Insulated Transmission Line Research Articles

Shape optimisation of basin insulator for DC gas insulated switchgear/gas insulated transmission lines based on artificial bee colony algorithm

AbstractThe surface charge accumulation on the basin insulator is a particular problem in high‐voltage direct current (HVDC) gas insulated switchgears (GIS) and gas insulated transmission lines (GIL), which restricts the development and application of extra/ultra high‐voltage (EHV/UHV) technology. The surface charge distribution and anti‐flashover performance are closely related to the insulator shape. To obtain the optimal basin insulator, a shape optimisation model based on artificial bee colony algorithm was established, and the minimisation objective functions based on capacitive (initial) electric field distribution and resistive (steady‐state) electric field distribution were respectively constructed. The optimisation results of the two objective functions were calculated, and the electric field and charge distribution on the surface of the insulator before and after optimisation were evaluated and compared using the simulation model. The results indicated that the optimisation based on the capacitive electric field can significantly inhibit the surface charge accumulation, and the optimised insulator can significantly reduce the surface normal electric field and make the distribution of the tangential electric field more uniform, and the maximum tangential electric field can be reduced by 10%; the optimisation based on resistive electric field can achieve similar results, but requires more computational resources. Based on the results, the ‘anti‐charge’ basin insulator structure suitable for the DC field was given.

Methyl and carbon–carbon double bond in anhydride molecules modulated surface charge trap depth of epoxy materials

Epoxy insulators in gas insulated switchgear and gas insulated transmission lines tend to accumulate surface charges, leading to insulation flashover. Improving the surface trap characteristics of epoxy materials, which can accelerate the surface charge dissipation of epoxy insulators, is a promising method to improve the surface insulation performance. The surface trap characteristics of epoxy materials are strongly influenced by the chemical groups in the acid anhydride molecules. In this work, by quantum chemical calculations and isothermal surface potential decay tests, taking six organic anhydrides that differ only in the methyl and carbon–carbon double bonds, we find the modulation laws of methyl and carbon–carbon double bonds on the charge trap depth within and between molecular chains. The regulation mechanism is revealed from the microscopic perspectives of electron energy structure and electron cloud offset. The changes of surface charge trap depth of epoxy materials are primarily attributed to the changes in the spatial distribution of the electron cloud density between and on the valence bonds caused by the interaction between the electron-donating methyl group and the electron-absorbing carbon–carbon double bond.

Current Loss Mechanism of Magnetic Insulation Transmission Line With Helical Inductance Support

Accurate adjustment of long magnetic insulation transmission lines (MITLs) is crucial in high-power pulsed systems, which must guarantee insulation and high transmission efficiency of current under complicated electromagnetic environment. In this study, a spiral inductance-supported insulation structure in a coaxial MITL was designed, which achieved high efficiency of current transmission up to 90%. Moreover, the current transmission efficiency of the MITL under various working states and specific mechanism of the current loss was investigated by combining circuit with particle-in-cell (PIC) simulations. Results show that the current loss of the inductance-supported MITL in the under-matched state is caused by the inductance current on the support structure, as the electron charge layer is strongly constrained by magnetic field. However, the charge constraint among MITL electrodes is relatively weak in self-limited flow. The electrons bombard the supporting rod, which results in charge accumulation and unneglectable current loss in self-limited flow. This work uncovers the mechanism of current loss in the spiral inductance-supported MITL, which provides guidance for engineering design.

Characterizing the HERMES-III Extended Magnetically Insulated Transmission Line

Modeling and simulation of the HERMES-III extended magnetically insulated transmission line (MITL) terminated by a diode with an annular cathode and a planar anode was performed with an electromagnetic particle-in-cell code, EMPIRE-PIC. Self-limited MITL theory was developed and implemented in order to model select shots in EMPIRE by allowing one to transform an experimental current trace into a voltage waveform that is input into EMPIRE. Simulating shots 11 132, 11 133, 11 134, 11 135, 11 136, and 11 146 in EMPIRE shows good agreement with experimental current traces at several locations along the MITL. New calibrations for the anode current shunts are proposed based on low-power shots fired into a short-circuited diode configuration (short-shots). The new calibrations show improved agreement with simulation results.

Space charge dynamics of epoxy/micro-Al2O3 composites under multi-physical fields

The direct current gas insulated transmission line (DC GIL) technique has emerged as a promising solution to achieve carbon neutrality and enable efficient long-distance transmission of renewable energy. However, the stable and long-term operation of DC GIL insulators poses a significant challenge that requires insulating materials capable of overcoming charge accumulation issues against multi-fields, including electric, temperature, and force. Here, we investigate the space charge dynamics and electrical conductivity of epoxy (EP) resin and EP/micro-Al2O3 composites under various stimuli of electrical, thermal, and mechanical stresses, using two modified pulsed electro-acoustic and electrical conductivity measurement systems with mechanical pressure control. It is found that stronger electric fields and higher temperature conditions have a more significant impact on space charge accumulation, while higher mechanical stress results in more shallow traps in EP composites. Furthermore, the bipolar carrier transport modeling and numerical calculations are performed to rationalize the experimental observations and reveal the mechanistic impacts of multi-physical fields on the space charge behavior of EP composites for DC GIL insulator use.

Impact of Air Gap Defects on the Electrical and Mechanical Properties of a 320 kV Direct Current Gas Insulated Transmission Line Spacer

Air gap defects inside a spacer reduce its insulation performance, resulting in stress concentration, partial discharge, and even flashover. If such gap defects are located at the interface between the insulation and conductor, a decrease in mechanical stress may occur. In this work, a finite element method-based simulation model is developed to analyze the influence of gap defects on the electrical and mechanical properties of a ±320 kV direct current gas insulated line (DC GIL) spacer. Present findings reveal that a radially distributed air gap produces a more significant effect on the electric field distribution, and an electric field strength 1.7 times greater than that of the maximum surface value is observed at the air gap. The axial distribution dominates the distortion of the surface stress by generating a stress concentration region in which the maximum stress of the air gap is twice the pressure in the surrounding area.

Mutually magnetically insulated two-species Brillouin flow

In this work, we use the Brillouin flow analytic framework to examine the physics of Magnetically Insulated Transmission Lines (MITL). We derive a model applicable to any particle species, including both positive and negative ions, in planar and cylindrical configurations. We then show how to self-consistently solve for two-species simultaneously, using magnetically insulated electrons and positive ions as an example. We require both layers to be spatially separated and magnetically insulated (mutually magnetically insulated); for a 7.5 cm gap with a 2 MV bias voltage, this condition requires magnetic fields in excess of 2.73 T. We see a close match between mutually insulated MITL performance and “superinsulated” (high degree of magnetic insulation) electron-only theory, as may be expected for these high magnetic fields. However, the presence of ions leads to several novel effects: (1) Opposite to electron-only theory, total electron currents increase rather than decrease as the degree of magnetic insulation becomes stronger. The common assumption of neglecting electrons for superinsulated MITL operation must be revisited when ions are present—we calculate up to 20× current enhancement. (2) The electron flow layer thickness increases up to double, due to ion space-charge enhancement. (3) The contributions from both ions and electrons to the MITL flow impedance are calculated. The flow impedance drops by over 50% when ions fill the gap, which can cause significant reflections at the load if not anticipated and degrade performance. Additional effects and results from the inclusion of the ion layer are discussed.

Experimental Study on the Frequency Tunability of a Magnetically Insulated Transmission Line Oscillator

In the present study, the frequency tunability of a magnetically insulated transmission line oscillator (MILO) is investigated by simulation in conjunction with an experiment. In the simulation, the tunable MILO reaches a maximum output power of approximately 5.0 GW with an applied voltage of 590 kV and an input power of 29 GW. The corresponding power efficiency is 17.2%, and the frequency is 1.575 GHz. In addition, by adjusting the outer radius of the vane from 77 to 150 mm, the 3-dB tunable frequency of the tunable MILO ranges from 0.85 to 2.12 GHz, and the tuning bandwidth is 85.5%. The experimental results show that when the voltage and current are 405 kV and 42 kA, respectively, the peak microwave power of the tunable MILO is approximately 2.3 GW with a microwave frequency of approximately 1.900 GHz. The difference in the two frequencies at the peak microwave power is significant because of the tuning mechanism. When the outer radius of the vane varies in the range of 76.6–93.0 mm, the 3-dB tunable frequency range is between 1.552 and 2.168 GHz, corresponding to a 3-dB tuning bandwidth of 33.1%. The experimental measurements verify the simulation results.

Fast Time Response Full Absorption Faraday Cup and Its Application in the Measurement of Intensive Electron Beam Diodes

A magnetically insulated transmission line (MITL) is an inevitable choice for ultra-high power density energy transmissions. Its working process is complex, with an obvious influence on the working process of electron beam diodes and other load devices. The power coupling process of an electron beam diode driven by an MITL is a difficult problem in pulse power applications. No research is available on the electron beam characteristics of its anode. In this paper, a fast time response full absorption Faraday cup was developed. An intense electron beam measurement waveform showing the multi-stage characteristics was obtained through measurements using the Faraday cup absorber as the anode of the electron beam diode. The stage characteristics of the beam were in good agreement with the vacuum transmission, magnetic insulation formation, and multi-stage process of the stable magnetic insulation. The beam intensity corresponded with the conduction current of the cathode. It was obviously smaller than the current of the anode. The results reflected the influence of the different processes of the magnetic insulation on the transmission line on the beam waveforms in the diode area and provided a reference for the power transmission of the power device and the load system design.

Study on the Capture Effect of Particle Trap in 1000 kV AC GIL

In this paper, a 1000 kV gas insulated transmission line (GIL) prototype with a basin insulator and a three-post insulator structure was designed and developed to study the particle motion behavior in an ultra high voltage (UHV) GIL. Particle traps were arranged on the convex side of the basin insulator and near the three-post insulator. Aluminum and copper particles were arranged in front of and on the surface of the traps. A short-time voltage application procedure and a graded long-time voltage application procedure were designed to investigate the difference in the capture effect of the traps on the two types of particles when the test voltage was applied to the prototype through these two voltage application procedures. The results of the study show that the particle motion behavior is complex under AC voltage. The placement of the particles, their own properties and the voltage application procedure all affect the motion behavior of the particles. The trap with the grid structure is effective in capturing particles near both the convex side of the basin insulator and the three-post insulator. At the end of the test, some of the particles went inside the trap and some of them gathered at the edge of the trap. The graded long-time voltage application procedure with a small voltage application gradient and a long duration enables the particles to move sufficiently at a lower voltage to be trapped.

Model of a 5-MA Linear-Transformer-Driver Accelerator: Comparison of MITL Performance for Bremsstrahlung Electron Beam Diodes and Z-Pinch Loads

A transmission line circuit model was conducted to compare the performances of the two-level 2.5 Ω magnetically insulated transmission lines (MITLs) system of a 5-MA linear-transformer-driver (LTD) accelerator for two kinds of typical loads, including bremsstrahlung electron beam diodes and Z-pinch loads. Both the electron current loss in the pulse front during the magnetic insulation setup process and the electron flow distribution in the magnetic insulation steady state were analyzed. When the accelerator drives an electron beam diode load with impedance of 1.20 Ω (a single level), the duration of the magnetic insulation setup is about 12 ns, the current loss is about 130 kA in a single MITL level, the maximum electron flow current is about 50 kA in the end of MITL, and its amplitude decreases gradually after the steady magnetic insulation is established. When the accelerator drives a Z-pinch load with length of 1.5 cm, radius of 1.2 cm, and mass of 0.3 mg/cm, the duration of the magnetic insulation setup is almost zero, the maximum electron flow current in the end of MITL can reach about 55 kA (a single level), and the waveform of the electron flow resembles a saddle shape, which reaches the peak at the pinch stagnation time.

An Iterative Method to Determine the Vacuum Impedance of Magnetically Insulated Transmission Line System

In this article, the dimensionless ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E/{\textit{cB}}$ </tex-math></inline-formula> at the starting point of constant-gap section of the magnetically insulated transmission line (MITL) at the time of peak MITL voltage was used as a figure of merit of a disk MITL. Taking a 15-MA driver as an example, the equivalent circuit model and the impedance iterative method are established by means of the TL-code circuit simulations. The vacuum impedance of the four-level outer-MITLs is determined. In particular, when the target ratio ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E/{\textit{cB}})_{i}$ </tex-math></inline-formula> is set to 0.25 (the ratio of flow current to anode current <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{f}/I_{a}$ </tex-math></inline-formula> is 3.12%), the impedance of the four-level MITLs is 1.70, 2.12, 2.88, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.16~\Omega $ </tex-math></inline-formula> . When ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E/{\textit{cB}})_{i}$ </tex-math></inline-formula> of the four-level MITLs is set to 0.15, 0.18, 0.20, and 0.22 (the ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{f}/I_{a}$ </tex-math></inline-formula> is 1.23%), the impedances are 2.68, 2.89, 3.06, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.02~\Omega $ </tex-math></inline-formula> , respectively.

Impact of power flow on Z-pinch loads

Magnetically insulated transmission lines (MITLs) are used to deliver tens of MA to a Z-pinch load. The MITLs suffer current losses due to contaminant plasma located in the anode–cathode gap which is swept toward the load along the power flow. The swept up contaminant plasma can deposit mass and energy onto the load resulting in deformations or the seeding of macroscopic instabilities. This paper discusses 2D fully kinetic simulations of the contaminant plasma evolution which predict the current losses and the flux of mass and energy onto the load. The effects of a dynamic, i.e., imploding, load are shown to increase both the current loss and the mass and energy flux. The MITL used is a conical, radially converging design which is a feature common to MA-scale Z-pinch accelerators.

A fiber-coupled dispersion interferometer for density measurements of pulsed power transmission line electron sheaths on Sandia's Z machine.

A fiber-coupled Dispersion Interferometer (DI) is being developed to measure the electron density of plasmas formed in power flow regions, such as magnetically insulated transmission lines, on Sandia National Laboratories (SNL's) Z machine [D. B. Sinars etal., Phys. Plasmas 27, 070501 (2020)]. The diagnostic operates using a fiber-coupled 1550nm CW laser with frequency-doubling to 775nm. The DI is expected to be capable of line-average density measurements between ∼1013 and 1019 cm-2. Initial testing has been performed on a well-characterized RF lab plasma [A. G. Lynn etal., Rev. Sci. Instrum. 80, 103501 (2009)] at the University of New Mexico to quantify the density resolution lower limits of the DI. Initial testing of the DI has demonstrated line-average electron density measurements within 9% of results acquired via a 94GHzmm wave interferometer for line densities of ∼1 × 1014cm-2, despite significant differences in probe beam geometries. The instrument will next be utilized for measurements on a ∼1 MA-scale pulsed power driver {MYKONOS [N. Bennett etal., Phys. Rev. Accel. Beams 22, 120401 (2019)] at SNL} before finally being deployed on SNL's Z machine. The close electrode spacing (mm scale) on Z requires probe beam sizes of ∼1mm, which can only be obtained with visible or near infrared optical systems, as opposed to longer wavelength mm wave systems that would normally be chosen for this range of density.

Evaluation of Explosive Emission Carbon Fiber Cathodes for High-Power Microwave Devices

Most high-power microwave (HPM) sources, such as the magnetically insulated transmission line oscillator (MILO) being developed at Texas Tech, utilize cold cathodes that generate electrons via explosive emission. Highly emissive cathodes such as the presented can generate current densities and currents greater than 1 kA/cm 2 and 10 kA, respectively, which are required for devices that can output radio frequency (RF) power greater than 100 MW. Typically, these cathodes are made of materials such as metal, silk or synthetic velvet, carbon fiber, and cesium iodine (CsI)-coated carbon fiber. In order to optimize the MILO performance, we fabricated carbon fiber velvet cathodes and compare their performance with other commercially available carbon fiber cathodes. Fabrication was done on a manual, mechanical loom using commercially available carbon fiber thread. Four carbon fiber cathodes were tested: in-house fabricated monomodal carbon fiber velvet, in-house fabricated bimodal carbon fiber velvet, in-house fabricated carbon fiber plain weave cloth, and bimodal carbon fiber velvet manufactured by ESLI Inc. Testing was performed in a vacuum chamber with variable AK gap in the high vacuum range (10−7 torr). High-speed optical imaging was performed in order to determine the uniformity of the generated plasma as well as the e-beam. Voltage and current measurements were performed to determine diode impedance and perveance.

Polishing Orientation Affecting Surface Charging and Flashover Characteristics of GIL/GIS Epoxy Spacer

In this article, the influence of polishing orientation on surface charge accumulation and flashover characteristics of the basin spacer in a downsized dc-gas insulated transmission line (GIL) are explored by experiments. Results show that the surface charge distribution of the polished spacers is different from the untreated one. Heteropolar charges can be observed in the polished area and outward expansion area under prestressing condition. The flashover voltages of all polished spacers are decreased. The radial polishing (RP) has the greatest impact on the flashover voltage, and the flashover voltage of the radially polished spacer is decreased by 11% under positive prestressing condition. The metal dust adsorptions of the spacers are also measured, which shows that the radially polished spacer is easier to absorb metal dust and the metal dust diffuses in the radial direction. SEM and energy spectrum scanning of different spacers show that there are more exposed alumina particles on the surface of the polished area. The microdischarge induced by alumina particles is the source of heteropolar charges. When the polished area of the spacers is repaired with epoxy paint, heteropolar charges disappear on the surface of the repaired spacer. It is confirmed that charge accumulation and flashover along the spacer surface are mainly affected by the change of surface morphology on the polished area.

Surface Charging Affecting Metal Particle Lifting Behaviors Around Epoxy Spacer of HVDC GIL/GIS

In this article, the mechanisms of surface charging and surface modification of epoxy spacer affecting the lifting voltage and the motion trajectory of spherical metal particles in a downsized dc gas insulated transmission line (GIL) are investigated by simulations and experiments. Results show that the lifting voltage can be predicted with much accuracy by calculating the electric field distribution under the capacitive condition. Prestressing leads to a 27% decrease of lifting voltage, which is traceable to that surface charging making a major contribution to the enhancement of electric field force. Direct fluorination is proven as an effective modification method to improve the insulation performance of epoxy spacer again. The lifting inclination is significantly inhibited after treatment because of less surface charge accumulation and more rationalized electric field distribution. The trajectory simulation results indicate that more serious surface charging can activate more metal particles to lift up and drive them to more collisions. The lifting probability and velocity of metal particles around the epoxy spacer are markedly decreased by fluorination, which is a promising treatment for dc GIL/gas insulated switchgear (GIS) application.

Designing HVDC GIS/GIL spacer to suppress charge accumulation

To suppress the charge accumulation on the spacer surface in direct current (DC) gas insulated transmission line (GIL) is an important and emergent issue for the development of a clean, safe and economic smart grid. A design method of the DC spacer is proposed, and a spacer prototype is prepared and evaluated both by simulation and type test. The DC withstand voltage test and polarity reversal test are performed using the new DC spacer compared with a commercialised 220 kV AC spacer. The simulation results indicate that the surface electric field and surface charge of the DC spacer are lower than those of the alternating current (AC) spacer under DC voltage. The test results verify that the surface flashover voltage of this DC spacer is higher than that of the AC spacer. The potential feasibility of the spacer design for HVDC is discussed. It is hoped that the content of this paper can bring new ideas in the development of HVDC gas insulated equipment.

Characteristics of Switch Prefire Generated Fault Voltages Transmitted in a Four-Stage LTD Module

Future large-scale pulsed-power accelerators are commonly designed based on linear transformer driver (LTD) topology. With the number of gas switches being significant (~ 100000), gas switch prefire might be an inevitable malfunction in an LTD accelerator. The transmission characteristics of switch prefire and its influences on major components in a multicavity LTD module remain to be investigated. In this article, the mechanism of fault voltage generation by switch prefire and inductive coupling effect of the magnetic core were thoroughly understood. It was found that switch prefire drove the cores to magnetize reversely toward negative saturation state ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- B_{s}$ </tex-math></inline-formula> ), raising the voltage on switches in adjacent stages. A circuit model reflecting both initial states and magnetization dynamics of cores was developed for numerical analysis of switch prefire. From previous tests of a four-stage LTD module with deionized water insulated transmission line, unexpected discharges were captured and recognized by D-dots installed in the internal electrode of the transmission line. Test results indicated that an evident manifestation of switch prefire was the polarity reversion of D-dot voltage pulses. Both theoretical and experimental analyses revealed that single brick prefire generated a peak voltage of 70 kV across the transmission line under the charge voltage of ±70 kV. Circuit simulations were in good accordance with experimental data of D-dot signals in the main pulse. However, discrepancies between simulations and experimental results implied that the model was not precise enough in the magnetization dynamics under saturated state.

Electromagnetic Analysis of an Axially Periodic Disk-Loaded Slow-Wave Structure With Low-Loss Dielectric Filling Between Disks for a Magnetically Insulated Line Oscillator

The partially dielectrically filled periodic disk-loaded co-axial structure as a slow wave interaction structure for magnetically insulated transmission line oscillator (MILO) has been electromagnetically analyzed using field matching technique. The slow wave structure (SWS) includes a periodic disk-loaded coaxial structure in which the low-loss dielectric material is partially filled between successive disks. In the analysis, the dispersion relationship and interaction impedance have been calculated taking into account all the harmonics generated inside the structure. The effect of the dielectric constant of the material on the dispersion characteristics and phase velocity has been investigated. Also, the effect of different structural parameters on the dispersion characteristics and the interaction impedance has been obtained. To validate the performed theoretical analysis, the analytically obtained dispersion results have been compared with previously known dispersion results and then also confirmed by the commercial code &#x201C;Computer Simulation Tool (CST) Studio Suite.&#x201D; The agreement between theoretical analysis and simulation with respect to the dispersion characteristics is within 5&#x0025;.