaluation is done by time-domain simulation by which the performance of each method is studied in both steady-state and transient cases. It is observed that using tolerance band methods not only reduces the switching frequency but also allows for handling severe fault cases in a grid connected system. Use of this method can reduce the switching losses and also allow for reduction of the cell capacitor size.

Optimal Control of Medium-Voltage Drives—An Overview

Abstract: Carrier modulation is the most common method for inverter control of ac drive systems Operation at switching frequencies of several kilohertz is customary to restrain the harmonic distortion of the motor currents Lower switching frequency is preferred for medium-voltage drives, owing to the higher switching losses of the semiconductor devices This calls for optimizing the performance of the pulsewidth modulator Improvements are achieved by abolishing carrier modulation with its equidistant time spacing of the voltage pulses Optimal pulse patterns can be precalculated for every steady-state operating point of the drive The patterns are stored in a memory of the drive system, from which they are retrieved and used for inverter control Control at transient operation is achieved by adapting the optimal steady-state pulse patterns to the respective situation Specific trajectories of the motor current space vector are then created by online optimization An alternative method of online optimization relies on predicting the space vector trajectories for the next possible inverter switching states The switching state that leads to minimum switching frequency is then selected Optimal pulsewidth modulation reduces harmonic distortion It permits operation at very low switching frequency and reduces the switching losses This increases the current-carrying capability of the semiconductor devices to the extent that the power rating of an inverter approximately doubles

Thermal Analysis and Cooling System Design of Dual Mechanical Port Machine for Wind Power Application

Abstract: Dual mechanical port (DMP) machine, a kind of electrical continuous variable transmission, is a very competitive alternative for vulnerable constant speed-ratio mechanical gearbox in wind power application and hybrid electrical vehicles. As the inner wound rotor is surrounded by the stator and the permanent magnet (PM) outer rotor, the DMP machine suffers from possible PM demagnetization and insulation failure under severe thermal condition. In this paper, thermal analysis and cooling system design of a DMP prototype machine are investigated. To predict the heat sources in actual operation, a transient cosimulation method is presented. Thermal parameters and the flow distribution in the cooling ducts are calculated in detail. Finite-element analysis of the thermal field is carried out to obtain the temperature distribution and two typical thermal contacts are considered. A robust fully forced-air cooling system with inner rotor teeth ducts is proposed for the DMP machine and a 10-kW DMP machine is prototyped. Experimental results at different working conditions are given to verify the theoretical analysis. Both simulation and experimental results reveal that the numerical approach and the proposed cooling system are not only more accurate and reliable, but can be referred to other electrical machine cooling system design.

Electrical switching dynamics and broadband microwave characteristics of VO2 RF devices

Abstract: Vanadium dioxide is a correlated electron system that features a metal-insulator phase transition (MIT) above room temperature and is of interest in high speed switching devices. Here, we integrate VO2 into two-terminal coplanar waveguides and demonstrate a large resistance modulation of the same magnitude (>10^3) in both electrically (i.e. by bias voltage, referred to as E-MIT) and thermally (T-MIT) driven transitions. We examine transient switching characteristics of the E-MIT and observe two distinguishable time scales for switching. We find an abrupt jump in conductivity with a rise time of the order of 10 ns followed by an oscillatory damping to steady state on the order of several {\mu}s. We characterize the RF power response in the On state and find that high RF input power drives VO2 further into the metallic phase, indicating that electromagnetic radiation-switching of the phase transition may be possible. We measure S-parameter RF properties up to 13.5 GHz. Insertion loss is markedly flat at 2.95 dB across the frequency range in the On state and sufficient isolation of over 25 dB is observed in the Off state. We are able to simulate the RF response accurately using both lumped element and 3D electromagnetic models. Extrapolation of our results suggests that optimizing device geometry can reduce insertion loss further and maintain broadband flatness up to 40 GHz.

Adaptive Power Capture Control of Variable-Speed Wind Energy Conversion Systems With Guaranteed Transient and Steady-State Performance

Abstract: This paper deals with the power capture control of variable-speed wind energy conversion systems. The control objective is to optimize the capture of wind energy by tracking the desired power output. Arbitrary steady-state performance is achieved in the sense that the tracking error is guaranteed to converge to any predefined small set. In addition, to maximize the wind energy capture, transient performance is enhanced such that the convergence rate can be larger than an arbitrary value, which further limits the maximum overshoot. First, an adaptive controller is designed for the case where known aerodynamic torque is assumed. Then, by utilizing an online approximator to estimate the uncertain aerodynamics, the need for the exact knowledge of the aerodynamic torque is waived to imitate the practical experience. With the aid of a novel output error transformation technique, both of the proposed controllers are capable of shaping the system performance arbitrarily on transient and steady-state stages. Meanwhile, it is also proved that all the signals in the closed-loop system are bounded via Lyapunov synthesis. Finally, the feasibility of the proposed controllers is demonstrated on an 1.5-MW three-blade wind turbine using the FAST (Fatigue, Aerodynamics, Structures, and Turbulence) code developed by the National Renewable Energy Laboratory.

Electrical switching dynamics and broadband microwave characteristics of VO2 radio frequency devices

Abstract: Vanadium dioxide (VO2) is a correlated electron system that features a metal-insulator phase transition (MIT) above room temperature and is of interest in high speed switching devices. Here, we integrate VO2 into two-terminal coplanar waveguides and demonstrate a large resistance modulation of the same magnitude (>103) in both electrically (i.e., by bias voltage, referred to as E-MIT) and thermally (T-MIT) driven transitions. We examine transient switching characteristics of the E-MIT and observe two distinguishable time scales for switching. We find an abrupt jump in conductivity with a rise time of the order of 10 ns followed by an oscillatory damping to steady state on the order of several μs. We characterize the RF power response in the On state and find that high RF input power drives VO2 further into the metallic phase, indicating that electromagnetic radiation-switching of the phase transition may be possible. We measure S-parameter RF properties up to 13.5 GHz. Insertion loss is markedly flat at 2.95 dB across the frequency range in the On state, and sufficient isolation of over 25 dB is observed in the Off state. We are able to simulate the RF response accurately using both lumped element and 3D electromagnetic models. Extrapolation of our results suggests that optimizing device geometry can reduce insertion loss further and maintain broadband flatness up to 40 GHz.

A Traveling-Wave Detection Method Based on Park's Transformation for Fault Locators

Abstract: A novel algorithm for transient detection based on Park's transformation is proposed. It is very simple, self-adapts to electrical noise and phase imbalances, and is appropriate to be used in connection with traveling-wave fault location (TWFL) methods. The proposed technique is evaluated through Electromagnetic Transients Program simulations. A 230-kV transmission system case study is carried out, in which power system voltage and current waveforms are picked up one sample at a time. It is shown that the method is very reliable and suitable for multiterminal TWFL algorithms.

Dynamic Analysis and Control for Resonant Currents in a Zone-Control Induction Heating System

Abstract: This paper presents a quick and accurate power control method for a zone-control induction heating (ZCIH) system consisting of multiple working coils connected to multiple H-bridge inverters. A uniform temperature profile can be achieved by adjusting the current in each working coil. This paper proposes a new current control method based on a circuit model using real and imaginary (Re-Im) current/voltage components. The method detects and controls the Re-Im components of the coil current instead of the current amplitude and phase angle. As a result, the proposed method enables the inverters to control the coil current independently from the others. Experiments using a six-coil ZCIH system are conducted to verify the validity of the proposed method. The experimental results confirmed that the proposed method makes it possible to improve the stability of the current feedback control, not only in steady states but also in transient states.

General Methodology for Analysis of Sub-Synchronous Interaction in Wind Power Plants

Abstract: This paper presents a general methodology for analysis of sub-synchronous interaction in wind power plants. These include appropriate frequency scanning method for the assessment of the sub-synchronous control interaction, and calculation of the electrical damping provided by the wind turbine generator for investigation of the sub-synchronous torsional interaction. A general formulation of both methods applicable to any given wind turbine and network is presented. A dynamic frequency scanning method for the turbine side is developed which takes account of the turbine nonlinearities and its active behavior. Various aspects that need to be considered when injecting a voltage or current signal into the system for dynamic frequency scanning are discussed in detail. The veracity of these methods is confirmed against electromagnetic transient analyses. The application of these tools and techniques is demonstrated on a practical power system comprising type 3 wind turbines and series compensated lines.

Frequency domain analysis of pipe fluid transient behaviour

Abstract: Pipe transient signals are hyperbolic in nature where key features of the signal repeat periodically and are well suited to the analysis in the frequency domain. For this reason, a number of studies have been conducted on the use of frequency domain approaches for a variety of purposes, from fault detection to the prediction of the unsteady system response. Despite the number of papers on the topic over the past decades, there are no detailed review of the developments in the frequency domain analysis of pipe transient signals. This paper provides an assessment and review of the relevant research and provides a critical discussion of both the strengths and weaknesses of this approach. A method for extracting a system's frequency response function using conventional valve closure signals is proposed and the influence of various faults, friction and pipe wall viscoelasticity on this response function are compared with the corresponding impacts in the time domain. This study shows that most changes o...

Parameters Identification and Modeling of High-Frequency Current Transducer for Partial Discharge Measurements

Abstract: Rogowski coil (RC) is a low-cost, air-cored, and flexible induction sensor for nonintrusive condition monitoring and thus can be used in a variety of applications. In this paper, a lumped parameter model of RC is presented and an experiment-based methodology is developed to determine its parameters. The performance of the RC is analyzed for detection and measurement of high-frequency (pulsed) signals such as partial discharge (PD) current pulses. A simple and efficient technique of numerical integration is adopted to avoid the conventional type of expensive and complex design analogue integrators. RC is modeled and simulated in the alternative transient program-electromagnetic transient program environment. The designed coil is tested to measure PDs in the laboratory. Simulated and experimental performance of RC is compared with a high-frequency current transformer. This comparison shows a good match and, hence, validates the design of RC for PD applications.

Transient analysis of Bernoulli serial lines: performance evaluation and system-theoretic properties

Abstract: Transient behavior of production systems has significant practical and theoretical implications. However, analytical methods for analysis and control of production systems during transients remain largely unexplored. In the framework of serial production lines with Bernoulli machines and finite buffers, this article develops a mathematical model for transient analysis and derives closed-form expressions for evaluating the production rate, consumption rate, work-in-process, and probabilities of machine starvation and blockage during transients. In addition, a computationally efficient procedure based on recursive aggregation is developed to approximate the transient performance measures with high accuracy. Finally, based on the mathematical model derived, system-theoretic properties of several important system transient characteristics are studied.