Ionization wave propagation and cathode sheath formation due to surface dielectric-barrier discharge sustained in pulsed mode

This work deals with the experimental study of a surface dielectric-barrier discharge,as a part of the ongoing interest in the control of plasma induced electro-fluid dynamic effects(e.g.plasma actuators).The discharge is generated using a plasma reactor consisting of a fused silica plate which is sandwiched between two printed circuit boards where the electrodes are developed.The reactor is driven by narrow high voltage square pulses of asymmetric rising(25 ns)and falling(2.5μs)parts,while the discharge evolution is considered in a temporarily and spatially resolved manner over these pulses.That is,conventional electrical and optical emission analyzes are combined with high resolution optical emission spectroscopy and ns-resolved imaging,unveiling main characteristics of the discharge with a special focus on its propagation along the dielectric-barrier surface.The voltage rising part leads to cathode-directed ionization waves,which propagate with a speed up to 105m s~(-1).The voltage falling part leads to cathode sheath formation on the driven electrode.Τhe polarization of the dielectric barrier appears critical for the discharge dynamics.

Study of the Electric Filed Around a Metal Oxide Surge Arrester： Measurement and Simulation

The study of the electric field around a surge arrester is useful for design procedures and diagnostic tests. The current work com- putes the electric field around a medium voltage gapless surge arrester using 2D and 3D representation of the arrester. The 2D simulation design, which is described in IEC 60099-4 Standard, cannot include the non symmetrical parts of the arrester geometry and the test arrange- ment. 3D simulation procedures have the advantage that takes into account these asymmetries, giving more accurate results for each measurement position. In order to confirm the suitability of the created models, the simulation results of the electric field, using the 2D and 3D edition of PC Opera, are compared with recorded measurements, which are obtained in laboratory using appropriate calibrated field meters.

Review of High Voltage Direct Current Cables

Increased renewable energy integration and international power trades have led to the construction and development of new HVDC transmission systems.HVDC cables,in particular,play an important role in undersea power transmission and offshore renewable energy integration having lower losses and higher reliability.In this paper,the current commercial feasibility of HVDC cables and the development of different types of HVDC cables and accessories are reviewed.The non-uniform electric field distribution caused by the applied voltage,temperature dependent conductivity,and space charge accumulation is briefly discussed.Current research in HVDC cable for higher operation voltage level and larger power capacity is also reviewed with specific focus on the methodologies of space charge suppression for XLPE extruded cables.

交、直流电场作用下交联聚乙烯中的空间电荷习性（英文）

Polyethylene is one of the widely studied polymeric insulation materials,which have been used extensively for cable insulation.One of the major issues related to polymeric materials is its easy formation of space charge,a high chance to cause electric field distortions.This phenomenon is more significant under high voltage direct current(HVDC)stresses.Space charge can also be observed under high voltage alternative current(HVAC)stresses but with much less intensity due to the limited charge injection period and the effect of charge recombination caused by the constantly variance of the external fields.When considering the situation of an AC voltage combined with a DC offset,a possible scenario in HVDC technology,there was little research on charge dynamics in the insulation in terms of both experimental and simulation work.In this paper,a numerical simulation based a bipolar charge injection/transport model is used to obtain characteristics of space charge in polyethylene under the combined AC and DC high voltage at room temperature.The bipolar charge injection/transport model,which is widely used in HVDC space charge simulation,is applied in the combined conditions.The overall applied voltage,consisted of root mean square(RMS)values of the AC voltage and DC voltage,is kept the same,while the DC component’s voltage ratio and AC component’s frequency are changed respectively,to illustrate their effects on the space charge dynamics within the insulation under combined electric fields.The simulated charge distributions present notable differences when DC offset is increasingly added in,while relatively small differences when AC component’s frequency altering,especially for the cases whose frequency exceeding 0.5 Hz.

Electrical Aging Phenomena of Power Cables Aged by Switching Impulses

Due to the insufficient information regarding the aging phenomenon of cables caused by switching impulses, we aged 15 kV XLPE and EPR cable samples by 10 000 switching impulses in experiments and tested them. Plus in order to compare the aging phenome- non under multi-stress conditions, additional EPR cable samples were aged by rated AC voltage and current with switching impulses superimposed. We used measurements of partial discharge parameters to monitor the cables’ conditions during their aging process, and the AC breakdown voltages measurement to evaluate the cables after aging. Moreover, the Fourier transform infrared (FTIR) spectroscopy measurements revealed the changes of insulation materials after aging. The measurement results confirm that the accelerated aging of cable samples had taken place. The impacts of each individual aging factor are shown through the selected measurements and comparison. The study also helps to assess the reliability of the XLPE and EPR cables under similar condition while serving in power systems.

NOx Reduction and Desorption Studies Under Electric Discharge Plasma Using a Simulated Gas Mixture:A Case Study on the Effect of Corona Electrodes

In this study, reduction and desorption of oxides of nitrogen （NOx） were conducted using an electrical discharge plasma technique. The study was carried out using a simulated gas mixture to explore the possibility of re-generation of used adsorbents by a nonthermal plasma desorption technique. Three different types of corona electrodes, namely, pipe, helical wire, and straight wire, were used for analyzing their effectiveness in NOx reduction/desorption. The pipe- type corona electrode exhibited a nitric oxide （NO） conversion of 50%, which is 1.5 times that of the straight-wire-type electrode at an energy density of 175 J/L. The helical-wire-type corona electrode exhibited a NOx desorption efficiency almost 4 times that of the pipe-type electrode, indicating the possibility that corona-generated species play a crucial role in desorption.

Effectiveness of Lightning Protection Devices

In addition to the conventional Franklin Rod,many non-conventional air terminals are being used as lightning protection devices.As cited in previous works,these non-conventional devices emit space charge in the vicinity of the terminals during the process of lightning stroke.A number of factors affect the performance of these lightning protection devices,among them are geometry and dimension of the devices,location of the device above the ground,height of the cloud above the ground,and polarity of the lightning stroke.The performance of these lightning protection devices has been a topic of discussion by researchers for many years.Some studies focused on the magnitude of emission current from these devices as a criterion to evaluate their performances.The critical flashover voltage(CFO)between the devices and a metal screen simulating cloud can also be used as another criterion to evaluate the performance of the devices.Laboratory measurements were conducted in controlled conditions on different types of lightning protection devices to compare their performance.Four different types of devices were used in the present study:Franklin Rod,TerraStat models TS 100,TS 400,and Spline Ball Ionizer.The study focused on the CFO voltage of the air gap between devices and the metal screen.The CFO voltage was evaluated using standard switching and lightning impulses.The measurements were recorded for positive as well as negative polarity.The air gap between the devices and metal screen was selected at 2 m and 3 m.The results obtained provide a better understanding of the electrical performance of lightning protection devices.

Experimental Study on Indoor Air Cleaning Technique of Nano-Titania Catalysis Under Plasma Discharge

In this study, a new technique of air cleaning by plasma combined with catalyst was proposed, which consisted of electrostatic precipitation, volatile organic compounds （VOCs） decomposition and sterilization. A novel indoor air purifier based on this technique was adopted. The experimental results showed that formaldehyde decomposition by the plasma-catalyst hybrid system was more efficient than that by plasma only, Positive discharge was better than negative discharge in formaldehyde removal. Meanwhile, the outlet concentration of ozone byproduct was effectively reduced by the nano-titania catalyst.

Achieving Better NO_x Removal in Discharge Plasma Reactor by Field Enhancement

Application of plasma chemistry for gas cleaning is gaining prominence in recent years, mainly from an energy efficiency point of view. In this paper we conducted a comparative study of NO/NOx removal using two different types of dielectric barrier discharge electrodes, wire- cylinder reactor, pipe-cylinder reactor. Investigations were first carried out with synthetic gases to obtain the baseline information on the NO/NOx removal with respect to the two geometries studied. Further, experiments were carried out with raw diesel exhaust under loaded condition. A high NOx removal efficiency of 90% was observed for the pipe-cylinder reactor as compared to that of 53.4% for the wire-cylinder reactor. Furthermore, for the same energy consumed per NO molecule （about 73 eV/NO molecule）, the removal efficiency increased from 67% for the wire- cylinder to about 98% for the pipe-cylinder which was quite appreciable.

Vibrational Temperature of Excited Nitrogen Molecules Detected in a 13.56 MHz Electrical Discharge by Sheath-Side Optical Emission Spectroscopy

Optical emission spectroscopy parallel to the axis of a low-pressure radio-frequency discharge in nitrogen was realized, i. e. end-on instead of customary side-on measurements. A specially designed optical feedthrough was fabricated and installed co-planar with the anode elec- trode, allowing non-invasive observations perpendicular to the plasma sheath. The principal ex- cited species flowing towards the grounded electrode were identified and, by assuming Boltzmann distributions, the vibrational temperature of the molecules on the X1 ∑＋/g and C3Ⅱu levels was estimated under various operating conditions.

关于在电力系统中局部放电对固体绝缘介质的破坏的研究

文章是对于发生在交流电场中在固体绝缘介质的空气间隙的老化进程的实验,通过制造一个在2mm厚度的硅胶样品包含直径1mm的单个的空气间隙,五个样品在12k V的交流电场下加压6小时而后加压到15k V直到有一个样品击穿。在加压过程中,局部放电数据被常规的记录下来。剩下的4个样品将被检查是否有老化的痕迹。初步结果显示,被切割的样品的老化部分有可能是凹凸部分从空气间隙中延伸出来。这篇论文详细介绍这个实验,以及包含获得的老化的样品的照片和详细介绍因为电老化过程对化学成分的变化在硅胶中的影响。

Accelerated thermal ageing on performance of polypropylene-based semiconducting screen for high voltage direct current cable applications-Effect of antioxidants

The potential of using eco-friendly thermoplastic polypropylene(PP)-based insulation for high voltage direct current(HVDC)cable has been widely investigated but much less work on the PP-based semiconducting screen(SC).Considering a long service life(>30 years)under high temperature and high electrical stress is required for typical HVDC cables,and investigations on the effect of antioxidant(AO)concentration and thermal oxidative sta-bility,mechanical,and electrical properties of PP-based SCs have been conducted.It has been demonstrated that an appropriate combination and amounts of AOs are critical for achieving high thermal stability and maintaining the mechanical properties of SC after ageing in a harsh environment(150°C,with Cu,in air,7 days).Although higher amounts of space charges have been observed in SC/PP/SC samples with higher AO concentrations,the impact on space charge behaviours is less after ageing,suggesting that ageing(or operating at high temperature)leads to microstructure evolution in SC and can potentially mitigate space accumulation in PP-based insulating materials.

Investigation on effect of semiconducting screen on space charge behaviour of polypropylene-based polymers for HVDC cables

High voltage direct current(HVDC)power transmission cable is critical for realising sustainability through renewable energy revolution.Eco-friendly thermoplastic polypropylene(PP)-based polymers/nanocomposites are regarded as promising candidates for replacing current thermoset crosslinked polyethylene(XLPE)cables.As an essential component of the extruded HVDC cable for improving conductor/insulation interface and suppressing charge injection to insulation at high DC electrical stresses,developing semiconducting(SC)screens that are compatible with PP-based insulation is of similar importance but has not been well studied yet.This work aims at designing PP-based semiconducting screens and investigating space charge behaviours of SC/PP/SC sandwich specimen to unfold the effect of semiconducting materials,bonding methods,applied DC electric field,and temperature on charge injection,accumulation,transportation,and dissipation in PP-based insulation.Although conventional thermal,mechanical,and low field electrical characterisations demonstrated that all of the developed semiconducting materials meet the performance criteria of commercial semiconducting materials,their space charge and local electric field distribution varied significantly at high DC fields.Compared with the traditional non-bonded configuration used at lab-scale,charge injection was enhanced in hot-pressed SC/PP/SC samples with tightly bonded interfaces,which better reflects the real situation in extruded cables.High temperature further intensified charge injections.Besides,our results also revealed that high temperature and electric field strongly influence charge mobilities and consequently their distribution and local electric field in PP-based insulations.

Influence of humidity on conduction processes in gas-insulated devices

Humidity has been considered as one of the main influencing factors that determine the conduction processes and electric strength of gas-insulated systems.Whereas in the past,various studies focused on the change in the partial discharge inception voltages,breakdown strength of homogeneous and inhomogeneous field arrangements,and insulator flashover voltage,recent studies have investigated the changes in ion currents measured through different gas gaps.In the framework of this contribution,a highly precise humidity control circuit has been developed to analyse the significance of humidity in the range from−25 to−5°C frost-point,which is fully applicable to operating gas-insulated devices.Using sulphur hexafluoride(SF6)as the insulation gas at 0.45 MPa,Al2O3-filled epoxy resin insulators,and technically rough electrodes,the humidity was found to significantly influence the intensity of microdischarges at interfaces.Charge generation from microdischarges at the interfaces substantially increased with increasing humidity.For an electric field of 5 kV/mm that were applicable to the dimensioning of gas-insulated devices,humidity strongly influences the charge provision from technically rough interfaces and potentially contributes to the surface-charge accumulation at insulator surfaces.On the other hand,for low-field conduction phenomena,no increase in the ion currents from natural ionisation or electrophoretic conduction was observed.For the investigated range of parameters,humidity is expected to be highly relevant for the design of gas-insulated devices.

Effect of organoclay loading on the dielectric properties and charge dynamics of a PP- rubber nanocomposite

The addition of organoclay to a polypropylene-rubber (PP-rubber) blend, primarily introduced to compatibilise the immiscible polymer blend, invokes contrasting dielectric and charge dynamic behaviour depending on the filler loading level. The authors report that at 0.5 wt.% loading, the organoclay decreases the DC conductivity, causes no significant dielectric losses, makes no significant difference to the space charge results compared to the unfilled system, and increases the reproducibility of the breakdown strength results, and hence the reliability of the material. These somewhat surprising results, contrasted by measurements of samples with 2.5 and 5 wt.%, lead us to conclude that trace amounts of organoclay improve the otherwise immiscible polymer blend making organoclay a suitable additive for HVDC applications.

Interruption limits of mechanical circuit breakers and circuit upgrades for current injection in HVDC circuit breakers

Current injection circuit breakers consist of a mechanical interrupter(MI)with a current injection and an energy dissipation branch in parallel.The performance of the complete device is largely determined by mechanical operation time and interruption performance of the MI.In the standard configuration,current injection is realised using a pre-charged inductor–capacitor circuit.A higher interruption performance of the MI makes it possible to scale down the resonant injection circuit,and thus have a more economical design.Additionally,the implementation of more complex injection circuits that quickly create zero crossings,while maintaining favourable conditions for interruption,can lead to economic benefits.In this study,the interruption performance of a model gas interrupter as part of a current injection topology is investigated.The results are used to verify a corresponding simulation model and two-dimensional upgrade circuits that influence the injection current to increase the range of interruptible fault currents.On the basis of experimental results,the simulation model is used to investigate the performance of upgrade circuits for the use in high-voltage direct current(HVDC)systems.The results indicate that using improved injection circuits can considerably increase the economic advantage of current injection circuit breakers compared with other topologies.

Improving interruption performance of mechanical circuit breakers by controlling pre-current-zero wave shape

Mechanical circuit breakers(MCBs)are the limiting component for current injection HVDC circuit breakers.Improving their interruption performance reduces requirements for capacitance and inductance needed in the injection circuit and thus space use and costs.Higher performance can be achieved by creating a period of low current gradient before zero crossing in the MCB,e.g.by using a saturable inductor(SI).In this paper,the impact of duration and steepness during the low current-gradient phase is linked to arc parameters of the investigated model gas circuit breaker.It is shown in a scaled experimental setup that an optimum design of the SI can be derived from arc time constant and interruption limits for constant current gradients.This optimisation results in a considerable increase of interruption performance.The feasibility of implementing an SI in a full-scale HVDC circuit breaker is demonstrated using simulations.Using an improved injection scheme,the stresses for the MCB can be reduced significantly.Consequently,the injection circuit components can be scaled down,making the topology more economical.The reduced interruption requirements might also make it possible to use a single gas interrupter instead of a series connection of vacuum interrupters,reducing the complexity of the mechanical switch.

Ageing behaviour of a polyethylene blend:influence of chemical defects and morphology on charge transport

Abstract:The properties of novel cable insulation systems will rely critically upon the morphology of the material.Here,a blend of high and low-density polyethylene(PE)was processed in order to generate three sets of samples with different morphologies.The influence of thermo-oxidative ageing at 120℃ was then considered.The resulting chemical changes included the introduction of unsaturation and oxygen-containing groups and were determined by antioxidant consumption and oxygen permeability.Such chemical defects were found to be concentrated in the fraction of each system that was molten at 120°C and,consequently,served to inhibit recrystallisation following ageing.The resulting spatial distribution of charge trapping sites was therefore strongly dependent on morphology.The electrical conductivity of each system varied non-monotonically with ageing:short times reduced the conductivity;a rapid increase in conductivity over five orders of magnitude occurred beyond a critical ageing threshold.Despite the pronounced structural differences between the morphologically distinct sets of samples,all exhibited comparable conductivity values beyond this threshold,implying that while charge transport is strongly influenced by chemical factors,crystallinity is relatively unimportant.This experimental finding appears at odds with theoretical studies of the electronic states in crystalline and amorphous PE.

Dielectric behaviours of bio-derived epoxy resins from cashew nutshell liquid

In this study,four commercially available bio-derived epoxy systems(extracted from cashew nutshell liquid)were prepared and characterised.The glass transition temperature(Tg),dielectric spectroscopy,DC conductivity and breakdown properties of these epoxy resins were studied.Differential scanning calorimetry(DSC)demonstrated that the T_(g) of the investigated systems ranged from 67 to 122°C.The DC conductivity was very low(<10^(-16) S cm^(-1))and comparable to the conventional dielectrics at room temperature(RT).However,all systems showed a strong temperature dependence of the electrical conductivity and exhibited sharp increase around their respective T_(g).Arrhenius analysis led to activation energy,E_(a),values around 1 eV;higher E_(a) values were observed in systems with a lower T_(g).Dielectric spectroscopy revealed a flat and low response at temperature below T_(g).However,both the real and imaginary permittivity increased with decreasing frequency at mid to low frequencies as the temperatures approached T_(g).The variations of AC breakdown strength of all samples were not statistically significant,but the DC breakdown strength of sample 2503Aþ2002B was higher than the others,which might be due to reduced charge transport in this system.The results indicate that novel bio-derived epoxy systems from renewable sources are potential alternatives for traditional petroleum-based epoxy systems in certain insulation applications.