Analysis on the Characteristics of Short-tail Voltage Wave of Insulator on the Double-circuit Transmission Line in Back Flashover and Design of the Experiment Circuit

Lightning striking on a transmission tower induces high ground potential rise and high voltage at tower arms in which potential is normally at ground level, and subsequently causes overvoltage across an insulator string. If this overvoltage is higher than the withstanding voltage of the insulator string according to the v-t (voltage-time) curve, back flashover phenomena will occur and this event may cause outage. The main objective of this paper is to study the factors influencing the back flashover phenomena. The computer program PSCAD/EMTDC (Power System Computer Aided Design/Electromagnetic Transients including DC) is used to simulate lightning striking on a transmission tower 115kV. Lightning current, transmission towers, ground resistance, insulator strings and back flashover phenomena are modeled. Main simulations are lightning striking on different towers, different soil resistivity, different lightning current magnitudes and wave shapes, different locations, and different phase angles of source voltage. Simulation results show that the higher tower encounters higher induced voltage. A back flashover occurs at the top tower arm easier than at the middle and lower arms. The higher soil resistivity induces higher voltage. The larger lightning current magnitude impacts on higher induced voltage. The longer rise time of lightning current generates lower induced voltage. Lightning strikes directly on tower generate higher voltage than that of striking on overhead ground wires.

Traditionally, the withstand voltage of an insulator string to lightning impacts is checked using the standard impulse 1.2/50 μs. First, the authors show that when a lightning stroke hits the tower or a ground wire, the resulting overvoltages are characterised by much shorter tail durations than the tail of incident lightning currents. The analysis of wave shapes, in particular tail durations of the overvoltages affecting the insulator strings, is carried out in the form of a detailed parametric study. Secondly, the authors describe the routine tests carried out in a HV laboratory to evaluate the withstand voltage of various insulator strings as a function of tail duration of the applied impulses. Five strings of different lengths were tested, with impulse tail durations between 50 μs and 4 μs. In conclusion, it clearly appears, when considering the mechanisms resulting in back flashover, that the withstand voltage of insulator strings is much higher than their conventional value for the 1.2/50 μs impulse.

In order to research the surface flashover characteristics of two typical insulating materials (nylon and synthetic glass) under hundred-nanosecond pulse in nitrogen gas, experimental device of surface flashover under hundred-nanosecond pulse was designed. The variation laws of the flashover voltage of the insulating material with air pressure and flashover distance have been researched via experimentation. The experimental results showed that the flashover voltage increased with the increasing of the air pressure. However, the flashover electric field intensity decreased with the increasing of the distance. In the same condition, the flashover voltage of PMMA is higher than that of nylon. The flashover voltage of the materials with different gas pressure and distance under hundred-nanosecond pulse were obtained via the surface flashover experiment.

The characteristics of grooved insulator flashover under pulsed voltage are discussed in this paper. The groove widths are determined by the secondary electron emission trajectory. The experiments results show that the flashover voltages increased while the insulator samples with 0.05 mm, 0.1 mm, or 0.7 mm wide grooves were used and the biggest voltage multiple was 1.4. The samples with 1 mm wide grooves had the same flashover voltage level as the flat ones. It is concluded that proper grooves design could increase the insulator flashover level by preventing the development of the secondary electron emission avalanche. Mechanism of inhibition was analyzed with calculation of electric field strength on the surface of insulator. Wider grooves, would trap the electrons and narrower ones would inhibit the motion of the initial electrons. Comparing the samples surface micrograms, a conclusion was given that the surface micro-flaw would drop the insulator flashover level.

- In order to monitor operating performance of contaminated outdoor insulators, a wavelet-based flashover prediction method is proposed. In most cases, the low-frequency components, namely, fundamental, 3 rd , and 5 th harmonic components have been mainly used for the sake of the spectral analysis of the leakage current. However, in this paper, the detail coefficients of wavelet transform representing high-frequency components are used as important information to predict a flashover in the contaminated insulator. Experimental results verify that the proposed method has a superior capability for flashover prediction.Key Words : Flashover Prediction, Contaminated Insulator, Leakage Current, Wavelet Transform, High-Frequency Components*정 회 원 : Kyungpook National University, School of Electrical Engineering and Computer ScienceE-mail : songyc03@ee.knu.ac.kr접수일자 : 2010년 1월 27일 최종완료 : 2010년 3월 3일 1. Introduction With increasing application in wet and contaminated conditions, such as salt-fog environments, outdoor insulators are exposed to long-term humidity and severe contamination. Once the moisture on the insulator surface is condensed to form continuous wet film, leakage current will be driven by source voltage to pass through the conductive layer. Thermal energy produced by the current cause evaporation of the layer and formation of dry-band gaps. Once the electric field at these gaps reaches the breakdown strength of air, dry-band discharges will occur. Due to initiation and development of the discharges, flashover may take place and affect operating reliability of power systems. From the viewpoint of reducing costs and minimizing risk of damage to people Camcorderand property, it is urgent to develop methods for monitoring outdoor insulator performance in contaminated environments. Current methods include the equivalent salt deposit density (ESDD), optical measurements, and leakage current measurements. In particular, the leakage current can provide much useful information on the state of a contaminated insulators, thus, several approaches using the leakage current have already been introduced to monitor the contamination conditions of outdoor insulators, for example, deterioration pattern analysis, leakage current property comparison under various circumstances, spectral analyses, and stochastic analysis using a Hilbert transform and the level crossing rate [1-6]. However, in most case, the low-frequency components, namely, fundamental, 3

The purpose of this study is to review the current famous fault location methods for double-circuit parallel transmission lines on the same pole. To begin with, fault classification and characteristics of double-circuit transmission lines are described. Then, depending on the extracted data from one or both ends of the transmission lines, different algorithms of fault location are divided into two classes, that is, single-ended algorithms and multi- ended algorithms. Concerning on the fault-location determination suffers influence from some factors, such as the positioning system operating mode, the distributed capacitance and the transition resistance, etc., these methods based on the different parameter model of double-circuit transmission lines mainly use six-sequence component method and heuristic algorithm to get the equations. Combined with the distinctive characteristics of differential component net and each sequence component network, these methods apply different numerical analysis methods for solving these equations with the aim to determine fault point location accurately. These two classes of algorithms are separately discussed and detailed analysis in the research, from which their respective advantages and disadvantages are summarized as follows. At last, development tendency of fault location algorithms on the double-circuit transmission lines on the same pole are prospected.

The statistical model is used as a typical method for vacuum insulator stack flashover probability calculation. We have calculated the stack flashover probability and the g factors by which a flashover will change the voltages on the each of the insulator rings with different conditions. The calculated results show that the real performance of insulator stack characteristics with the multi-stage and large circumferential transit time. The matrix a in the statistical model was predigested to reduce the g calculation time and make the probability calculation accurate. Finally, analysis based on fixed distance between insulation rings showed that there were some optimized ring numbers with maximum voltage amplitude and electrical field amplitude. This conclusion could be used for evaluation of insulation ring number in insulator stack design.

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A conventional power supply was designed with two stages, the former stage function as a power factor corrector (PFC), and the latter stage is a DC/DC converter which regulates the output voltage of the system. However, the cost and energy conversion efficiency of the two-stage system is higher than a single-stage one. LLC resonant converters not only have characteristics of high efficiency and low noise but also are easy to control. This circuit topology is becoming more and more widely used in application of power conversion. The duty cycle of both MOSFETs of a LLC converter is 50% that is different from a traditional PWM converter. The output regulation is control by modulation of frequency, in the ZVS region, the energy from primary side to load increases as the switching frequency decreases. In this thesis, a single-stage LLC resonant converter which combines a boost-type PFC cell and an LLC resonant DC/DC cell is proposed. The control method of it is the same as a conventional LLC resonant converter that regulate output voltage by modulating switching frequency. When the load increases, the switching frequency decreases so that the LLC gain increases and more energy can be transferred by input inductor due to longer charging time. The proposed circuit could not only reduce components and circuit size but also increase the power conversion efficiency. It has no control of the voltage of the DC-bus capacitor because there is no separate converter that can control it as there is in a two-stage converter. The capacitor voltage is dependent on the energy equilibrium established between the energy flowing into the capacitor from the input inductor and the energy flowing out of the capacitor and transferred to the output. Therefore, the characteristics of LLC tank are investigated and a new LLC design consideration is proposed, which insures the DC-bus capacitor voltage can be kept in a tolerable region. Finally, an experimental circuit is implemented to verify the analysis.

Design, Fabrication, and Characterization of Monolithically Integrated Acoustic and Photonic Devices on Lithium Niobate Over Insulator (LNOI) Platform

In many insulation systems, overvoltages below the flashover level can give rise to discharges which produce free charge carriers. These charges may be deposited on insulation surfaces, resulting in a high surface field and consequent distortion of the normal field. This may result in increase, or decrease, in flashover voltage. Investigations were undertaken to examine the factors and mechanisms governing the production of surface. charge and the resulting field distortion. The experimental equipment consisted of a simple point-insulation-plane electrode assembly, mounted in a glass test cell filled with air, to which unidirectional impulse voltages were applied. Polycarbonate and polymethylmethacrylate samples were tested. Measurements of the discharge currents, residual surface field and resulting dust figure patterns were carried out. Under both positive and negative impulse voltages, discharge current pulses occurred resulting in high surface fields on the insulation sample. On the wavetail of the applied impulse discharge, currents occurred in a reverse direction to those originally produced by the applied impulse. With repeated impulses, the stored surface charge produced by one impulse significantly affected the discharge activity of the following impulse. Under flashover conditions, a substantial surface field remained following the flashover.

Aiming at the front door coupling effect of high-power microwave on millimeter wave fuze, the electromagnetic simulation software was used to irradiate a certain millimeter wave FM continuous wave fuze model, and the joint simulation was carried out with the front-end limiting circuit of the fuze. On this basis, orthogonal experiments are designed to analyze the influence level of signal parameters. When the high-power microwave signal frequency is aligned with the fuze working frequency, the simulation results show that the maximum coupling voltage at the end of the antenna can reach 188 V when the peak radiation field intensity is 60 kV/m. When the peak radiation field intensity is 40 kV/m, changing the characteristic parameters of the radiation signal and finds that the long pulse width signal is more likely to cause the thermal breakdown effect of the limiter; the rising time of the signal will affect the end of the antenna. When the peak value and pulse width of the coupled voltage waveform are fixed, the spike leakage voltage caused by the input signal with rise time of 5 ns is about 5.94 V, while the spike leakage voltage is 18.4 V when rise time is 0.1 ns, and the limiting circuit reaches saturation state faster. The orthogonal experiment shows that the spike leakage peak voltage is most affected by the rise time of the signal, and the signal peak is the largest, meanwhile the value has the second effect on it.

Abstract— This paper discusses the potential application of ANN techniques for detection of single line to ground faults and fault type classification on double circuit transmission lines with remote end infeed. Distance protection of double circuit transmission lines has been a very challenging task. The problems arise principally as a result of the mutual coupling between the two circuits under different fault conditions. An accurate algorithm for fault detection and classification of single line-to-ground faults (A1N, A2N, B1N, B2N, C1N & C2N) in double circuit transmission line considering the effects of mutual coupling, high fault resistance, varying fault location, fault inception angle and remote source infeed is presented using feed forward neural network (FFNN) algorithm. The algorithm employs the fundamental components of voltage and current signals. This technique neither requires communication link to retrieve the remote end data nor zero sequence current compensation for healthy phases are required. This is a major advantage of the proposed algorithm for protection of double circuit line fed from both the ends. Results of study on a 220 kV transmission line are presented as an illustration. Simulation results indicate that algorithm is immune to the effect of mutual coupling, fault type, fault inception angle, fault resistance, fault location and remote end infeed. Index Terms— Artificial neural network, Double circuit transmission line, Fault detection & classification, High impedance fault, Single line-to-ground fault.

The performance of a transverse field electromagnetic flowmeter in a steady two-phase flow was investigated analytically for a disperse and an annular flow regime. In both cases the flowmeter output voltage was found to be proportional to the mean velocity of the liquid phase. Experiments in a steady air-water mixture showed good agreement with the analysis. An impedance void fraction meter was designed and built to conduct measurements of unsteady void fractions. Short electrodes excited by voltages of opposite polarity were used in combination with a highly sensitive signal processor. The steady state calibration indicated that the meter was somewhat sensitive to the void fraction distribution for the bubbly flow regime. However, the transition to a churn turbulent regime greatly affected the meter steady state response. The dynamic capability of the void fraction meter was estimated by comparison of the statistical properties of the voltage fluctuations in a nominally steady bubbly flow with those of a shot-noise process. The filter function associated with the finite volume of the electric field within the fluid cell could be determined from the measured autocorrelation function and was shown to be mainly a function of the velocity of the disperse phase. Also some properties of the disperse phase could be inferred from the statistical analysis. Two void fraction meters were used to measure the propagation speed of kinematic shocks in an air-water bubbly mixture for various void fractions and water flow rates. The relative velocity of the disperse phase calculated from these measurements decreased with an increase in the disperse phase concentration. However, this effect disappeared at higher water flow rates and the relative velocity became independent of void fraction. Measurements of the propagation speed of shocks of decreasing strength provided a good verification of the kinematic wave theory. The shock thicknesses could also be determined leading to the conclusion that an important diffusion mechanism was responsible for arresting the steepening of the wave. Cross-correlations of the fluctuating voltage of two void fraction meters in a steady bubbly flow were determined. The speed measured by this technique was identified as the infinitesimal wave speed of the void fraction and not the velocity of the dispersed phase as postulated by some authors. The normalized cross-correlation maxima showed that the small amplitude void fraction disturbances were short-lived structures, which were created and diffused on a continuous basis. The cross spectral density revealed that the waves present in these disturbances were nondispersive.

In order to solve the problems of poor insulation, low resolution and sparking etc., a retarding field energy analyzer is upgraded, simulated and analyzed to test and analyze the electron beam parameters accurately, which derive from the guns of traveling wave tubes and beams after beam-wave interaction, providing a reference for the design, performance improvement and development of the traveling wave tubes. The retarding energy analyzer with a controllable focusing voltage insulates high-voltage cylinder electrically from retarding mesh and applies a controllable small voltage between them. The second electrostatic lens accelerates and focuses the diverging beam near the area of the mesh, to weaken too much expansion in the region of the weak part that the first lens functions, due to space-particle effects and beam emittance, etc. and optimize the slope of the particle trajectory and resolution. Meanwhile, the whole structure tends to miniaturizition and lightweight. A computer code called CST is used to simulate the particle distribution in particle interfaces and the whole electric field distribution, and Matlab code helps data processing and exports the key parameters. Besides, the paper focuses on analyzing the optimization and improvement of the performance reflected by the resolution and Full Width at Half Maximum, etc. The result shows that this energy analyzer is competent to apply to the experiments of electron beams.