Showing papers on "Transient (oscillation) published in 2014"

A Machine Learning and Wavelet-Based Fault Location Method for Hybrid Transmission Lines

Abstract: This paper presents a single-ended traveling wave-based fault location method for a hybrid transmission line: an overhead line combined with an underground cable. Discrete wavelet transformation (DWT) is used to extract transient information from the measured voltages. Support vector machine (SVM) classifiers are utilized to identify the faulty-section and faulty-half. Bewley diagrams are observed for the traveling wave patterns and the wavelet coefficients of the aerial mode voltage are used to locate the fault. The transient simulation for different fault types and locations are obtained by ATP using frequency-dependent line and cable models. MATLAB is used to process the simulated transients and apply the proposed method. The performance of the method is tested for different fault inception angles (FIA), different fault resistances, non-linear high impedance faults (NLHIF), and non-ideal faults with satisfactory results. The impact of cable aging on the proposed method accuracy is also investigated.

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Abstract: Charge generation in organic solar cells is a fundamental yet heavily debated issue. This article gives a balanced review of different mechanisms proposed to explain efficient charge generation in polymer–fullerene bulk-heterojunction solar cells. We discuss the effect of charge-transfer states, excess energy, external electric field, temperature, disorder of the materials, and delocalisation of the charge carriers on charge generation. Although a general consensus has not been reached yet, recent findings, based on both steady-state and transient measurements, have significantly advanced our understanding of this process.

A Nine-Level Grid-Connected Converter Topology for Single-Phase Transformerless PV Systems

Abstract: This paper presents a single-phase transformerless grid-connected photovoltaic converter based on two cascaded full bridges with different dc-link voltages. The converter can synthesize up to nine voltage levels with a single dc bus, since one of the full bridges is supplied by a flying capacitor. The multilevel output reduces harmonic distortion and electromagnetic interference. A suitable switching strategy is employed to regulate the flying-capacitor voltage, improve the efficiency (most devices switch at the grid frequency), and minimize the common-mode leakage current with the help of a novel dedicated circuit (transient circuit). Simulations and experiments confirm the feasibility and good performance of the proposed converter.

Broadband energy harvesting using acoustic black hole structural tailoring

Abstract: This paper explores the concept of an acoustic black hole (ABH) as a main design framework for performing dynamic structural tailoring of mechanical systems for vibration energy harvesting applications. The ABH is an integral feature embedded in the host structure that allows for a smooth reduction of the phase velocity, theoretically approaching zero, while minimizing the reflected energy. This mechanism results in structural areas with high energy density that can be effectively exploited to develop enhanced vibration-based energy harvesting. Fully coupled electro-mechanical models of an ABH tapered structure with surface mounted piezo-transducers are developed to numerically simulate the response of the system to both steady state and transient excitations. The design performances are numerically evaluated using structural intensity data as well as the instantaneous voltage/power and energy output produced by the piezo-transducer network. Results show that the dynamically tailored structural design enables a drastic increase in the harvested energy as compared to traditional structures, both under steady state and transient excitation conditions.

State Detection of Bond Wires in IGBT Modules Using Eddy Current Pulsed Thermography

Abstract: Insulated gate bipolar transistor (IGBT) modules are important safety critical components in electrical power systems. Bond wire lift-off, a plastic deformation between wire bond and adjacent layers of a device caused by repeated power/thermal cycles, is the most common failure mechanism in IGBT modules. For the early detection and characterization of such failures, it is important to constantly detect or monitor the health state of IGBT modules, and the state of bond wires in particular. This paper introduces eddy current pulsed thermography (ECPT), a nondestructive evaluation technique, for the state detection and characterization of bond wire lift-off in IGBT modules. After the introduction of the experimental ECPT system, numerical simulation work is reported. The presented simulations are based on the 3-D electromagnetic-thermal coupling finite-element method and analyze transient temperature distribution within the bond wires. This paper illustrates the thermal patterns of bond wires using inductive heating with different wire statuses (lifted-off or well bonded) under two excitation conditions: nonuniform and uniform magnetic field excitations. Experimental results show that uniform excitation of healthy bonding wires, using a Helmholtz coil, provides the same eddy currents on each, while different eddy currents are seen on faulty wires. Both experimental and numerical results show that ECPT can be used for the detection and characterization of bond wires in power semiconductors through the analysis of the transient heating patterns of the wires. The main impact of this paper is that it is the first time electromagnetic induction thermography, so-called ECPT, has been employed on power/electronic devices. Because of its capability of contactless inspection of multiple wires in a single pass, and as such it opens a wide field of investigation in power/electronic devices for failure detection, performance characterization, and health monitoring.

25th Anniversary Article: Materials for High‐Performance Biodegradable Semiconductor Devices

Abstract: We review recent progress in a class of silicon-based electronics that is capable of complete, controlled dissolution when immersed in water or bio-fluids. This type of technology, referred to in a broader sense as transient electronics, has potential applications in resorbable biomedical devices, eco-friendly electronics, environmental sensors, secure hardware systems and others. New results reported here include studies of the kinetics of hydrolysis of nanomembranes of single crystalline silicon in bio-fluids and aqueous solutions at various pH levels and temperatures. Evaluations of toxicity using live animal models and test coupons of transient electronic materials provide some evidence of their biocompatibility, thereby suggesting potential for use in bioresorbable electronic implants.

Electromechanical Transient Modeling of Modular Multilevel Converter Based Multi-Terminal HVDC Systems

Abstract: This paper studies the techniques for modeling modular multilevel converter (MMC) based multi-terminal HVDC (MTDC) systems in the electromechanical transient mode. Firstly, the mathematical model of the MMC and its corresponding equivalent circuit are established, which are similar to those of the two level converters. Then, a power flow calculation method for AC/DC systems containing MMC-MTDC systems is developed. Two dynamic models for MMC-MTDC systems are developed in the paper. One is the detailed model, taking into account of the AC side circuit, the inner controllers, the modulation strategies, the outer controllers and the MTDC circuit. The other is the simplified model, which only reserves the outer controllers and partial dynamics of the MTDC circuit based on a quantitative analysis of the detailed model's dynamic processes, and it can be used in electromechanical transient simulation with a larger step size. Both the detailed and the simplified models are implemented on PSS/E and compared with the accurate electromagnetic transient models on PSCAD in a four terminal MMC-MTDC system; the result proves the validity of the developed models. Lastly, a stability study of a modified New England 39-bus system is executed, and the result shows that the AC fault can be isolated well in an MMC-MTDC asynchronously connected AC grid.

Impacts of Three MMC-HVDC Configurations on AC System Stability Under DC Line Faults

Abstract: This paper analyzes the dc line fault transient stability characteristics of AC/DC power systems with three different modular multilevel converter based high voltage direct current (MMC-HVDC) configurations. The first configuration is half bridge sub-module based MMC (H-MMC) HVDC configuration, which clears dc line faults by tripping the ac circuit breakers. The clamp double sub-module (CDSM) based MMC (C-MMC) HVDC configuration with dc line fault clearance ability constitutes the second configuration. A line-commutated converter (LCC) and MMC hybrid HVDC configuration with dc line fault clearance ability, named LCC-diode-MMC (LCC-D-MMC), is the third configuration. The detailed processes of clearing dc line faults in three MMC-HVDC configurations are analyzed. The equal area criterion is utilized to analyze the dc line fault transient processes. Besides, to evaluate the power system transient stability characteristics in three MMC-HVDC configurations, an index, called critical ac transmitted power (P ac_critical ), is proposed. Under the same dc line to ground fault, transient stability characteristics of three test systems based on the three different MMC-HVDC configurations are compared. Excellent performance of the LCC-D-MMC HVDC configuration under the dc line fault is demonstrated through comparison of P ac_critical in the three test systems. Finally, the study is extended to a modified New England 39-bus system, and the simulation results also correspond with the theoretical analysis.

Natural Frequency-Based Line Fault Location in HVDC Lines

Abstract: To predict the location of dc line-faults in an HVDC system, a novel algorithm based on traveling-wave natural frequency using 10-ms current data from the single end is proposed in this paper. The relationship among the natural frequency of traveling wave, fault distance, and reflect coefficient at the terminal of the transmission line is presented. The multiple signal classification method is employed to extract the dominant natural frequency through the spectrum analysis of the traveling wave. The traveling-wave velocity and reflect coefficient under the dominant natural frequency are calculated to identify the fault location. Performance of the proposed fault-location algorithm is studied through detailed simulations carried out by using the electromagnetic transient simulation software PSCAD/EMTDC. The method does not need to identify the surge arrival time and it is more robust compared to the traditional traveling-wave-based fault-location methods. The simulation results have shown that the proposed method is valid and can locate the faults occurring on HVDC transmission lines accurately.

Study of Physically Transient Insulating Materials as a Potential Platform for Transient Electronics and Bioelectronics

Abstract: Acar, H., Cinar, S., Thunga, M., Kessler, M. R., Hashemi, N. and Montazami, R. (2014), Study of Physically Transient Insulating Materials as a Potential Platform for Transient Electronics and Bioelectronics. Adv. Funct. Mater., 24: 4135–4143. doi: 10.1002/adfm.201304186

An Optimized Phase-Shift Modulation For Fast Transient Response in a Dual-Active-Bridge Converter

Abstract: In this letter, two different phase-shift modulation used for the transient response of a dual-active-bridge dc/dc converter and their effects on the dynamics of the converter are discussed and compared It is found that the settling time for both cases depends on the value of the equivalent series resistance of the power inductor To minimize the transient time, a modified asymmetric double-side modulation is proposed, which enables the converter to transfer smoothly from one steady state to another one regardless of the equivalent series resistance of the power inductor Experimental results of the dynamics of the converter using different transient modulation techniques are also included for the purpose of validation

Specific emitter identification based on Hilbert-Huang transform-based time-frequency-energy distribution features

Abstract: A novel specific emitter identification method based on transient communication signal's time-frequency-energy distribution obtained by Hilbert-Huang transform (HHT) is proposed. The transient starting point is detected using the phase-based method and the transient endpoint is detected using a self-adaptive threshold based on the HHT-based energy trajectory. Thirteen features that represent both overall and subtle transient characteristics are proposed to form a radio frequency (RF) fingerprint. The principal component analysis method is used to reduce the dimension of the feature vector and a support vector machine is used for classification. A signal acquisition system is designed to capture the signals from eight mobile phones to test the performance of the proposed method. Experimental results demonstrate that the method is effective and the proposed RF fingerprint can represent more subtle characteristics than the RF fingerprints based on instantaneous amplitude, phase, frequency and energy envelope. This method can be equally applicable for any wireless emitter to enhance the security of the wireless networks.

Sharing Transient Loads : Causes of Unequal Transient Load Sharing in Islanded Microgrid Operation

Abstract: When inverters in voltage control mode are paired with synchronous generators (SGs), they exhibit poor transient load sharing in islanded operation. It is well known that power electronics respond more quickly where the inverter initially picks up the majority of every load step, and the generator's output slowly increases until they reach their steady-state load sharing given by droop settings. This excessive transient power output from the inverter constrains its rating relative to the largest load step and could negatively impact the battery life in battery energy-storage inverters. This article provides a detailed examination of the differences between the frequency-regulation characteristics of inverters and generators to explain, in a novel way, the cause of the poor transient power sharing. The tradeoff between an improved transient load sharing and increased voltage and frequency transients is highlighted, and it is demonstrated that equal transient load sharing can be achieved by emulating an SG using a power hardware-in-the-loop (HIL) approach. Validation is provided by simulation and experimental results.

New Repetitive Control With Improved Steady-State Performance and Accelerated Transient

Abstract: In repetitive control (RC), the enhanced servo performance at the fundamental frequency and its higher order harmonics is usually followed by undesired error amplifications at other frequencies. In this paper, we discuss a new structural configuration of the internal model in RC, wherein designers have more flexibility in the repetitive loop-shaping design, and the amplification of nonrepetitive errors can be largely reduced. Compared to conventional RC, the proposed scheme is especially advantageous when the repetitive task is subject to large amounts of nonperiodic disturbances. An additional benefit is that the transient response of this plug-in RC can be easily controlled, leading to an accelerated transient with reduced overshoots. Verification of the algorithm is provided by simulation of a benchmark regulation problem in hard disk drives, and by tracking-control experiments on a laboratory testbed of an industrial wafer scanner.

Compositional Transient Stability Analysis of Multimachine Power Networks

Abstract: During the normal operation of a power system, all the voltages and currents are sinusoids with a frequency of 60 Hz in America and parts of Asia or of 50 Hz in the rest of the world. Forcing all the currents and voltages to be sinusoids with the right frequency is one of the most important problems in power systems. This problem is known as the transient stability problem in the power systems literature. The classical models used to study transient stability are based on several implicit assumptions that are violated when transients occur. One such assumption is the use of phasors to study transients. While phasors require sinusoidal waveforms to be well defined, there is no guarantee that waveforms will remain sinusoidal during transients. In this paper, we use energy-based models derived from first principles that are not subject to hard-to-justify classical assumptions. In addition to eliminate assumptions that are known not to hold during transient stages, we derive intuitive conditions ensuring the transient stability of power systems with lossy transmission lines. Furthermore, the conditions for transient stability are compositional in the sense that one infers transient stability of a large power system by checking simple conditions for individual generators.

Modular multilevel converter models for electromagnetic transients

Abstract: Modular multilevel converters (MMCs) may contain numerous IGBTs. The modeling of such converters for electromagnetic transient type (EMT-type) simulations is complex. Detailed models used in MMC-HVDC simulations may require very large computing times. Simplified and averaged models have been proposed in the past to overcome this problem. In this paper existing averaged and simplified models are improved in order to increase their range of applications. The models are compared and analyzed for different transient events on a MMC-HVDC system.

Investigation of the Rotor Demagnetization Characteristics of Interior PM Synchronous Machines During Fault Conditions

Abstract: This paper investigates the transient magnetic behavior of an interior permanent-magnet (IPM) synchronous machine following a large transient current caused by a fault condition. A combination of finite-element (FE) analysis and numerical integration of the machine equations is used to calculate the currents resulting from a symmetrical three-phase short-circuit fault at the machine terminals, including magnetic saturation effects. FE analysis is further used to simulate the magnetic behavior of the machine when subjected to the fault currents, with a particular focus on the propagation of the demagnetizing MMF throughout the conductive laminations as well as the conductive PM material. Eddy currents induced in the laminations and magnet material are shown to impede the progression of the demagnetizing MMF. However, the transient analysis illustrates the manner in which the fault currents ultimately trigger the onset of irreversible demagnetization of significant portions of the rotor magnets in the analyzed machine. The transient demagnetization analysis method is extended to investigate the influence of several key design variables, including machine operating temperature and segmentation of the rotor magnets.

A Grid-Level High-Power BTB (Back-To-Back) System Using Modular Multilevel Cascade Converters WithoutCommon DC-Link Capacitor

Abstract: This paper provides an intensive discussion on analysis, simulation, and experiment of a back-to-back (BTB) system unifying two modular multilevel cascade converters based on double-star chopper cells (MMCCs-DSCCs). Each of the two DSCCs connected back-to-back consists of multiple cascaded chopper cells and a center-tapped inductor per leg. Low voltage steps bring significant reductions in harmonic voltage and current to the BTB system. Neither dc-link capacitor nor voltage sensor is required for regulating the dc-link voltage and controlling the dc-link current. A three-phase 200-V, 10-kW, 50-Hz downscaled BTB system with phase-shifted PWM is designed, constructed, and tested to verify its operating principles and performance. Analytical, simulated, and experimental results agree well with each other in steady and transient states. Experimental waveforms confirm the effectiveness of a self-starting/restarting procedure.

Electromechanical transient modeling of modular multilevel converter based multi-terminal HVDC systems

Abstract: This paper studies the techniques for modeling modular multilevel converter (MMC) based multi-terminal HVDC (MTDC) systems in the electromechanical transient mode. Firstly, the mathematical model of the MMC and its corresponding equivalent circuit are established, which are similar to those of the two level converters. Then, a power flow calculation method for AC/DC systems containing MMC-MTDC systems is developed. Two dynamic models for MMC-MTDC systems are developed in the paper. One is the detailed model, taking into account of the AC side circuit, the inner controllers, the modulation strategies, the outer controllers and the MTDC circuit. The other is the simplified model, which only reserves the outer controllers and partial dynamics of the MTDC circuit based on a quantitative analysis of the detailed model's dynamic processes, and it can be used in electromechanical transient simulation with a larger step size. Both the detailed and the simplified models are implemented on PSS/E and compared with the accurate electromagnetic transient models on PSCAD in a four terminal MMC-MTDC system; the result proves the validity of the developed models. Lastly, a stability study of a modified New England 39-bus system is executed, and the result shows that the AC fault can be isolated well in an MMC-MTDC asynchronously connected AC grid.

Small-Signal Analysis and Optimal Design of External Ramp for Constant On-Time V $^{\bf 2}$ Control With Multilayer Ceramic Caps

Abstract: Recently, constant on-time V2 control, and its variety named constant on-time control, or constant on-time ripple-based control is more and more popular in industry products due to features of high light-load efficiency, simple implementation, and fast transient response. However, subharmonic oscillation occurs when using multilayer ceramic caps due to the lagging phase of the capacitor voltage relative to the inductor current. External ramp compensation is one simple solution to solve the instability issue. However, the characteristics of constant on-time V2 control with external ramp are not fully understood and no explicit design guidelines for the external ramp are provided. This paper investigates the small-signal characteristics of constant on-time V2 control with external ramp compensation by providing a factorized, easy-to-use small-signal model. The external ramp is a critical parameter because it directly affects the position and damping of two pairs of double poles. Based on this fact, design guidelines of the external ramp for optimal dynamic performance are provided. Moreover, the effect of duty cycle is investigated. Finally, the small-signal experimental results and load transient performance are presented to verify the small-signal analysis and proposed design guideline.

Decentralized Control of Large-Scale Storage-Based Renewable Energy Systems

Abstract: Intermittent nature of renewable energies and random nature of moving loads negatively influence the power system transient stability. Energy storage can enhance the system response by providing short-term energy sources. However, fast dynamic storage systems require advanced controllers to stabilize the transients. This paper introduces the application of a decentralized overlapping decomposition controller in transient stability of renewable energy penetrated storage-based power systems. In a battery-based power system there exists strongly coupled state variables shared among areas. Large-scale controllers can decouple the system state variables and stabilize the oscillations. Here, the model of battery storage is obtained considering the battery's state of charge and the droop in power electronic interfaces. The overlapping decomposition decentralized controller demonstrates an enhanced transient stability performance under variable state of charge in various capacity of storage devices.

Stability of power grids: An overview

Abstract: Transient stability and steady-state (small signal) stability in power girds are reviewed. Transient stability concepts are illustrated with simple examples; in particular, we consider three methods for computing region of attraction: time-simulations, extended Lyapunov function, and sum of squares optimization method. We discuss steady state stability in power systems, and present an example of a feedback control via a communication network for the 10 Unit 39 Bus New England Test system.

A comprehensive cell capacitor energy control strategy of a modular multilevel converter (MMC) without a stiff DC bus voltage source

Abstract: Cell capacitor energy control of a Modular Multilevel Converter (MMC) is conventionally done by controlling leg current and modulation strategy. In most of literatures, leg current transient is analyzed under an assumption that the DC bus is a stiff DC voltage source. In a real MMC-based HVDC transmission system, however, there's no such virtual stiff DC voltage source and the conventional regulation method can lead to poor dynamics of cell capacitor energy control and even make system unstable. In this paper, the MMC model is revised for circulating current transient analysis. Based on the revised model, a new comprehensive cell capacitor energy control strategy is proposed by updating leg capacitor energy reference on-line and injecting positive and negative sequence circulating currents. Validity of the proposed method is verified by a 7-level scaled version prototype experimental setup.

Simulation of the load rejection transient process of a francis turbine by using a 1-D-3-D coupling approach

Abstract: This paper presents the simulation and the analysis of the transient process of a Francis turbine during the load rejection by employing a one-dimensional and three-dimensional (1-D-3-D) coupling approach. The coupling is realized by partly overlapping the 1-D and 3-D parts, the water hammer wave is modeled by defining the pressure dependent density, and the guide vane closure is treated by a dynamic mesh method. To verify the results of the coupling approach, the transient parameters for both typical models and a real power station are compared with the data obtained by the 1-D approach, and good agreements are found. To investigate the differences between the transient and steady states at the corresponding operating parameters, the flow characteristics inside a turbine of the real power station are simulated by both transient and steady methods, and the results are analyzed in details. Our analysis suggests that there are just a little differences in the turbine outer characteristics, thus the traditional 1-D method is in general acceptable. However, the flow patterns in the spiral casing, the draft tube, and the runner passages are quite different: the transient situation has obvious water hammer waves, the water inertia, and some other effects. These may be crucial for the draft tube pulsation and need further studies.