Impulse Overvoltage Research Articles

Electrical damage mechanism analysis of traction motor bearings on high-speed trains considering overvoltage impulse

ABSTRACT As the critical rotating knuckle, traction motor bearings play a vital role in transmitting torque from motors to wheels for high-speed trains, while some electrochemical corrosion signs appear in traction motor bearings. To maintain the safety of the traction system, it is essential to explore the electrical damage mechanism of traction motor bearings considering the impact brought from current or overvoltage impulses. Herein, via analysing the electrochemical corrosion evolutionary process of bearings, main influencing factors are determined that both transient overvoltage impulse and long-term grounding current bring dramatic impact on bearings. To assess overvoltage distribution on different carriages, a ‘catenary-train-substation’ traction power supply model is launched based on practically collected impedances. A ‘carriage-bogie-motor’ circuit model is built to evaluate the overvoltage propagation path, meanwhile, the 3D finite element geometry of the bogie with bearings is launched to observe the overvoltage distribution in the bearings for evaluating breakdown risk.

Analysis and suppression of operational overvoltage and inrush current for high‐speed trains by automatic phase‐switching technique

AbstractWhen a high‐speed train approaches the insulated phase‐splitting section embedded between neighbouring power supply arms, the vacuum circuit breaker (VCB) installed on trains must be disconnected to maintain the traction power supply system as a no‐load condition for completing phase‐switching action, as the train passes through the phase‐splitting section depending on its inertia. However, when operating VCBs, the arc is easily triggered between the mobile contacts inside, accompanied by an overvoltage impulse. Herein, to explore the generating mechanism of inrush current and operational overvoltage, a model describing the ‘substation‐catenary‐train’ traction power supply system is launched based on an equivalent circuit modelling technique. Through the analysis of the transient VCB operational process, the phase of catenary voltage is directly related to the characteristics of the VCB switching‐on overvoltage, as the traction current's phase angle is relevant to the amplitude‐frequency characteristics of the VCB switching‐off overvoltage. Inrush current as a noteworthy element is related to the traction transformer's remanence. The automatic phase‐switching technique is utilised to suppress both operational overvoltage and inrush current, as the optimal combination of VCB switching‐on and switching‐off phases considering the balance between overvoltage and inrush current is achieved based on the particle swarm algorithm.

Analysis of overvoltage impact on shell-type on-board traction transformer by multi-physics field simulation

With the rapid development and increasing demands for the high-speed railway in China, it is of great significance to examine the defects of the high-speed train and ensure the safety of the operating railway. In the actual operation of the high-speed train, there is a split-phase section to transit the train to the next substation power supply arm. In this particular stage, operational overvoltage would occur as the VCB and the pantograph switch on and off, which threatens the safety of the operation of high-speed trains and contributes to the breakdown of transformer windings, which is indispensable in transformer operation. In this work, a finite element model of shell type on board traction transformer is constructed and multi-physics field simulation results, including electromagnetic field, temperature field, and flow velocity field, are analyzed to investigate the potential risky spots in the practical operation of high-speed train under overvoltage impulse, which could be supportive in maintaining the devices and detecting the winding defects.

Experimental Study of High Frequency Impulse Overvoltages at Sayano-Shushenskaya HPP

High-frequency switching overvoltages with an oscillation frequency of up to several MHz are most dangerous for power transformers and cable bushings with XLPE insulation. A working group in charge for studying the influence of high-frequency impulse overvoltages on the operational reliability of power transformers has been established within the CIGRE framework. Experimental studies of high-frequency impulse overvoltages in the 500 and 15.75 kV networks at the Sayano-Shushenskaya HPP were carried out. A new digital recording system equipped with contactless capacitive sensors was used for measuring impulse overvoltages. By using such recording system, it became possible to obtain data on the overvoltages experienced simultaneously by cable bushings and power transformers on the 500 and 15.75 kV voltage sides. The study results have shown that the highest peak values of high-frequency overvoltages are induced by switching operations of disconnectors in the 500 kV GIS. The overvoltages emerging in the 500 kV network are transferred to the 15.75 kV voltage level via the capacitances between the 500/15.75 kV transformer windings. The resulting overvoltage level may be more than a factor of ten higher than the amplitude value of the nominal 15.75 kV voltage. An analysis model was developed in the ATP-EMTP software, which has been verified against the experimental data. The accomplished numerical analyses and experiments have shown that the use of RC circuits is the most efficient protection measure against high-frequency overvoltages.

ВЛИЯНИЕ РЕЗОНАНСНЫХ ЯВЛЕНИЙ НА ТЕХНИЧЕСКИЙ РЕСУРС АСИНХРОННЫХ ЭЛЕКТРОДВИГАТЕЛЕЙ ПРИ ИМПУЛЬСНЫХ ПЕРЕНАПРЯЖЕНИЯХ

The relevance The work is devoted to assessing the impulse overvoltages effect at the input of the supply cable on the asynchronous machines technical resource as a solution to the electrical equipment failure problem caused by the resource loss. Aim of research Reducing the asynchronous machines accident rate by monitoring their technical condition and the implementation of electrical equipment riskbased maintenance during impulse overvoltages. Research methods The asynchronous electric motor, a supply cable, lightning effects and surge arrestor simulation modeling was used; Fourier analysis was applied. Results The simulation model of electric motor supply system section with installed surge arrestors in transformer substation cells was developed. Spectral composition of input voltage and current of asynchronous machine depending on cable line length was obtained. Assessment of technical resource of asynchronous machine based on voltages and currents oscillograms on its input and the related value of energy transmitted to its insulation during impulse overvoltages was realized.

Study of the waves propagation of lightning overvoltage impulse in the catenary system of electrified railways

Introduction. The article is devoted to the study of the waves propagation of lightning overvoltage impulse in the catenary system of electrified railways. The purpose of the work is to determine the zone of waves propagation of lightning overvoltage impulse along the catenary system, the degree of attenuation and the influence of the soil condition on the mode of their propagation.Materials and methods. The authors carried out tests on the waves propagation of lightning overvoltage impulse along the catenary system of the 1st (ring) track and the 15th (dead end) track of the Experimental Loop of the Railway Research Institute at the Shcherbinka station using a high-voltage impulse voltage generator.Results. During the tests, the influence of the soil condition on the mode of waves propagation of lightning overvoltage impulse was determined. The degree of attenuation of the lightning overvoltage impulse wave in the catenary system in the looped and dead end sections is determined, considering the influence of the reverse (reflected) overvoltage wave, as well as changes in the voltage level during the wave process occurring in the catenary system. The data obtained during the tests enable to determine the propagation speed of the lightning impulse wave.Discussion and conclusion. The obtained test results will be useful for improving the reliability of lightning protection devices of the catenary system against external overvoltages and can be used to improve insulating elements.

Overvoltage Response at the Terminal for Impulse Current Injection to 10 kV Distribution Line

This paper presents the three-phase impulse overvoltage measured at the terminal of a 10 kV distribution line when the impulse current was injected into the phase conductor of the distribution line. The overvoltage observation station was installed at the terminal of the line to record the three-phase overvoltage waveforms. It is shown that the struck phase overvoltage consists of three components including the negative pulse, the stage of residual voltage maintenance and the overshoot oscillation part. The observed negative peak values of the struck phase overvoltage range from −85.4 kV to −96.0 kV in three different injected impulse levels at 90,120,150 kV charging voltage. The duration of residual voltage depends on the duration time of injected current. Furthermore, there is a strong positive relationship between the oscillation amplitude of the struck phase overvoltage and the injected current amplitude. The peak amplitudes of three-phase overvoltage always have the following magnitude relationship: struck phase C > induced phase B > induced phase A. The voltage coupling coefficients of induced phases will not exceed 0.4. The test method proposed in this paper will be helpful to the research on the lightning overvoltage response characteristics of the distribution line and the protection analysis of arresters.

A Mathematical Modeling Approach to Characterize the Growth of the Electrical Tree in XLPE Insulation Under Lightning Impulse Overvoltage

A Mathematical Modeling Approach to Characterize the Growth of the Electrical Tree in XLPE Insulation Under Lightning Impulse Overvoltage

METHODS FOR REDUCING VOLTAGE IN ELECTRIC NETWORKS OF INDUSTRIAL ENTERPRISES TO ENSURE RELIABLE WORK OF ELECTRICAL EQUIPMENT

The article discusses the method of limiting overvoltages in the electrical networks of industrial enterprises to ensure reliable operation of electrical equipment. The features of overvoltages in high-voltage networks are analyzed and measures to limit them are proposed. In the conditions of constantly complicated technological processes, an increase in the degree of responsibility for a break in the power supply, the importance of ensuring the reliable operation of the insulating structure, on which the whole reliability of the functioning of the electric power system depends, increases. In the process of a robot of an electric power system for an electric power supply, a constant power supply is required. In viscous-voltage lancing rods, the head jerel of the external overload is є discharge bliskavki. Impulse overvoltage is broadened to the meaning of the state of the world. Hvilі, you can make it easy for you to do electrical installation, electric wisdom is lower, lower in linear. Vibration of electricity (promotion by electrodes) Power supply and power supply are guilty but economically correct. The robotic complex for coordination and control of the installation for lowering the price of the shredded electronic copying has been spread.

Study on Laminated Bus Bar for High-pulse Power Converter Module

Aiming at the problems of high switching overvoltage of power semiconductor devices and unbalanced current of parallel power devices in high-pulse power converters, this paper proposes a laminated bus bar of symmetrical path on the basis of the theoretical analysis and key points of laminated bus bar. The finite element simulation software was used to analyze the stray inductance of laminated bus bars. The actual stray inductance and overvoltage of the converter commutation circuit were measured by single pulse experiment. The results show that the laminated bus bar of symmetrical path can effectively reduce stray inductance, suppress switching impulse overvoltage and improve the current balance of the parallel power devices. It is of great significance to analyze the performance of laminated bus bar of symmetrical path, and it can be used for high-pulse power conversion technologies.

Enhancing the Protective Performance of Surge Arresters against Indirect Lightning Strikes via an Inductor-Based Filter

Preventing the medium voltage (MV) transformer fault by protecting transformers against indirect lightning strikes plays a crucial role in enhancing the continuous service to electricity consumers. Surge arresters, if selected properly, are efficient devices in providing adequate protection for MV transformers against transient overvoltage impulses while preventing unwanted service interruptions. However, compared to other protective devices such as the spark gap, their prices are relatively high. The higher the surge arrester rating and energy absorption capacity are, the higher the prices go. This paper proposes an inductor-based filter to limit the energy pushed into the surge arrester, and consequently to prevent any unwanted failure. An energy-controlled switch is proposed to simulate the fault of the surge arrester. Surge arresters with different ratings, e.g., 12 kV, 18 kV, 24 kV, 30 kV, 36 kV, and 42 kV with two different classes of energy, namely, type a and type b, are tested under different indirect lightning impulses such as 100 kV, 125 kV, 150 kV, 175 kV, 200 kV, 250 kV, 300 kV, and 500 kV. Furthermore, these surge arresters are equipped with different filter sizes of 100 μH, 250 μH, 500 μH, and 1 mH. Results prove that equipping a surge arrester with a proper filter size enhances the performance of the surge arrester significantly such that a high rating and somewhat expensive surge arrester can be replaced by a low rating and cheap surge arrester while providing similar or even better protective performance for MV transformers. Therefore, such configurations not only enhance the protective capability of surge arrester, but also reduce the planning and operating costs of MV networks.

Evaluation of Filtered Spark Gap on the Lightning Protection of Distribution Transformers: Experimental and Simulation Study

Protection of transformers, as one of the most expensive equipment in the power system, against lightning overvoltage impulses is a vital task. This paper, for the first time so far, investigates the effects of a filtered spark gap on the protection level of transformers against lightning overvoltage impulses. The filter is an inductor that is placed in series with the transformer and before the spark gap aiming to reduce the voltage at the connection point of the spark gap, and hence, enhancing the protection level of the transformer under lightning overvoltages. The experimental laboratory tests are accomplished on a 400 kVA, 22/0.4 kV, Delta-Star ( Δ − Y ) connection type transformer under 110 kV, and 125 kV overvoltage impulses, whereas the size of the spark gap is set to 80 mm and two inductors of 35 μ H and 119 μ H are considered. In order to perform a more in-depth analysis, a model that works reasonably close to the empirical case is developed in the EMTP-RV software. An optimization algorithm is used to determine the sensitive parameters of the double-exponential function, which is used to reproduce the applied laboratory lightning impulse voltages in the EMTP-RV environment. Moreover, the transformer is modeled according to the Cigre Guidelines (Working Group 02 of Study Committee 33). The behavior of the spark gap is simulated as close as the practical situation using the disruptive effect method. The preciseness of the simulated filtered spark gap model is verified by comparing the results of the simulated model in the EMTP-RV with the results of experimental tests. After verifying the model, different sizes of inductors are studied in the EMTP-RV environment to investigate whether larger or smaller inductors provide better protection for the transformer under lightning conditions. A comparison is performed among the conventional spark gap, surge arrester, and the filtered spark gap to provide a better analysis of the potential of the proposed device. The results indicate that proper sizing of the inductor, within an effective range, slightly enhances the protection level of the transformer.

Protection of Asynchronous Motors From Internal Closes of Stator Winding

A method for constructing a high-speed effective protection of large asynchronous motors from arc faults in the stator winding arising from weakening and electrical breakdown of insulation on the machine body and caused by the action of switching or impulse overvoltages has been developed and substantiated. The proposed scheme for detecting arc faults is based on monitoring the active and capacitive conductivity of the winding in-sulation relative to the machine body and violation of the conduction star in case of insulation damage. It is shown that the asynchronous motor protection according to the proposed method will effectively operate both in case of single-phase short-circuits to the case, and inter-turn and phase-to-phase closures, leading to signifi-cant damage to the insulation and winding conductors. The proposed protection has a speed that is inaccessi-ble to modern means of relay protection of motors from such damages. It is proposed to build protection against internal faults on a two-level principle, due to an additional device for quickly extinguishing the residual EMF in the stator winding, induced in it after the motor is disconnected from the network.

Высокочастотные коммутационные перенапряжения в сети среднего напряжения

Conditions under which high-frequency impulse overvoltages occur in medium-voltage (MV) networks with insulated and resistively grounded neutrals are considered. It has been shown from numerical analysis results that voltage resonance may occur at high frequencies. In that case, the overvoltages may reach ten times the nominal voltage value. Owing to the use of arc chutes, resistively grounded neutrals, and metal-oxide surge arresters, the failure rate in MV networks has been decreased; however, faults due to high frequency overvoltages still have not been eliminated. Results from experimental investigation of impulse voltage distribution along the transformer winding are presented. The results of full-scale and simulation experiments have confirmed the possibility of resonance to occur at the medium-voltage level in carrying out switching operations at the high-voltage level. Under the conditions of high-frequency overvoltages, metal-oxide surge arresters are able to protect a limited area. A digital recorder equipped with contactless voltage sensors has been developed for recording high-frequency overvoltages. The overvoltages excited by switching operations at a 10 kV substation have been recorded.

Breakdown Characteristics of C4F7N/CO2 Mixtures Under Steep-Wavefront Impulse Voltages

Steep-wavefront impulse overvoltages may be generated, including fast-front overvoltage (FFO) with wavefront time of 0.1~20 μs, during the operations of disconnector switch in gas insulated equipment (GIE), threatening its insulation. To study the breakdown characteristics of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> alternative C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixtures under FFO, the influence of FFO wavefront time, pressures and polarities on the breakdown characteristics of C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in slightly non-uniform and extremely non-uniform electric field were investigated. The results show that the effects of FFO wavefront time, polarities and pressures on the insulation properties of C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> are similar. In the slightly non-uniform electric field, the insulation performance of 9%C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/91% CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> reaches 78% SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> under the same conditions. The 50% breakdown voltage of gas under impulse voltage (U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> ) of the C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> decreases first then rise with the FFO wavefront time showing a U-curve. It is also found that the voltage polarity has little effect on the trend of the 50% breakdown voltage curve with wavefront time of impulse voltage (U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> -Tf) and the 50% breakdown voltage curve with gas pressure (U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> -P) in the slightly non-uniform electric field. In the extremely non-uniform electric field, the U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> of the C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tends to decrease first and then stabilize with the increase of wavefront time. With the increase of pressure, the U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> of C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> increases linearly in the slightly non-uniform electric field. Under the positive FFO, in the extremely non-uniform electric field, the U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> of the C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> changes with the rise in pressure showed a tendency to increases first, then decreases and rises again, resembling the N letter. Under the negative FFO, the U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50%</sub> of C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> N/CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> rises with the increase of pressure and tends to be saturated.

Аппроксимация переходных резонансных напряжений и токов в обмотках силовых трансформаторов для определения собственных частот колебаний и коэффициентов затухани

Transient interaction between power transformers and power cable lines may give rise to resonance overvoltages in the transformer primary windings. To develop protection measures against resonance overvoltages and to design transformers resistant to resonance overvoltages, it is necessary to know the natural frequencies of the transformer windings. Recent years have seen very rapid development of transformer windings high-frequency models. However, the mathematical models used in practice, which came from calculations of impulse overvoltages in transformer windings, reproduce the frequency dependences of losses and damping at natural frequencies with insufficient accuracy. To verify and improve the mathematical models used for analyzing high-frequency processes in transformer windings, it is necessary to have sufficient experimental data on the values of natural frequencies and damping factors. Methods for experimentally determining the natural frequencies and damping factors of power transformer windings are considered. Theoretical principles and analytical expressions for transient voltages and currents obtained for simplified equivalent circuits of windings with lumped parameters are given. An approach is proposed, according to which the transient voltages and currents in the winding are represented as the sum of steady-state and free components. The free component is then approximated using the theoretical expressions obtained for the equivalent circuits of the windings. The results of applying the approach to approximating the transient voltage at the midpoint and the current in the neutral of a dry-type transformer’s high-voltage winding are presented.

Investigation on the Resonant Oscillations in an 11 kV Distribution Transformer under Standard and Chopped Lightning Impulse Overvoltages with Different Shield Placement Configurations

This paper presents an investigation on the resonant oscillations of an 11 kV layer-type winding transformer under standard and chopped lightning impulse overvoltage conditions based on calculated parameters. The resistances, inductances and capacitances were calculated in order to develop the transformer winding equivalent circuit. The impulse overvoltages were applied to the high voltage (HV) winding and the resonant oscillations were simulated for each of the layers based on different electrostatic shield placement configurations. It is found that the placement of grounded shields between layer 13 and layer 14 results in the highest resonant oscillation and non-linear initial voltage distribution. The oscillation and linear stress distributions are at the lowest for shield placement between the HV and low voltage (LV) windings.

Electrical Tree Characteristics in Polypropylene Under Impulse Superimposed DC Voltage in LN2

During the operation of the high-temperature superconducting (HTS) dc cable, due to the operation of the power electronic device, the cable insulation is subjected to the repeated impulse overvoltage while being subjected to the rated dc voltage, which easily generates the electrical tree and causes insulation failure. Polypropylene, the primary substrate for polypropylene laminated paper, faces a working environment in liquid nitrogen (LN 2 ). The insulation performance of polypropylene in a low-temperature environment may change and affect its degradation process, threatening the safe and stable operation of power system. In this paper, a needle-plane electrode system was used to study the electrical tree characteristics in LN 2 . The results show that the tree morphology changes with different impulse frequency under dc-impulse voltage. With the increase in the positive dc voltage, the treeing process is accelerated. However, with the increase in the negative dc amplitude, the tree length decreases. With the impulse frequency increasing, the tree length increases significantly.

Improving the Electric Field Distribution in Stress Cone of HTS DC Cable Terminals by Nonlinear Conductive Epoxy/ZnO Composites

Nonuniform electric field distribution is one of the key issues for the superconducting cable terminals. This paper tries to modify the electric field distribution in HTS dc cable terminals by the nonlinear conductive epoxy/ZnO insulation, instead of the insulation with a fixed conductivity. The nonlinear conductivity of epoxy/ZnO composites is measured, and a finite element simulation model of the +100 kV HTS dc cable terminal is established to analyze the electric field distribution at the steady and transient state. The results show that the field-dependent conductivity has a great effect on the dc electric field distribution. If the nonlinear coefficient is excessively large, the electric field in the polypropylene laminated paper (PPLP) will be extremely strong; otherwise, the electric field in the terminal insulation will be extremely strong. The epoxy/ZnO composites with an appropriate nonlinear conductivity can effectively uniform the electric field distribution at the triple-junction and the stress cone surface. When the switching overvoltage impulse superimposed dc voltage is applied, the maximum field is always located in the cable insulation of PPLP, which is because that the electric field distribution in the stress cone is mainly dependent on the capacitance characteristics of the insulation instead of the conductivity. The nonlinear conductive epoxy/ZnO composites could play an important role in promoting the development of stress cone of HTS dc cable terminals.

Design and Test of 500-kV Lightning Protection Insulator

In this paper, based on insulation coordination and simulation, a lightning protection composite insulator is proposed, made up of an insulation part and an arrester part connected in series, whose length is close to that of the standard composite insulator, which met the window size requirements of the tower. The arrester part contains zinc oxide resistors and an internal core rod, which has good explosion-proof performance and tensile strength, in the meantime overcomes the problem that arresters cannot bear the tensile load of transmission lines. Electrical simulations and experiments have shown that the shielding failure current can be released through the proposed lightning protection insulator to the ground and the counterattack current can be prevented from entering the line. The dry-arc distances of the arrester part and the insulation part are 2.4 and 2.2 m respectively, and the grading ring distance is 1.8 m. The maximum AC withstanding voltage reaches 700 kV, and the switching overvoltage impulse withstanding voltage reaches 1200 kV. The proposed insulator can prevent lightning accidents from happening. Moreover, its length, tensile load, and installation mode are the same as existing line insulators, which has the advantages of low cost and ease of installation and maintenance.