Hasil untuk "Electricity and magnetism"

Weihua Jiang

This review article is the third of a three-article introductory series on virtual cathode oscillators. The first article has laid the theoretical ground for understanding the physical properties of the virtual cathode, and the second article has provided a numerical tool for studying virtual cathode oscillation. This third article focuses on the interaction between the electron beam and electromagnetic field. The virtual cathode oscillator has been studied for decades with the aim of developing it as high-power microwave source. The beam-field interaction has been one of the core issues that always perplexes both experimentalists and theorists. Using the physical model established in the first article and the numerical method described in the second article, this article is an attempt to answer some of the key questions based on a more comprehensive description of the device and its interaction process. This article is expected to serve as a reference for young researchers and students working on high-power microwaves and pulsed particle beams.

Jean Nepomuscene Twahirwa, Celestin Ntivuguruzwa

This study investigates the effects of smart classrooms, comprehensive assessment management information systems, and remediation teaching techniques on teachers ‘and student’s conceptual understanding of physics. The participants included 67 students from Senior One, Senior Two, and Senior Three, purposively sampled from G.S Mushongi in Kirehe district, along with 40 physics teachers selected purposively from the same district. Data was collected using two instruments: the Physics Achievement Test and the Teacher Mixed Multiple-Choice Questionnaire. Both students and teachers underwent remediation and training, respectively. The study focuses on taught specific physics units for each grade level: Current Electricity and Qualitative Analysis of Linear Motion for Senior One; Magnetism and Demagnetization and Reflection of Light in Curved Mirrors for Senior Two; and Heat Transfer and Quantity of Heat, Renewable Energy and Non-Renewable Energy, and Introduction to Electromagnetic Induction for Senior Three. Descriptive and inferential statistics were computed to analyze the data, and the results were presented using tables and figures created using Microsoft Excel and Origin version 2019b software. For Senior-One, Senior-Three students, the post-test scores demonstrated a highly statistically significant improvement (p < .001), suggesting that the remediation positively influenced their performance. However, for Senior Two students, the data analysis did not show any statistically significant improvement in their performance after remediation (p > .05). The results indicated a significant difference after training (p < .001) for the teachers. This research highlights the importance of targeted remediation and smart classrooms in enhancing academic performance and teachers’ perceptions.

Md Zakaria Habib, Nathaniel Taylor

We propose a method for the fault passage indication of earth faults in resonant-earthed networks, based on phase current measurements alone. This is particularly relevant for electricity distribution systems at medium-voltage levels. The method is based on the relative magnitudes of the phasor changes in the phase currents due to the fault. It is tested for various network types and operation configurations by simulating the network in <span style="font-variant: small-caps;">pscad</span> and using the simulated currents as the input for an implementation of the method in <span style="font-variant: small-caps;">matlab</span>. In over-compensated networks, the method shows reliable detection of the fault passage, with good selectivity and sensitivity for both homogeneous and mixed (cable and overhead line) feeders. However, for the less common under-compensated systems, it has limitations that are described further in this study. The method has good potential for being cost effective since it requires only current measurements, from a single location, at a moderate sampling rate.

Yanchun XU, Zhongyao FAN, Sihan SUN et al.

Since the fault current output from the doubly-fed wind farm has frequency deviation characteristics and contains large harmonic components, when an internal fault occurs in the transmission transformer for the doubly-fed wind farm, the ratio of the second harmonic to the fundamental wave in the differential current of the transformer increases, which makes the differential protection of the transformer face the risk of delay action. Moreover, when the system fails, a large number of non-periodic components in the fault current output from the doubly-fed wind farm will make the current transformer in the transmission transformer more prone to saturation, resulting in reduced reliability of the differential protection of traditional transformers. This paper proposes a differential protection scheme for the transmission transformer for large-scale wind farms based on detrended analysis. Firstly, the sampling current is processed by detrended analysis through the sliding data window to obtain the detrended residual function, and then the slope characteristics of the current waveform are utilized to complete the effective distinction between the magnetizing inrush current and the fault differential current (including the current transformer saturation state) of the transformer. The proposed protection scheme is validated to be applicable under different operating conditions by building a transmission system for the doubly-fed wind farm in PSCAD.

Yongxin Zhang, Qikun Feng, Shaolong Zhong et al.

Abstract In the process of coping with energy and environmental protection issues, technologies such as energy materials, energy devices, and energy systems have made great progress. With excellent performance, film capacitors play an increasingly important role in energy‐related fields. With the increase of application scenarios and the continuous development of film material technology, it is urgent to establish a better theoretical connection from films to capacitors. First, the main components of the capacitor including the film and the positional relationships among them are given. Then, from the two perspectives of indirect calculation according to the volume and the direct calculation according to the winding process, the equation between the dielectric constant of films and the corresponding capacitance of capacitors is established. Further, the measurement data and error analysis results of the built test platform prove the accuracy and great potential of the proposed calculation methods. In addition, error sources, including film thickness uniformity, are listed. Finally, the challenges faced by the proposed calculation methods and the paths that can be referenced for future research are summarised and discussed.

Xiaofeng YANG, Yihang FANG, Pengzhen ZHAO et al.

In order to achieve the goal of “double carbon”, the development of wind power generation technology is essential. At the same time, with the increasing complexity of power grid, the real-time detection and accurate evaluation of the state of wind turbines and other power equipment are becoming increasingly important. In recent years, the development of big data technology and the improvement of power equipment data monitoring technology makes possible the application of big data technology in power equipment state recognition. Compared with the conventional methods, the above-mentioned methods are independent of accurate empirical thresholds or quantitative models, and have better adaptability to the rapid increase and variability of data. Thus, this paper applies the unsupervised (K-means) and supervised (BPNN) machine learning methods to state recognition of wind turbines, while exploring the variation of accuracy and computational efficiency after the application of dimensionality reduction methods. The results show that both machine learning methods are effective in state recognition of wind turbines, while the dimensionality reduction method can effectively improve the computational efficiency with limited accuracy loss.

Cheng ZHONG, Di ZHAI, Yang LU et al.

To address the problem of reduced communication quality in narrow underground power pipe gallery, where wireless sensor network coverage is affected by irregular shapes, obstacles, and electromagnetic interference, a power monitoring coverage sensing model is constructed based on the minimum access rate constraint, and an improved gray wolf coverage optimization algorithm is proposed by combining neuron mapping and differential evolution. Firstly, a uniform initial population is generated by neuron chaos mapping. Then, the nonlinear convergence factor is used to balance the global and local search ability. And finally, a differential evolution algorithm is introduced to mutate the gray wolf individuals. A comparative simulation analysis is made of various coverage optimization methods, and the results show that the proposed algorithm has robust search capabilities and it can significantly improve the network coverage performance in the narrow underground power pipe galleries, while effectively satisfying the communication needs of the monitored nodes.

Zhulin Liang, Ming Lu, Weiheng Zhao et al.

Aiming at the problem that the control performance of the frame system will become worse and even lose stability after the control moment gyroscope is installed on the vibration isolation platform, this paper conducts co-simulation based on ADAMS, ANSYS and MATLAB to study the Kinematics and dynamics of the control moment gyroscope under different flexible support conditions. The results of the paper can be used to simulate the operation of control torque gyroscopes under various conditions such as multiple operating conditions, variable loads, variable damping, variable stiffness, and zero gravity. Reverse guidance is provided for the design of isolation platforms and control torque gyroscopes, which has important engineering application significance for achieving low disturbance and high-precision control performance of control torque gyroscopes, and thus achieving high-precision control performance of spacecraft attitude.

Fan Wu, Fei Cao, Xurong Zhang et al.

Abstract To solve the problem of the mismatch of the wideband underdetermined direction of arrival (DOA) estimation under the condition of the On‐grid model, this paper extends the narrowband Off‐Grid model to wideband and proposes a new DOA estimation algorithm for Off‐Grid sources based on group sparsity. The proposed algorithm first obtains the preliminary estimation result under the current predefined discrete grid through the group sparsity wideband DOA estimation algorithm. Then, the Off‐Grid optimisation problem is adopted to calculate the Off‐Grid deviation vector. It is also assumed that the off‐grid deviation vectors of each frequency subband are exactly the same, thereby reducing the number of parameters to be estimated. Therefore, the proposed algorithm can not only maintain similar or even better estimation accuracy but also greatly reduce the computational complexity. Finally, simulation is conducted and the results verify the effectiveness and performance of the proposed method.

Viktor Burdovitsin, Ilya Bakeev, Kirill Karpov et al.

We describe our investigations of a plasma-cathode electron source designed for the deposition of oxide coatings by the electron-beam evaporation of dielectric materials. Tests carried out using oxygen as the working gas showed that the source is operable without a change in parameters for at least ten hours of continuous operation. The current–voltage characteristics of the hollow-cathode plasma source in oxygen displayed a monotonically increasing character, and the voltage dependence of the discharge current was exponential. At the same time, for argon, nitrogen, and helium, the discharge voltage remained unchanged over a current ranging from 0.1 A to 1 A. A possible reason for these differences is the formation of oxides on the electrode surfaces for operation in the oxygen, impeding the discharge operation and requiring higher voltages for the same current as the other gases. The dependencies of the electron beam current on the accelerating voltage were monotonically increasing curves for all the gases except for helium, for which the beam current remained unchanged with increasing voltage over a range from two to ten kilovolts.

Lei CUI, Tong YANG, Ruliang ZHANG et al.

Insulated gate bipolar transistor (IGBT) is usually used in combination with power diode in power electric circuit because it has no reverse blocking ability. In order to reduce cost and parasitic inductance, the freewheeling diode and IGBT are integrated by process method, and a reverse blocking insulated gate bipolar transistor (RB-IGBT) is thus proposed. For reducing the terminal area of conventional reverse blocking IGBT, an improved composite terminal structure is proposed. The double doped field limiting ring is used to introduce a n-type low doping region near the p-type field limiting ring, which can reduce the lateral expansion rate of depletion region, increase device reliability, improve terminal efficiency and save terminal size.

Qiyou LIN, Wenliang JIANG, Yuanyuan LI et al.

In a DC microgrid system, distributed units are usually connected to the DC bus in a decentralized manner, and distributed power sources and loads are easily affected by the external environment, which degrades the bus voltage stability of the DC microgrid. In order to solve this problem, a coordinated control strategy for DC microgrid system based on bus voltage stratification is proposed in this paper. The strategy builds a DC microgrid system structure mainly based on photovoltaic power generation, and then divides the bus voltage into five levels. Under the corresponding operation mode of each level, the operation mode and the control strategy are studied. Finally, the effectiveness of the proposed control strategy in suppressing the fluctuation of the system bus voltage is verified when the fluctuation of the photovoltaic power generation and the load is caused by the change of environmental factors.

Arega Getaneh Abate, Rossana Riccardi, Carlos Ruiz

The interplay between risk aversion and financial derivatives has received increasing attention since the advent of electricity market liberalization. One important challenge in this context is how to develop economically efficient and cost-effective models to integrate renewable energy sources (RES) in the electricity market, which constitutes a relatively new and exciting field of research. This paper proposes a game-theoretical equilibrium model that characterizes the interactions between oligopolistic generators in a two-stage electricity market under the presence of high RES penetration. Given conventional generators with generation cost uncertainty and renewable generators with intermittent and stochastic capacity, we consider a single futures contract market that is cleared prior to a spot market where the energy delivery takes place. We introduce physical and financial contracts to evaluate their performance assess their impact on the electricity market outcomes and examine how these depend on the level of RES penetration. Since market participants are usually risk-averse, a coherent risk measure is introduced to deal with both risk-neutral and risk-averse generators. We derive analytical relationships between contracts, study the implications of uncertainties, test the performance of the proposed equilibrium model and its main properties through numerical examples. Our results show that overall electricity prices, generation costs, profits, and quantities for conventional generators decrease, whereas quantities and profits for RES generators increase with RES penetration. Hence, both physical and financial contracts efficiently mitigate the impact of uncertainties and help the integration of RES into the electricity system.

Sihua WANG, Junjun WANG, Lei ZHAO et al.

Surface pollution of insulators is the main cause of pollution flashover, and the pollution components have different effects on the electrical performances of insulators. In order to study the influence of pollution components on the surface electric field of insulators, a simplified model of composite insulators was established by taking Golmud city along the Qinghai-Tibet Railway as the actual operation environment of insulators. It is found through simulation that when the contaminants are dry, their influence on the surface electric field of insulator follows the tendency from high to low as CaSO4＞KNO3＞NaNO3＞K2SO4＞NaCl＞MgSO4, which is mainly determined by the relative dielectric constant of the medium; when the contaminants are wet, their influence follows the tendency as sodium chloride ＞ nitrate ＞ sulfate, and the conductivity of each polluted liquid becomes the main factor affecting the electric field distribution. The hydrophobicity also affects the electric field distribution of insulators, and the overall electric intensity of insulator with drying zone is higher than that without drying zone. The simulation results were verified by the experimental results. The conclusion explains the influence of pollution components on insulator’s insulation performance from the perspective of electric field distribution, and can provide a new idea for regional pollution classification and insulator contamination test in laboratory.

Ming‐Jian Li, Meng Li, Yu‐Fei Liu et al.

Abstract Front‐fed parabolic reflectors are among the most commonly used antennas in the industry. While in spaceborne applications, membrane reflectors are very promising due to their lightweight and foldable features. However, considering the large size, small thickness and low stiffness, solar radiation and microwave radiation will have considerable influences on the antennas’ shape accuracy as well as the radiation characteristics. In this article, a theoretical approach is presented to solve the multi‐physical effects of the parabolic antenna. The deformation of the reflector is derived by the shallow shell theory, taking into account the solar pressure, the microwave pressure and the thermal effects due to solar and microwave heating. The far‐field electromagnetic radiation pattern is then obtained by considering the deformation of the reflector. On the other hand, a numerical approach combining the finite element method, the multi‐level fast multipole method, and the large element physical optics is also presented. Numerical examples suggest good agreement between the theoretical and numerical results. The methods have been applied into the analysis of design models in the Space Solar Power Station project. Also, these approaches can be directly extended into other space membrane reflector antennas.

Alan Richards

As students learn physics, they are often required to reason about the behavior of macroscopic and microscopic phenomena, and to synthesize prior knowledge from several different areas of physics to construct understanding of new ideas. This can be a tremendously difficult cognitive task for novice students, especially when the unfamiliar phenomena described are potentially microscopic or abstract. Therefore, it can be very helpful for physics instructors to develop and employ pedagogical techniques that help students to visualize and to reason productively about these concepts. A particularly effective strategy uses kinesthetic learning activities (KLAs).

Fiona Armstrong, W. Teo

Yuan Hwang, Hyanghee Jeon, Geon Yeoung Wang et al.

Bacterial vaginosis is a common female disease caused by a vaginal infection due to an overgrowth of bacteria that naturally live in the vaginal tract. Bacterial vaginosis has frequently been treated with the oral or vaginal administration of antibiotics and topical disinfectants. However, hygienic application of topical treatment deep in the vagina remains difficult. Herein, we introduce a novel vaginal cleaning device using plasma-activated water generated from supplied water. Remarkably, plasma source generation at atmospheric pressure is well known to eradicate bacterial infection through the generation of free radicals and/or chlorine chemicals with antimicrobial activity. The device was designed to alleviate a bacterial infection by spraying plasma-activated water generated from a cleaning solution container with plasma modules. The spray nozzle contains both a clean outlet and a suction outlet to spray and recover the plasma water, respectively, and is connected to a disposable silicone tube. The other nozzle, which has a laser light and air pump, can perform a second sterilization and dry the vagina after washing. Free chlorine chemicals with antibacterial activity were detected in the plasma-activated water by the device. Clinical application in patients with bacterial vaginosis confirmed the stability and effectiveness of our device. Therefore, these results show a novel clinical application of atmospheric pressure plasma to medical field as a plasma medicine.

Muhammad Alifyan Zulkarnain, Muhammad Fajri Raharjo, Meylanie Olivya

The conventional question and answer method has limited time, space and level of understanding from each student so that it becomes an obstacle to the ineffective teaching and learning process. Global pandemic events that have a negative impact on the education sector. So that the teaching and learning process is temporarily stopped at school and diverted online. This study aims to produce a chatbot application as a media for distance learning that helps the teaching and learning process of teachers and students . In practice, This research uses telegram as a chatbot media. The method used for data collection is by interview, literature study, and observation. Testing will be carried out with a black box, calculation of accuracy, and a user acceptance test in the form of distributing questionnaires. The results of this study produce a Chatbot Application as an E-Learning Media for Students that can help teachers in providing distance learning to students efficiently and easily.

Huankun ZHANG, Junyi LI, Bin ZHANG

As an important component of transmission lines, insulator plays an essential role in the stable operation of the power grid. However, the outdoor environment in which the insulators are located can easily lead to the hanging of foreign objects. This paper proposes a novel method for foreign object detection on insulators based on the improved YOLO v3: Dense-YOLO v3. A dense network is designed to replace one of the convolutional layers of the original network in order to realize the multi-layer feature reuse and fusion of the insulator, which improves the detection accuracy. In addition, we amplify the training set to improve the training effect of the network and propose a wrong detection cost function to measure the risk of false detection. The experiment shows that the proposed algorithm has a detection precision rate reaching up to 94.54%. Meanwhile, the Dense-YOLO v3 outperforms YOLO v3 and Faster R-CNN, both in terms of detection accuracy and wrong detection cost. The result shows that the presented approach can be applied to the UAV inspection of transmission lines.