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The short-circuit withstanding performance of busbar system is one of the most important safety indexes for low-voltage (LV) switchgear. The resonance characteristics, short-circuit displacement, and stress concentration of four typical busbar system arrangements are numerically analysed in this study. First, modal analysis is used to calculate the vibration modes and natural frequencies of the busbar systems. The influence of span length and phase-to-phase distance is discussed and thresholds for resonance prevention are given. Then, electromagnetic-structural coupled models are built to simulate the short-circuit mechanical response. The time-varying displacement and stress are obtained and the dynamic stability of typical arrangements is compared. The proposed results can provide theoretical reference for the dynamic stability design of busbar system in LV switchgear.

This paper introduces a contact resistance model of bolt structures based on virtual material method. The main idea of the model is to interpose a virtual layer between the overlapped bus bar and define properties including equivalent resistivity and equivalent modulus of elasticity to describe the dynamic characteristics of the distributed contact resistance. The determining method of these properties is introduced, and the validity of the model is demonstrated by comparing the calculated contact resistance with the measured results. The model is further applied to the coupled electromagnetic–stress–thermal finite-element calculation to analyze the thermal stability of the bolt structure. The temperature rise process inside the bolt structure is studied, and the strengthening effect of the electrodynamic force on the thermal field is revealed. Besides, the influences of contact loosening and oxidation are also researched with the method.

This study aims to monitor the temperature inside power cable joint, with strong robustness to variable thermal environments and uncertain thermal parameters of the joint. The model consists of two sequential steps, radial-direction temperature inversion in the cable and axial-direction temperature inversion in the conductor. The former part was improved by coating the cable with a heat insulated layer, whereas the latter one was optimised through a proper parameter selection. Afterwards, an outdoor cable joint temperature-rise test was carried out and the temperature obtained from the proposed approach agrees well with the measured one. With the great change in ambient temperature and wind speed during the test, the model accuracy remains almost constant, and this is due to the heat insulated layer and low sensitivity coefficient of the algorithm. Besides, if the thermal conductivity and volumetric specific heat of waterproof compound in cable joint are assumed to increase by nearly five times, the maximum changes in temperature monitoring are only 1.4 and 0.4°C, respectively. The results indicate that this approach can effectively overcome the effects of variable thermal environments and thermal parameter dispersion in joint, thus promoting practical applications in temperature monitoring for cable joints.

Full-scale discharge tests of transmission line–tower air gaps are costly and time-consuming, and they cannot exhaustively simulate all the gap configurations in practical engineering. In this paper, a machine learning model established by support vector machine is introduced to predict the switching impulse discharge voltages of the ultra-high-voltage (UHV) AC transmission line–tower air gaps. The three-dimensional finite element models of a UHV cup-type tower and a UHV compact transmission line were established for electric field calculation, and some features were extracted from the hypothetic discharge channel and the shortest path between the bundled conductor and the tower. These features under a given voltage were normalised and input to the SVM model, while the output is two binary values, respectively, representing gap withstanding or breakdown. Trained by experimental data of one type of the UHV transmission line–tower gaps, the SVM model is able to predict the discharge voltages of another gap type. The mean absolute percentage errors of the two engineering gap types, under different gap distances, are 8.31 and 4.86%, respectively, which are acceptable for engineering applications. The results provide a possible way to obtain the discharge voltages of complicated engineering gaps by mathematical calculations.

This paper proposed a prediction method to predict a 10-kV oil-immersed transformer hot spot temperature (HST). A set of feature temperature points on the transformer iron shell is proposed based on fluid-thermal field calculation. These feature points, as well as transformer load rate, are taken as the input parameters of a machine learning model established by support vector regression (SVR), thus to describe their relationships with the HST. This model is trained by nine samples selected by L9(34) orthogonal array and applied to predict the HST of 20 test samples. The training samples are all obtained by simulation, and the test samples have consisted of simulation and transformer temperature rise test results. With effective parameter optimization of the SVR model, the predicted results agree well with the experimental and simulation data, the mean absolute percentage error (MAPE) is 1.55%, and the maximum temperature difference is less than 3 °C. The results validated the validity and the generalization performance of the prediction model.

The hybrid high-voltage direct current (HVDC) breaker combines mechanical and power electronics switching that enables it to interrupt power flows within a few milliseconds. Mechanical switch is a key component of hybrid HVDC breaker and has a number of serially connected interrupter units which ideally would divide the voltage equally. The static voltage distribution characteristics and voltage sharing design of a multi-break mechanical switch were discussed in this study. A finite-element model was developed to study the static voltage distribution characteristics and capacitance parameters of multi-break mechanical switch (which actually consists of resistance and capacitance parameters under direct current) as a preliminary study. Comparisons were made under the simulation of vertical and U-shaped arrangement forms. The results indicate that the static voltage distribution of the high-voltage terminal is at least more than 65%, whereas the severe non-uniform voltage distribution can be well improved by means of the method proposed in this study.

Purpose The purpose of this paper is numerical calculation of total electric field in oil-paper insulation. Now, there is no effective method to consider the influence of space charges when calculating the total electric field distribution in the main insulation system of the valve-side winding of an ultra-high-voltage direct current converter transformer. Design/methodology/approach To calculate the total electric field in an oil-paper insulation system, a new simulation method in single-layer oil-paper insulation based on the transient upstream finite element method (TUFEM) is proposed, in which the time variable is considered. The TUFEM is used to calculate the total electric field in an oil-paper insulation system by considering the move law of space charges. The simulation method is verified by comparing the simulation results to the test data. The move law of space charges and distribution characteristics of the electric field under difference voltage values in single-layer oil-paper insulation were presented. Findings The results show that the TUFEM has an excellent accuracy compared with the test data. When carrier mobility is a constant, the time to reach the steady state is inversely correlated with the initial electric field intensity, and the distortion rate of the internal total electric field is positively correlated with the initial electric field intensity. Originality/value This paper provides an exploratory research on the simulation of space charge transport phenomenon in oil-paper and has guiding significance to the design of oil-paper insulation.

A quasi-3D coupled-field method is introduced and applied on a ventilated dry-type transformer to study temperature rise of windings in this study. A simplified 3D model was first established to calculate energy loss of core and velocity distribution in a plane above the lower yoke. Then two accurate 2D models were built up to figure out energy losses in the windings. With a combination of indirect and sequential coupling, energy losses of both windings and core were used as heat source, and velocities for both 2D models were applied as boundary condition for analysing fluid-thermal field. Final results of temperature rise were calculated with temperature rise of two 2D models. In the end, numerical results were compared with experimental data to prove the effectiveness of this method.

A series of experiments are conducted on a 252-kV pantograph disconnector offline to monitor its mechanical condition. Three parameters, namely the dynamic strain of the post insulator, the rotation angle of the operating shaft, and the stator current of the driving motor, are measured by resistance strain gauges, magnetic-sensing angle transducer, and Hall current sensor, respectively. Measurement results of the three parameters under mechanical fault conditions including clamping stagnation of the conduct tube knuckle, failure of the balance spring, and uncompleted closing operation are compared with those in normal operating condition of the disconnector. The results indicate that the dynamic strain waveform, the rotation angle curve, and the motor current envelope waveform may get distorted with increased peak value and offset peak time when mechanical faults happen to the transmission or rotating mechanism of the disconnector, and more component of the side frequency will appear in the spectrum of the motor current waveform. The proposed methods are validated to be effective for mechanical fault diagnosis and condition assessment of pantograph disconnector, and are also of reference significance for other types of disconnectors. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Monitoring temperature inside the power cable joint is of great meaning to the safety of power distribution, while measuring it directly is infeasible. In this paper, the authors propose a new method to invert real-time hot spot temperature of conductor in the single-core power cable joint. This method consists of two parts, the radial-direction temperature inversion (RDTI) in the cable and axial-direction temperature inversion (ADTI) in the conductor. With this method, hot spot temperature of conductor in the cable joint could be figured out with the surface temperatures of the cable nearby the cable joint and load current, both of which are measurable. Then, the authors carry it out on an actual power cable joint in the laboratory, and the inversed temperatures are compared with experiment data, which come from the indoor experiment platform, to validate the availability of this method. The maximum error of this method is 6%, and it shows that this method for temperature inversion is feasible in laboratory scale.

Live-line work methods using helicopters in the vicinity of 1000-kV transmission lines were simulated with finite-element modeling of the electric-field distributions. The effect of the approach path of the helicopter live-line work platform toward the energized conductor was computed. Predicted electric fields on the line worker head, body, arms, and legs were compared with measured results from a single-phase ultra-high-voltage (UHV) mock-up, and with calculation results from the simulation of an extra-high-voltage (EHV) 500-kV transmission-line geometry. The UHV conductor-to-work platform discharge occurred at a distance of about 2 m, in line with standard flashover models. Additional shielding efficiency of about 4 dB is recommended for the electrically conductive clothing when working on the UHV lines in China, compared to EHV lines.

Corona inception can be viewed as the release of capacitive energy stored in the electric field. A new prediction method of the corona onset voltage based on electric field features and support vector machine (SVM) is proposed in this study. This method was used to predict the positive DC corona onset voltage of the rod-plane air gap. The predicted results were compared with the experimental values and the calculated values of the existing methods. Then, the corona onset voltage of the shielding sphere used in a valve hall of ±660 kV DC converter station was measured and predicted by the SVM model and the photoionisation model. The safe values of corona inception electric field strength were obtained. By comparing the finite element numerical calculation results of the electric field strength with the safe value, it was proved that the corona would not incept on the surface of the shielding sphere under operating voltage. This method provides a new possible way to predict corona onset voltages of valve hall fittings used in HVDC transmission projects.

This paper attempts to establish the grey correlation between the breakdown voltage of air gap and its energy storage status before discharge inception. A new method is presented for the breakdown voltage prediction of short air gaps. This method is based on an orthogonal design array and support vector machine (SVM). A hybrid model is established to predict the power frequency breakdown voltage of three typical air gaps including sphere-sphere, rod-plane, and rod-rod gaps. The input data are fifty parameters extracted from the static electric field distribution of the air gap, and the output of the model is whether the gap will breakdown under a given voltage. This model is trained by 9 samples selected by L 9 (33) orthogonal array and applied to predict the breakdown voltage of 90 test samples. With effective feature dimension reduction and parameter optimization of the SVM model, the predicted results coincide well with the experimental data, and the mean absolute percentage error (MAPE) is only 3.9%. The SVM model is also applied to predict the breakdown voltage of some atypical short air gaps including sphere-plane gaps, rod-sphere gaps, sphere-sphere gaps with different diameters, and rod-plane-sphere gaps. The MAPEs are respectively 4.3, 3.2, 4.2, and 6.4%, which verifies the validity and generalization performance of the proposed method. This model provides a new way to gain the critical breakdown voltage of air gaps, and therefore contributes to reducing the required test work.

Transient recovery voltage (TRV) distribution ratio and voltage sharing measure of series arrangement double- and triple-break vacuum circuit breakers (VCBs) were studied in this paper. The mathematical analysis of the TRV distribution ratio in a double-break VCB shows that the uneven TRV distribution ratio is caused by the stray capacitances and the imbalanced postarc current as a result of the difference of the postarc plasma characteristics in each vacuum interrupter. This has been proven by the synthetic breaking tests and simulation results based on a modified vacuum arc interruption model of a double-break VCB. On this basis, voltage sharing measure for a 126-kV triple-break VCB designed for the 110-kV power network has been discussed. The synthetic breaking test results indicated that the 126-kV triple-break VCB with 1000-pF grading capacitors has a substantially uniform TRV distribution, and can successfully break the rated short-circuit interrupting current (40 kArms).

The background of air insulation prediction research is briefly reviewed in this chapter, including the research and development on air discharge tests, discharge theories and physical models. Some beneficial inspirations extracted from the existing researches are concluded. The research assumption of air insulation prediction is described, mainly including the research ideas, implementation method and key technologies. The contents of this book is briefly introduced.

Air gap breakdown voltage is an important basis for the design of air insulation structure, which is related to applied voltage type and voltage waveform. The power frequency AC voltage and DC voltage are collectively called steady-state voltage or continuous voltage. Their change rates are very small, so the discharge development time is negligible compared with the voltage change rate. In this chapter, the air gap breakdown characteristics under steady-state voltage are introduced firstly, and then the air insulation prediction model is used to predict the power frequency breakdown voltages of air gaps with typical and atypical electrodes.

The hybrid high voltage direct current (HVDC) breaker is a key equipment of the DC grid. The high speed mechanical switch is a key component of the hybrid HVDC breaker and has a number of serially connected switch units. The serially connected units should divide the voltage uniformly. The static voltage distributions and distributed capacitances of a 500 kV hybrid HVDC breaker mechanical switch are studied in this paper. A three dimensional finite element model, considering the actual dimension parameters of six 110 kV high speed mechanical units in serially vertical arrangement, is established to study the static voltage distribution characteristics and capacitance parameters of a 500 kV multi-break high-speed mechanical switch, with six 110 kV high speed mechanical switches in serially vertical arrangement. The calculated results indicate that, under vertical arrangement, from the point of capacitive voltage distribution, the static voltage distributions of the six mechanical switch units are non-uniform, and the high voltage unit is more than 75% of the whole voltage.

The static and dynamic voltage distribution characteristics and voltage sharing design of a 126-kV modular triple-break vacuum circuit breaker (VCB) was discussed in this paper. The finite-element method and power frequency tests were used to calculate and verify the static voltage distribution ratios and distributed capacitance parameters of multi-break VCBs, respectively. A model combining the post arc current model and equivalent capacitance model was proposed for simulations of dynamic transient recovery voltage (TRV) distribution characteristics of the triple-break VCB. The simulation results indicated that besides the stray capacitances, the TRV sharing design should take the influence of residual charge (RC) into account, and the influence of RC on the TRV distribution is relevant to the differences of RC parameters among the series interrupters. Therefore, the appropriate value of grading capacitors should meet the requirement of the worst case. Based on the investigation of this paper, a 126 kV U-shaped triple-break VCB prototype with 1000 pF grading capacitors has been produced, and the successful large current breaking tests indicate the good breaking capacity and suitable voltage sharing design of the triple-break VCB.

Breakdown voltage of the air gap is of vital importance for the design of the external insulation in high-voltage transmission and transformation projects. In this paper, a new prediction method for the breakdown voltages of typical air gaps based on the electric field features and support vector machine (SVM) was proposed. According to the finite element calculation results of static electric field distribution, the electric field values in the whole region, discharge channel, surface of the electrode and the shortest path were extracted and post-processed, which constituted the electric field features characterizing the gap structure. Then, the breakdown voltage prediction model of the air gap was established by using electric field features as the input parameters to SVM, and whether the gap breakdown would happen as the output parameters of SVM, which changing the regression problem to a binary classification problem. This model was applied to predict the power frequency breakdown voltages of different short air gaps including sphere-sphere gaps, rod-plane gaps, sphere-plane gaps and sphereplane- sphere gaps. The power frequency breakdown voltages of longer air gaps which are affected by corona, and the 50% positive switching impulse breakdown voltages of long sphere-plane gaps and rod-plane gaps were predicted as well. The predicted results agree well with experimental values and simulated results of the published models, which validate the effectiveness of the proposed model. This method supplies a new possible way to obtain the breakdown voltage of air gaps.

This paper analyzes the influences of the factors in a capacitor-driven multi-stage synchronous induction coilgun (SICG) and optimal combination of the design parameters based on orthogonal experiment design (OED). Finite element transient solver is used to calculate the motion characteristics of a multi-stage SICG. The muzzle speed is set to be the target of the optimization. System parameters (factors), levels and orthogonal array (OA) are determined by the order. Three optimization schemes are compared, including stage-by-stage optimization, section optimization and full optimization. Optimal combination of the three-stage SICG is obtained, as well as important order of factors. It shows that a set of comparatively better parameters and relatively higher efficiency could be achieved through orthogonal optimum. Wire diameter and coil diameter are the most important factors that influence the system efficiency of the launcher. Besides them, length of coil, trigger position and number of turns have less influence for the system in the important order. Section optimization could reduce the amount of computations compared to full optimization and a set of comparatively better parameters and relatively higher efficiency could be achieved. It is suggested to optimize a multi-stage SICG. How to choose the factors from lots of parameters is according to the engineering practice. Improper choices of the value in levels may cause unable to reach an optimum. The conclusion will be useful for coilgun design.

There has been a 1000 kV ultra-high-voltage (UHV) AC transmission line running in China, but the related papers introducing the approach path of the helicopter live-line work platform (HLLWP) are still not presented. The model of the HLLWP approaching the transmission lines were established in this paper, based on the real parameters of helicopter MD500 and 1000 kV UHV AC transmission lines. Through calculating electrostatic field with the finite element method, the results of the electric field were obtained when the HLLWP was at different locations nearby a certain phase and approaching different phases. The surface electric fields on the HLLWP and operator’s conductive clothing at different locations were analyzed and then the optimal approaching paths of different phases based on the consideration of the HLLWP and operator’s safety were proposed to provide suggestions for the operations of the HLLWP. In the end, the experimental data was compared with the numerical result to prove the reliability of this method.

With the development and increasing use of intelligent vacuum circuit breaker (VCB), the electromagnetic compatibility problem between VCBs and their low-voltage electronic equipment is becoming increasingly prominent. Switching operations of circuit breakers are known to be serious sources of electromagnetic interference (EMI) to their secondary equipment, which can lead to the control and protection devices display error and malfunction. Hence, it is very important to study the EMI problem of the secondary equipment (such as current transformer (CT), PT, and controller) caused by the operations of circuit breakers. This paper carried out the tests of a 10 kV medium-voltage outdoor pole-mounted switch by a synthetic test circuit with large current and high impulse voltage sources, and collected the current signals of the secondary circuit at the same time, then obtained the frequency and energy distributions of the disturbance signals, caused by the breaking operation of pole-mounted switch, by time and frequency characteristic analysis with the wavelet analysis. Results show that the secondary side of the CT and the feeder terminal unit (FTU) controller are bearing EMI when pole-mounted switch is in operation and the distribution of the frequency spectrum is wide. The most serious disturbance appears at the time of arc current passing zero and the frequency distribution is mainly in 7.81–15.63 MHz. The EMI caused by transient recovery voltage is mainly distribution in 31.25–125.0 MHz. The maximum amplitude of disturbance pulses is about 4.5 times as large as that of the normal case. The results can provide a reference to the measures against the EMI for the pole-mounted switch's FTU controller.

In the launching experiment of a coilgun, the driving coil, including winding and shielding, is always destroyed after several launching times. To improve the service life of a coilgun, the cumulative damage mechanism of the shielding and the fatigue life assessment of the winding are investigated in this paper. First, the electromagnetic field of a coilgun, considering the kinematic influence, is simulated using the field-circuit time-stepping finite-element method. Afterward, the result of the electromagnetic force (EMF) is applied to the structural field as the load to calculate the stress and displacement distributions. In addition, using the static strength assessment, it is determined whether there is a defect in the materials. For the shielding made of a nonmetallic material, if the stress is greater than the ultimate tensile strength (UTS), cracks must emerge. For the winding made of a metallic material, even though the stress is less than UTS, the fatigue failures still occur with the application of repetitive loads. The analysis results of the cumulative failure show that the shielding is destroyed after several launching times. According to the stress-life characteristics (expressed as S-N curves) of the winding material and the analysis results of fatigue, the service life of the winding is much longer than that of the shielding. Finally, a type of insulating material with higher UTS is proposed to improve the service life of the coilgun.

This paper focuses on the security condition of line worker during the implementation of helicopter live-line work by platform method on 1000 kV ultrahigh voltage (UHV) transmission lines. First, a model of helicopter live-line work is presented, and the electric field distribution of line worker when performing live-line work on 1000 kV UHV transmission lines is analyzed. Then, the capacitances between helicopter live-line work platform, transmission lines, and ground are obtained by energy method, and an equivalent circuit model is presented to analyze the current characteristic of bonding procedure. At the end of this paper, an experiment of helicopter live-line work is performed, the electric field distributions of line worker and the discharge current are measured and compared with simulation results, which will be used to judge the security condition of line worker and the feasibility of helicopter live-line work by platform method on 1000 kV UHV transmission lines.

This paper presents a fluid-thermal coupled analysis method to compute the temperature distribution in a 31.5 MVA/110 kV oil natural air natural (ONAN) three-phase three-limb transformer. The power losses of windings and core are measured by load-loss test and no-load test respectively. The convective heat transfer process, radiation and oil flow inside the transformer are investigated by finite volume method (FVM). In order to validate the feasibility and accuracy of the presented method, the temperature measuring system based on fiber Brag grating (FBG) sensor is constructed for the temperature rise test of the 31.5 MVA/110 kV ONAN transformer. The simulation results deduced from the proposed method agree well with experimental data. This model can be applied to optimizing design and load scheduling.

This paper takes a typical 220 kV three-phase three-limb oil-immersed transformer as an example, this paper building transient field-circuit coupled model and 3D coupled magneto -fluid-thermal model. Considering a nonlinear B–H curve, the magneto model uses the field-circuit coupled finite element method (FEM) to calculate the magnetic flux distribution of the core and the current distribution of the windings when the transformer is at a rated current and under direct current (DC) bias. Taking the electric power losses of the core and windings as a heat source, the temperature inside the transformer and the velocity of the transformer oil are analyzed by the finite volume method (FVM) in a fluid-thermal field. In order to improve the accuracy of the calculation results, the influence of temperature on the electrical resistivity of the windings and the physical parameter of the transformer oil are taken into account in the paper. Meanwhile, the convective heat transfer coefficient of the FVM model boundary is determined by its temperature. By iterative computations, the model is updated according to the thermal field calculation result until the maximum difference in hot spot temperature between the two adjacent steps is less than 0.01 K. The result calculated by the coupling method agrees well with the empirical equation result according to IEC 60076-7.

SUMMARY With the fast development of China's power grid construction, many transmission lines' corridors are inevitable across mountain with luxuriant vegetation. The occurrence of forest fires under transmission lines may lead to power interruptions, seriously affecting the stability of the grid operation. The characteristics of fire-induced flashovers are analyzed by statistical data of 220–500-kV lines trips in recent years. The line trip out rate caused by forest fires varied according to seasons and climate. The fire-induced flashover is much more serious than lighting trips. AC and DC voltage discharge tests of transmission line gap under different fire conditions are carried out by using wood crib fire. Test results indicate that: the gap's insulation strength under fire conditions is reduced to about 20% of that under pure air; the values of breakdown voltage have been decreased about 30% after alkali metal added; the gap breakdown under DC voltage has obvious polarity effect, and the flame shape and leakage current waveform are different under positive DC voltage and negative DC voltage. The analysis of gap breakdown mechanism under fire conditions shows that temperature, flame conductivity and ashes are main factors. Their combined effect significantly reduces the gap's insulation strength. Copyright © 2014 John Wiley & Sons, Ltd.

Multiprojectile railguns are novel electromagnetic launchers because of their ability to launch synchronously many projectiles per shot. Salvo performance, which means that all projectiles can leave the muzzle at the same time and hit the target simultaneously, is very important to increase lethality. However, several armature factors caused the asymmetric conditions to reduce the performance, such as different initial position, friction, and mass. To acquire a better salvo performance, we proposed and compared two types of double-projectile railgun structures, in which rail pairs are stacked and paralleled, respectively. With different armature positions, a 3-D harmonic eddy current field analysis is conducted to calculate the self- and mutual-inductance gradients. The results show that the values of the mutual-inductance gradient of the two pairs of rails $(M_{12}^{\prime}, M_{21}^{\prime})$ are different $(M_{12}^{\prime}\!\ne\!M_{21}^{\prime})$ . The circuit simulation was conducted to verify the influence of different initial position, friction, and mass in the launching process. It is found that the armature falling behind always gets larger propulsive force than the front one for stacked railguns, but the situation is opposite for paralleled railguns. Finally, two small-size prototypes are designed and manufactured to compare their salvo performance. The results indicate that the salvo performance of the stacked railgun is better than that of the paralleled railgun. Therefore, this configuration should be considered in the design of a multiprojectile railgun.

Shielding enclosure is very important for a coil launcher system. Shielding performance directly determines the electromagnetic performance of a coil launcher. In this paper, the formulation of the eddy current field involving movement is deduced, and the time step transient finite element method calculation is applied to investigate the shielding performance of a coil launcher. The simulation model of a three-stage coaxial induction coil launcher is built based on ANSOFT MAXWELL software. The transient-simulation model, which is involved in moving conductor eddy current, the pulsed power generator, and the power electronic component, is built as a circuit simulation in MAXWELL schematic module. The circuit model is well coupled with the field model in a transient solver. In every time step, the value of current obtained by the circuit simulation in advance is loaded in the field model as an exciting source to the coils. Three-typical shielding materials, iron, steel, and ferrite are used in 2-D field model to study the influence of shielding enclosure on the electromagnetic performance of the coil launcher. Silicon steel (laminated sheets) is taken in the 3-D field model. Different geometric dimensions of shielding enclosure are also taken into consideration. The speed of the armature and the magnetic field are calculated. The current distribution in the armature and shielding enclosing are displayed as well. Effects of shielding enclosure on the electromagnetic performance are then concluded. A ferrite shielding enclosure will increase the speed of armature obviously. The conductive material will induce eddy currents and decrease the leakage magnetic field outside, that shows excellent shielding performance. The silicon steel shielding enclosure has a similar impact on the speed of the armature to the ferrite, whereas it has better shielding effectiveness than the latter. Research results show that the silicon steel is suitable for the coil launcher shielding enclosure.

During the past few decades, various calculation models have been introduced into studying solid armature railgun. In this paper, four models, including constant resistance, speed resistance, current resistance, and multifactor resistance, were studied in detail. To verify their validity, experimental and simulation methods were adopted in both low- and high-velocity cases. By conducting a 3-D eddy current electromagnetic field simulation with the finite-element numerical method, the self-inductance gradient $(L^{ \prime}_{r})$ was got. The control equations of these different resistance models can be solved by the Runge–Kutta method. Comparisons of experimental and simulation results of these different resistance models indicated that an appropriate resistance model according to the experimental condition can improve the accuracy of simulation analysis. The friction coefficient resistance model can describe armature resistance during the movement process in both low- and high-velocity cases.

The eddy current and force distribution on the plunger of a single-stage induction damper is analyzed for the numerical investigation and design improvement of induction dampers. A 3-D finite-element method is applied to the eddy current analysis of the magnetic field, and the Lorentz force on the plunger is calculated using the nodal force method. The simulation results show that the Lorentz force on the plunger can be increased by installing a ferromagnetic enclosure around the coil. Furthermore, an induction damper model is proposed considering the skin effect of eddy current on the plunger. The results show that the flux interlinking the plunger is increased by reducing the thickness of the plunger. Finally, the dynamic simulation is carried out to verify the design and investigate the dynamic characteristics of the induction damper. The damper with a ferromagnetic groove enclosing the coil is proven to be more effective.

Scaling methods are widely used in railgun technology. But nonlinear problems are seldom studied because of the complexity and uncertainty. To employ the scaling method better, this paper brings the nonlinear drag model to the solid armature railgun circuit. Several drag models are compared including friction drag model (FDM), atmosphere drag model (ADM), and current drag model (CDM). Then, nonlinear scaling relationships of these drag models are discussed. Furthermore, a novel experiment is constructed to compare the drag models at low velocity. At last, the nonlinear scaling method is verified by an example at high velocity. FDM is accurate enough to simulate a railgun system at any condition. Coefficient $\varepsilon$ in FDM should be used considering different test conditions. Scaling relationships of FDM could not be derived at low velocity because of nonlinearity. Although at high velocity, FDM could be simplified to CDM and realize linear scaling method (LSM). Further research indicates that the FDM could use the scaling method directly and realize approximate LSM. CDM is workable which satisfies the conditions of LSM in nature. Due to the work range of the solid armature, usually under 2.5 km/s, nonlinear scaling method could be used in a railgun with FDM. For a solid armature railgun, the $f_{\rm ADM}$ is too small compared with $f_{\rm FDM}$ and it could be ignored in analysis of a railgun, especially at low velocity. What should be noted is that the velocity scaling factor must be controlled above about one-quarter to avoid the nonlinear field.

Performance analysis of a coil launcher is very important for experimental research and electromagnetic optimization design. To check the effect of the improvement and analyze electromagnetic transient in a coil launcher, a field coupling circuit simulation method is introduced in this paper. Initially, the current of drive coils is calculated by the circuit model based on improved current filament method, and then the exciting current is loaded in 2-D axisymmetric field model. The field model is built based on nonoverlapping mortar finite-element method (NO-MFEM). NO-MFEM divides the whole domain into two subdomains: one contains the movable part (armature), and the other contains the source current (coils). The two subdomains are discretized independently and the two sets of meshes are nonconforming on the interface. When the movable part changes its location, it is necessary only to change the node coordinates in movable subdomains and information of mortar nodes and elements. In this paper, performance analysis of a three-stage coaxial induction coil launcher is carried out based on the proposed field-circuit method. The correspondence of the simulation results proves the validity of the field-circuit method.

The conventional metallic grounding material such as copper, steel and alloys plating used in power system nowadays are facing lots of problems such as corrosion, bad contact with soil due to its hardness, high construction cost and so on. To solve these problems, a new type of grounding material — the flexible graphite-cooper composited grounding material (FGCG) was developed. Firstly, ingredients and structure of FGCG was described briefly. Secondly, the grounding and electromagnetic characteristics of FGCG was studied by simulation and experiment, the advantage of FGCG in terms of close contact with soil was researched. Finally, the feasibility of applying FGCG in the transmission tower grounding grid is analyzed and a brief illustration of the application of FGCG in 500kV transmission tower grounding grid is given.

The breakdown voltage of typical electrode structures is important for the design of external insulation, but little attention is paid to the analysis of floating metals, which is common in high voltage transmission and transformation projects. Thus, photoionization criterion was adopted in this paper to calculate the breakdown voltage of sphere-plane-sphere air gap in slightly uneven electric field with the consideration of floating metals, and the power-frequency withstand voltage tests were carried out for the comparison. The results indicate that the calculation results agreed well with the test ones, where the mean absolute percentage errors are less than 3%. The study verifies the validity and generalization values of photoionization criterion within a certain range, and contributes to explaining the discharge mechanism of complex air gap in slightly uneven electric field.

This paper take the receiving-end power grid of ± 800kV Xiluodu-Zhexi (Xizhe) UHVDC project as an example, According to the topological structure of Zhejiang power grid and wide-earth model, a DC bias model of Xizhe UHVDC receiving-end power grid was established based on field-circuit coupling method. The scope considered in this simulation model: regard the grounding electrode as the center, for substation of 500kV and 220kV with earthed neutrals within 100km and substation of 110kV with earthed neutrals within 50km, the simulate calculation model is constructed. The soil structure was measured by Magneto-telluric method. Based on the measured data, the soil structure was considered as a horizontal multi-layer soil model. An experiment was conducted to test the validity of simulation model: measure the DC current flow through neutral point of each transformer of all sites while Jinhua grounding electrode was working in monopole operation and ground current was 500A DC. Experiment data shows that theoretical analysis results consistent with the experimental results. Suppression strategy was proposed and proved to be effective.

When the fault Arc inside the medium-voltage (MV) metal enclosed switchgear happens, the arc heats the filling gas in the switchgear, resulting in pressure rise, which may seriously damage the electrical equipment and building and may even endanger personnel. In order to put forward an appropriate calculation method to calculate the pressure effect caused by the internal arc in the MV switchgear with larger size and complicated structure, the arcing characteristic was introduced and an indirect multi-physics coupling analysis method based on CFD has been put forward. The simulation and test models with a small size were established, and the internal pressure rise was calculated under different arc sizes and the surface areas of pressure relief opening. The results show that, when using the indirect coupling analysis method, the relative error of average pressure between test and simulation calculation is just about 2%. The size of arc has no effect on the pressure distribution, but large influence on the temperature distribution. The change of pressure distribution in the simulation switchgear is bigger than temperature before and after the pressure relief disc opens. Hence, in order to simplify the calculation, the pressure distribution caused by internal arc in the MV switchgear obtained by the indirect coupling analysis method is feasible.

Corona discharge will influence the partial discharge measurement during direct current (DC) withstand voltage test for ultra high voltage (UHV) converter transformers. This paper discussed the corona control measures for corona ring and foil expansion pipe, through finite element method (FEM) simulation. An 800 kV converter transformer was taken as object. The influences of the size of corona rings and arrangement of foil expansion pipe on their surface electric field intensity during the test were discussed. Maximum surface electric field intensity values under different arrangement are given. The results of this paper can provide some reference to actual test of UHV DC transmission projects.

External insulation characteristic is the key issues in the design and operation process of high voltage transportation and transfer facilities, whose fundamental base is the breakdown characteristic of air gap. Applying artificial intelligence algorithm using machine learning to design external insulation of transmission line is one of the new directions in the study of external insulation. Therefore, a new method using the breakdown voltage of typical electrode gaps to predict the breakdown voltage of complex gaps is proposed, which is based on electric field features and support vector machine (SVM). The electric field distribution was calculated by finite element method and the electric field features were extracted to characterize the gap geometries. The prediction model was established by SVM, and the electric field features were set as input parameters of the model, while the output was whether the gap would breakdown under a given voltage. Applying this method, sphere gap and rod-plane gap is chosen as training samples to predict the breakdown voltage of ring-plane and ring-ring gaps and the calculated results are in good agreement with the experimental values by comparison. This method provides a new way for getting the breakdown voltage of complex air gaps. So it is conducive to decrease test work and reduce experimental cost.

Although in recent years, the electromagnetic repulsion mechanism based on the eddy current repulsive force quickly become a hotspot, the mechanism can't apply to circuit breaker of long clearance between open contacts and high voltage. So it's vital to find a new one used for circuit breaker of long clearance between open contacts. In this dissertation, a new fast repulsion mechanism of double-coil-structure with its mathematic model and movement characteristics simulations have been put forward. Compared with the electromagnetic repulsion mechanism, the result shows more apparent advantage of the mechanism and its energy use efficiency is higher. Finally, in terms of the parameter requirements of 40.5 kV vacuum circuit breaker, the comparative analysis of effects of parameters on the motion characteristics shows that the most important parameter is initial clearance between the two coils, followed by the axial number of turns of coils and radial number of turns of coils. This article is significant in the research of the motion characteristics of new fast repulsion mechanism.

Air gap breakdown is the result of the synthetic effect of electric field distribution and atmospheric environment. The breakdown voltage prediction model of rod-plane air gaps under rain conditions was established by support vector machine (SVM). The electric field features and atmospheric parameters were taken as the input of the prediction model. Based on the idea of binary classification, the withstanding or breakdown of an air gap under applied voltage was characterized by −1 and 1, which was taken as the output of the prediction model. 9 training samples were selected based on the idea of orthogonal design, and the power frequency breakdown voltages of other 29 test samples were predicted by the proposed model. The predicted results coincide well with the experimental values, and the average error of the 29 test samples is only 0.74%. The influence rules of the atmospheric parameters on breakdown voltage obtained by analysing the predicted results are almost the same with those obtained by experiments, which validates the accuracy and rationality of the proposed method. The research results can provide reference for breakdown voltage prediction and discharge characteristic study of air gaps in complex atmospheric environment.

The article simulates the typical tower grounding resistance using CDEGS. First of all, it compares the difference between the flexible graphite grounding electrode, round steel and manganin, in power frequency grounding performance and impulse grounding performance. The results show that flexible graphite grounding electrode is slightly higher than the round steel in the power frequency grounding resistance. But due to its non-magnetism, weaker skin effect, the impulse grounding resistance is lower than that of the round steel. Thus, flexible graphite grounding electrode has good lightning protection grounding characteristics. Take an application of flexible graphite grounding electrode in typical transmission lines in Foshan demonstration project for example, the practical engineering construction process and technology of flexible graphite grounding electrode is introduced, providing references for other similar engineering and test.

DC transmission system in monopole-grounded return operation mode may lead to DC magnetic bias of some transformers in AC power system. In order to develop the management measures of DC bias effectively, it is necessary to study the distribution of DC magnetic bias current in AC power system. Based on the direct field-circuit coupled method, considering the effect of multi-layer soil model on the electrical current field and the impact of the transmission lines on the earth surface potential (ESP) distribution, the DC magnetic bias calculation models of ±800kV Xi-Zhe and ±800kV Ling-Shao UHV transmission project were built. The modeling range and the ground grid parameters were determined, while the soil model is modified as well. The calculated values of DC biasing current were compared with the test result and the total average error of each testing station is no more than 5%, which verifies the accuracy of the calculation model. This research can provide reference to the calculation of DC current distribution in large AC power system and the management of DC magnetic bias.

This paper builds a field model of induction coil gun based on finite-element and boundary-element coupling method (FE-BECM). The fundamental of FE-BECM is introduced, and it is used in open-boundary eddy-current problem to prove its validity and find a way to reduce a nonconforming error. In the coil gun's FE-BECM model, the coil and projectile are discretized by finite elements; boundary elements are used to discretize the free space and associate unconnected finite-element regions. In FE-BECM model, the coordinates of nodes that belong to moving bodies change when movement occurs. The distribution of magnetic flux density and eddy current during the launch process is displayed. The result of FE-BECM is compared with experimental data and results of other numerical methods.

In order to improve the performance of a coil launcher, various cores and shells are considered in the structure design in this paper. Different geometry shapes and materials are loaded on the cores and the shells. A single-stage coaxial induction coil launcher is analyzed for simplicity based on Ansoft 2-D simulations. The conclusion can be summarized as follows. A magnetic core will improve the performance of a coil launcher greatly. Coil current and resistance loss of the system will be reduced while kinetic energy conversion rate and efficiency will be increased. A conductive core will reduce the performance because of eddy-current effect. A magnetic conductive core combines magnetization and eddy-current effect. Geometry of a core will also influence the performance of a coil launcher. In general, the greater the cross-sectional area and the length is, the better the performance. A shell takes little effect on the performance of a coil launcher, no matter what material and geometry is used. A magnetic nonconductive core with large cross-sectional area and enough length is suggested to be used in a coil launcher design. A detailed study will be presented in this paper.

This paper focuses on the scaling study of a capacitor-driven railgun. Railgun is an important kind of electromagnetic launcher. Although the basic principle is relatively simple, due to complicated technologies and extreme conditions, railgun is still far away from practical application. Scaling method is widely used in different field projects, including railgun systems. Subscale-system's test results and experiences can be extremely valuable for the actual system. This paper derives the circuit-scaling relationships based on circuit equations and momentum equations in a capacitor-driven railgun system. Four circuit linear-scaling methods (LSMs) are introduced, and a further approximate LSM is discussed. Finally, numerical verifications of the three models were given, including an original, a full LSM model, and an approximate full LSM model. It is shown that the performances of the full LSM model can be extrapolated perfectly to the original. The results of the full approximate LSM model can get almost the same result as the full LSM model, although some circuit equations are violated. All of the analyses demonstrate the flexibility and exactness of the circuit-scaling method. A detailed derivation and numerical validation are presented in this paper.