Transient AC Research Articles

Transient AC Overvoltage Suppression Orientated Reactive Power Control of the Wind Turbine in the LCC-HVDC Sending Grid

Transient AC Overvoltage Suppression Orientated Reactive Power Control of the Wind Turbine in the LCC-HVDC Sending Grid

Strategy for Suppressing Commutation Failures in High-Voltage Direct Current Inverter Station Based on Transient Overvoltage

Commutation failures in high-voltage direct current (HVDC) transmission systems often occur within inverter stations, posing challenges to the safe and consistent operation of HVDC transmission projects. This paper examines data gathered from a wide-spectrum transient electromagnetic voltage monitoring apparatus. The findings uncover a notable correlation between transient AC bus voltage disturbances, stemming from faults, and the occurrence of commutation failures in HVDC transmission systems. Leveraging this correlation, we propose a commutation failure mitigation strategy for HVDC systems centered around AC bus voltage disturbances within inverter stations. Through fault validation simulations conducted across three HVDC transmission systems, our strategy demonstrates effective suppression of commutation failure incidents within HVDC transmission projects.

Formulation of a Mechanical Stress Dependent Macroscopic Magnetic Model for Incremental Permeability Simulation

Electromagnetic based systems receive ever-growing interest for on-line assessment of steel strip properties for major manufacturers, especially when Advanced High Strength Steel (AHSS) products for automotive purposes are considered. Manufacturing AHSS need optimal control of the thermo-mechanical processing using in-line sensors for real time monitoring. IMPOC, 3MA and PropertyMon are some of the well-known electromagnetic sensors. Identification of a mechanical or metallurgical state from a magnetic signal is of great interest, requiring however sophisticated models to mimic the real behaviour and its sensitivity to many influencing parameters. Our paper presents a proposition to add the dependency of mechanical stress to the Jiles-Atherton hysteresis model. Experimental data consist of B(H) curves of carbon steels strips subjected to different levels of mechanical stress applied along the magnetic measurement direction. The JilesAtherton model has then been used to find out a set of stress-dependent coefficients. The variation of coefficients with stress is in close relationship with the variations of coercive field, remanent induction and permeability with stress. A coupled magneto-mechanical approach allows physics-based functions to be proposed to describe the evolution of JilesAtherton coefficients with stress. In this paper, experimental working conditions will be described, as well as the stressdependent Jiles-Atherton model. The model is finally implemented in FEM software, allowing more complex magneto-mechanical situations to be modelled. After matching with measurement, the new model is applied on 3MA modelling where magnetic transient analysis and magnetic AC steady state analysis are linked through incremental permeability.

An Evaluation of Co-Simulation for Modeling Coupled Natural Gas and Electricity Networks

Reliance on natural gas for power generation has increased the coupling between gas and power networks. While this coupling can bring operational and economic benefits, it can also yield challenges, as the constraints in one system can impact the other. Co-simulation can capture the constraints and interactions between these systems, but so far, there has been limited comparison of co-simulation results to those of an integrated model. In this work, we develop a new co-simulation framework using the HELICS platform and the SAInt tool for modeling transient gas and AC optimal power flow. We evaluate this co-simulation framework against a fully integrated version of the SAInt power and gas simulators, thus providing a benchmarking of the co-simulation approach. We compare results across the two approaches for two test networks and a network representing the Belgian power and gas networks, testing both normal operating conditions and cases with compressor disruptions. In each of the cases tested, we find nearly identical results from the two approaches across various metrics of interest, such as nodal pressure, gas flow rates, and active power generation. This alignment suggests that co-simulation can yield comparable results to fully integrated models for modeling coupled gas and electricity networks.

Transient and steady-state faults location in intelligent distribution networks compensated with D-STATCOM using time-domain equations and distributed line model

Intelligent electric distribution networks compensated with D-STATCOM are active networks in which data of terminals is important for better distribution of energy. Similar to all electricity network devices, energy distribution lines in such networks are also subject to transient and steady-state AC faults, e.g., short circuit. Sections of the network in which the fault occurs are separated by relays. In such a condition, fault location and restoring the normal status of the network are important. Fault location in these networks is performed based on synchronous data of two terminals because data of all buses are transmitted to the main bus In this paper, a time-domain equation method (TDEM) is proposed for locating faults on smart distribution grid (SDG) using distributed line model based on time-domain equations. The proposed method locates the point of transient and steady-state fault in SDG considering DGs and compensators. In this paper, the D-STATCOM and wind power plant are considered in SDG. The suggested method does not need the D-STATCOM and DGs model for applying the algorithm. It only uses data of less than half of the cycle for executing the algorithm. The numerical simulation confirms that the proposed fault location method is accurate and it provides a reliable solution for automatic fault location problem in distribution networks.

Transient Stability Constrained Optimal Power Flow in Mixed AC/Multi‐Terminal VSC HVDC System

This paper proposes a hierarchical transient stability constrained optimal power flow (TSCOPF) analysis in a power system with a multi‐terminal voltage‐source converter (VSC) high‐voltage direct current (HVDC) system and large‐scale wind farms. This aims at determining the reinforcement and operation of the power system using the multi‐terminal VSC HVDC (MTDC) system. By the method, the wind farms can be installed without any transient instability issues. On the AC grid side, transient rotor‐angle stability and transient AC voltage stability are included as additional constraints. On top of that, the constraints of DC bus voltage fluctuations and DC transmission line current deviations on the DC grid side are also taken into account. Time‐domain simulation approach is applied to investigate these transient stability constraints. The MTDC system reinforcement is designed considering wind power transmission and economic operation of the system model. The influences of the transient stability constraints on the system operation costs with high and low wind power output patterns are discussed. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Numerical simulation of flux avalanches in type-II superconducting thin films under transient AC magnetic fields

Flux avalanches induced from thermomagnetic instability are crucial challenges for the application of superconducting thin film devices. In this paper, flux avalanches in a type-II superconducting thin film exposed to a transient AC magnetic field are numerically simulated by solving the coupled nonlinear Maxwell’s equations and the heat diffusion equation based on the fast Fourier transform (FFT) method. The dependence of the threshold magnetic field on the ambient temperature, film thickness and magnetic field ramp rate are obtained through these numerical simulations, which show good agreement with experimental results. A linear increase in the threshold field as the film thickness increases and a nonmonotonic increase in the threshold field as the ambient temperature increases have also been found. Our numerical results demonstrate that the threshold field decreases exponentially as the ramp rate increases. Flux avalanche patterns observed in magneto-optical imaging (MOI) measurements for a film exposed to an AC magnetic field are reproduced. The hysteresis magnetization and maximum temperature jump curves are also illustrated. We find that fingering instability plays an important role in the thermomagnetic response of superconducting thin films under transient AC magnetic fields, especially for high magnetic field ramp rates.

Microscopic mechanism of thermomagnetic instabilities in type-II superconducting thin films under AC magnetic fields

Thermomagnetic instability is one of the significant challenges for the application of superconducting devices. In this paper, the microscopic mechanism of thermomagnetic instability in superconducting films subjected to a transient AC magnetic field is numerically investigated by coupling the generalized time dependent Ginzburg–Landau equations and the heat diffusion equation. The influences of magnetic field ramp rate, ambient temperature, and nanometer-sized artificial pinning on the vortex matter are considered in our simulations. It has been found that vortex alignment and repulsion play significant roles in the branching of the penetration trajectories of the magnetic flux. Under fast ramping magnetic fields, the increase in the temperature and instability in the vortex matter are more significant. However, the rising temperature and jump size in the magnetization weaken as the ambient temperature increases. Pronounced hysteresis in the vortex dynamics has been found in the film subjected to AC magnetic fields. As the AC cycle proceeds, the vortex penetration process gets more unstable. We have also found that the nanometer sized pinning strongly modulates the penetration of vortices and the vortex matter is highly correlated with the lattice structure of the pinning sites. Our results provide new insights into vortex dynamics and give a mesoscopic understanding on the channeling and branching in the vortex penetration paths in superconductors under AC magnetic fields.

Guest‐Tunable Dielectric Sensing Using a Single Crystal of HKUST‐1

AbstractThere is rising interest on low‐k dielectric materials based on porous metal–organic frameworks (MOFs) for improved electrical insulation in microelectronics. Herein, the concept of MOF dielectric sensor built from a single crystal of HKUST‐1 is demonstrated. Guest encapsulation effects of polar and non‐polar molecules are studied, by monitoring the transient dielectric response and AC conductivity of the crystal exposed to different vapors (water, iodine, methanol, and ethanol). The dielectric properties are measured along the <100> crystal direction in the frequency range of 100 Hz to 2 MHz. The dielectric data show the efficacy of MOF dielectric sensor for discriminating the guest analytes. The time‐dependent transient response reveals dynamics of the molecular inclusion and exclusion processes in the nanoscale pores. Since dielectric response is ubiquitous to all MOF materials (unlike DC conductivity and fluorescence), the results demonstrate the potential of dielectric MOF sensors compared to resistive sensors and luminescence‐based approaches.

HVAC Corona Current Characteristics and Audible Noise During Rain

Corona Audible Noise is a major factor regarding the public acceptance of overhead lines. In this article, 2-conductor bundle is used to study the influence of rain on AC corona discharge. Rainfall is simulated by a large rain dispenser, which can be adjusted over a wide range. Using this experimental setup, the transient AC audible noise and corona current pulse characteristics under rainfall condition were studied. It was found that there are three different stages of audible noise level during rain, in which the audible noise level during the steady state stage is the highest. This transient characteristic could be explained by two different types of corona discharges for discharges above and below the conductor. Furthermore, the steady state audible noise level was studied under a wide range of rain rates and applied surface electric fields. It was found that the sensitivity of the audible noise level to the rain rate decreases with the applied electric field. This finding is of vital importance as the rain intensity is regarded as one of the most important factors influencing the HVAC audible noise, and a higher rain rate is usually considered to cause a higher audible noise level. A possible explanation of this phenomenon is attributed to the effect of a “critical number of corona sources”, which is well known from DC corona and possible evidence is provided based on long-exposure photos.

Novel protection scheme for high‐voltage direct‐current transmission lines based on one‐terminal transient AC voltage

This paper considers the distributed capacitance of transmission lines to research the protection of high-voltage direct-current (HVDC) transmission line. According to the superposition theorem, the post-fault superimposed network of HVDC system is used to analyse the characteristic of the root mean square (RMS) of specific frequency voltage (SFV) during the transient period of internal and external faults. At the parallel resonant frequency of DC filter, for internal faults, the fluctuation of the RMS of transient AC voltage is larger than that of external faults. Based on the fluctuation characteristic of single-terminal transient AC voltage, a novel protection scheme for HVDC transmission lines is proposed. Various simulation results show that the proposed protection is immune to fault resistance and performs well under different fault types.

Temperature Field Simulations of a ReBCO Pancake Coil Under Pulsed Overcurrent Conditions

Under pulsed overcurrent conditions, some of the superconducting layers inside a ReBCO pancake coil are quenched and very high ac losses are generated due to nonlinear quenching resistances. Transient ac loss of each ReBCO layer is modeled in this paper by using an electromagnetic-superconducting coupled finite-element-modeling (FEM) method. Temperature field distributions inside and around the whole ReBCO pancake coil caused by the distributed ac losses are further discussed to investigate the thermal operating characteristics under pulsed overcurrent conditions. For a ReBCO pancake coil having 20 coil turns, the maximum ac loss value locates in the sixth coil turn from inner wall to outer wall of the whole coil. By considering the solid-to-solid and solid-to-fluid heat transfer behaviors, local overheated point having the highest operating temperature appears inside the middle and lower part of the sixth coil turn. A current-dependent power function for the highest temperature estimation is also concluded to depict the nonlinear relations between the operating temperatures and currents inside the ReBCO pancake coil. These electromagnetic superconducting thermal coupled characteristics lay some technical bases to design and evaluate proper overcurrent and overheating schemes for various ReBCO pancake coils and magnets.

Parameter Extraction and Power/Performance Analysis of Monolithic 3-D Inverter (M3INV)

An equivalent circuit model of monolithic 3-D inverter (M3INV) considering the electrical coupling between the stacked metal–oxide–semiconductor field-effect transistors (MOSFETs) is proposed. We conduct the parameter extraction with technology computer-aided design (TCAD) simulations, where LETI-UTSOI model in HSPICE is used for the bottom PFET and the top NFETs. Their parameters are extracted by fitting their current–voltage (for dc analysis) and capacitance–voltage (for transient ac analysis) characteristics. The parameters extracted from the LETI-UTSOI model contain the electrical coupling at the gate of the bottom MOSFET. In order to extract external capacitances such as monolithic intertier via (MIV)-to-MIV and MIV-to-contact in M3INV, we use two structures, the first that contains MIVs and metal lines and the second that does not. We observe that the dc and transient characteristics of M3INVs built using our extracted parameters match the TCAD mixed-mode circuit simulation results considerably well. Finally, we build and analyze ring oscillators using our M3INV to demonstrate the coupling impact on power and performance.

The investigation of the unseen interrelationship of grain size, ionic defects, device physics and performance of perovskite solar cells

Controlling the phenomenological morphology effects on the performance of the perovskite solar cell (PSC) is a continuing concern due to its photo-physical complexity and the existing contrary reports. Distinguishing the effect of the formed electron and hole traps in the bulk and at surface/interfaces of the perovskite layer from their impact on the performance of the device can be beneficial in optimizing fabrication methods. Here, the transient AC and steady state DC measurements, and morphology characterizations confirm the variation of performance parameters with respect to grain boundaries growth. The device physics is uncovered with respect to the grain size (GS) of the perovskite layer employing the theoretical drift-diffusion framework incorporating the electronic and ionic contributions. The increase of open circuit voltage () for devices with large GS can be associated to the density of defect states. The findings here suggest a more pronounced role of interfaces in efficiency enhancement of the PSCs with the emphasis on the impact of the hole transport layer (HTL)/perovskite layer interface which is also found to be accountable to the difference between the device internal voltage and the terminal voltage and minimizing this difference can lead to an enhancement of approximately 100 mV in . Additionally, the electron traps in the bulk of the perovskite layer play a distinguishable role in the reduction of for the device with the smallest GS. The ionic defect density is also estimated. Considering our results and previous reports, the performance of the PSC is remarkably dependent on the method of fabrication and the associated perovskite conversion mechanism, and not necessarily on GS. The results are expected to deliver important guidelines for the development of more efficient PSCs by further enhancement of the towards its thermodynamic limit of 1.32V, via creating optimal interfaces.

Evaluation of complications and visual outcome in various nucleus delivery techniques of manual small incision cataract surgery.

Purpose:To evaluate various methods of nucleus delivery in manual small incision cataract surgery, with reference to visual outcome, intraoperative, and postoperative complications.Methods:In this prospective randomized interventional study, five groups of 40 cases each were constituted, with reference to nucleus delivery technique: (a) phacosandwich, (b) fishhook, (c) irrigating vectis, (d) viscoexpression, and (e) anterior chamber maintainer (ACM). Visual outcome, intraoperative, and postoperative complications were evaluated in detail. Follow-up was done on first and seventh postoperative days (PODs) and then at fourth and eighth postoperative weeks.Results:The most common intraoperative complication was intraoperative miosis, followed by intraoperative hyphema, seen more in phacosandwich and irrigating vectis groups. The most common postoperative complication was striate keratopathy followed by transient postoperative corneal edema and AC inflammatory response, seen more in phacosandwich and fishhook groups. With reference to visual acuity, on the first POD 95% cases of ACM group achieved visual acuity >+0.5 logMAR unit. The difference in the visual outcome among groups was statistically significant. On fourth and eighth postoperative weeks, best-corrected visual acuity among various groups was comparable.Conclusion:ACM and viscoexpression are effective techniques for early visual rehabilitation. Fishhook has limited utility in softer nuclear grades and black cataracts. Phacosandwich is more suitable for nuclear sclerosis Grades 3–4. Irrigating vectis, viscoexpression, and ACM technique are effective techniques for all grades of nucleus Postoperative surgical-induced astigmatism was comparable in all techniques.

Enhancing LVRT Capability of DFIG-Based Wind Turbine Systems with SMES Series in the Rotor Side

The necessary Low Voltage Ride Through (LVRT) capability is very important to wind turbines. This paper presents a method to enhance LVRT capability of doubly fed induction generators- (DFIGs-) based wind turbine systems with series superconducting magnetic energy storage (SMES) in the rotor side. When grid fault occurs, series SMES in the rotor side is utilized to produce a desired output voltage and absorbs energy. Compared with other methods which enhance LVRT capability with Superconducting Fault-Current Limiter-Magnetic Energy Storage System (SFCL-MESS), this strategy can control the output voltage of SMES to suppress the transient AC voltage component in the rotor directly, which is more effective and rapid. Theoretical study of the DFIG under low voltage fault is developed; the simulation results are operated by MATLAB/Simulink.

A Transient Fault Recognition Method for an AC-DC Hybrid Transmission System Based on MMC Information Fusion

At present, the research is still in the primary stage in the process of fault disturbance energy transfer in the multilevel modular converter based high voltage direct current (HVDC-MMC). An urgent problem is how to extract and analyze the fault features hidden in MMC electrical information in further studies on the HVDC system. Aiming at the above, this article analyzes the influence of AC transient disturbance on electrical signals of MMC. At the same time, it is found that the energy distribution of electrical signals in MMC is different for different arms in the same frequency bands after the discrete wavelet packet transformation (DWPT). Renyi wavelet packet energy entropy (RWPEE) and Renyi wavelet packet time entropy (RWPTE) are proposed and applied to AC transient fault feature extraction from electrical signals in MMC. Using the feature extraction results of Renyi wavelet packet entropy (RWPE), a novel recognition method is put forward to recognize AC transient faults using the information fusion technology. Theoretical analysis and experimental results show that the proposed method is available to recognize transient AC faults.

A perovskite based plug and play AC photovoltaic device with ionic liquid induced transient opto-electronic conversion

Transient AC optoelectronic conversion at the perovskite–ionic liquid interface associated with induced electric double layers is implemented for energy harvesting.

Protection scheme for high‐voltage direct‐current transmission lines based on transient AC current

This study presents a new protection scheme for high-voltage direct-current (HVDC) transmission lines which only uses a specific frequency AC current (SFAC) at one of the line terminals during fault transients. The fluctuation characteristic of the root mean square (RMS) of SFAC is analysed under different fault conditions. The fluctuation in the RMS of SFAC during the transient period of internal and external faults is quite different. A fluctuation coefficient (FC) is proposed to measure the fluctuation. Fault types and the faulted line can be distinguished by calculating FC. A HVDC test system modelled in PSCAD/EMTDC is used to validate the proposed scheme considering different fault types, operation modes, sampling frequency and interference. Comprehensive test studies show that the performance of the proposed protection scheme is inspiring, and it is simpler and is of higher reliability compared with some traditional protections based on harmonic currents.

A New Self-Heat Modeling Approach for LDMOS Devices

The purpose of this research is to characterize and model the self-heating effect of n-channel Laterally Diffused Metal-Oxide-Semiconductor Field-Effect Transistors (LDMOS). In order to improve the accuracy and convergence of the self-heating (SH) simulations in DC, transient and small signal AC (S-Parameter) domains, a new SH model has been developed and implemented in Verilog-A version of BSIM4 model to simulate an LDMOS device. The new SH model does not require a separate thermal network. It has been verified with measurements in static DC, transient, and small signal AC (S-parameter). Excellent agreements between measured and simulated results have been obtained without sacrificing simulation speed and convergence performance.