Reliability improvement of gas discharge tube by suppressing the formation of short-circuit pathways

After cumulative discharge of gas discharge tube(GDT),it is easy to form a short circuit pathway between the two electrodes,which increases the failure risk and causes severe influences on the protected object.To reduce the failure risk of GDT and improve cumulative discharge times before failure,this work aims to suppress the formation of two short-circuit pathways by optimizing the tube wall structure,the electrode materials and the electrode structure.A total of five improved GDT samples are designed by focusing on the insulation resistance change that occurs after the improvement;then,by combining these designs with the microscopic morphology changes inside the cavity and the differences in deposition composition,the reasons for the differences in the GDT failure risk are also analyzed.The experimental results show that compared with GDT of traditional structure and material,the method of adding grooves at both ends of the tube wall can effectively block the deposition pathway of the tube wall,and the cumulative discharge time before device failure is increased by 149%.On this basis,when the iron-nickel electrode is replaced with a tungsten-copper electrode,the difference in the electrode’s surface splash characteristics further extends the discharge time before failure by 183%.In addition,when compared with the traditional electrode structure,the method of adding an annular structure at the electrode edge to block the splashing pathway for the particles on the electrode surface shows no positive effect,and the cumulative discharge time before the failure of the two structures is reduced by 22.8%and 49.7%,respectively.Among these improved structures,the samples with grooves at both ends of the tube wall and tungsten-copper as their electrode material have the lowest failure risk.

Fully Decoupled Branch Energy Balancing Control Method for Modular Multilevel Matrix Converter Based on Sequence Circulating Components

The modular multilevel matrix converter(M3C)is a potential frequency converter for low-frequency AC transmission.However,capacitor voltage control of high-voltage and largecapacity M3C is more difficult,especially for voltage balancing between branches.To solve this problem,this paper defines sequence circulating components and theoretically analyzes the influence mechanism of different sequence circulating components on branch capacitor voltage.A fully decoupled branch energy balancing control method based on four groups of sequence circulating components is proposed.This method can control capacitor voltages of nine branches in horizontal,vertical and diagonal directions.Considering influences of both circulating current and voltage,a cross decoupled control is designed to improve control precision.Simulation results are taken from a low-frequency transmission system based on PSCAD/EMTDC,and effectiveness and precision of the proposed branch energy balancing control method are verified in the case of nonuniform parameters and an unbalanced power system.

Analysis of Conducted Disturbance via Current Transformer Due to Switch Operation of GIS Disconnector in UHV Substation

The characteristics of conducted disturbances via current transformer(CT)caused by switch operations in 1000 kV UHV substations are studied in this paper.Since the broadband circuit models might have limitations on the stability and accuracy for CT conducted disturbance simulation,a frequencydomain simulation method without circuit synthesis is proposed based on VFTC spectrum and CT transfer characteristics.To improve VFTC simulation accuracy,both the bushing radiation loss and frequency-dependent earth impedance are taken into consideration.Transfer characteristics of three types of windings for the 1000 kV GIS CT are ingeniously measured from a combination of fast and slow impulse tests.Through conducted disturbances simulations in UHV Wuhu substation,bushing radiation loss is found to have a suppression effect on the highfrequency response but the effect of frequency-dependent earth impedance can be ignored.As a result,the maximum amplitude of 84.5 A for the conducted disturbance is obtained,thus a fast damped oscillatory wave immunity test with no lower than Level 4 from IEC standard is suggested to test the anti-interference performance of the secondary equipment in UHV substation.

Dispersion characteristics of switching transients from large‐scale experiments in a 1100 kV gas insulated switchgear circuit

The switching operation of gas insulated switchgear(GIS)disconnector will produce multiple random spark discharges between the gas gaps of the disconnector.Each spark discharge can be affected by various random factors,such as trapped charge on the load side,the initial operating phase,gap distance and operating speed of the disconnector,leading to dispersion characteristics.At present,some studies have mentioned this dispersion,but its impact is still unclear due to the lack of experimental data,resulting in a deviation between the simulation results and the measured results.In this study,thousands of switching operations were carried out based on the 1100 kV full‐scale GIS circuit in Wuhan ultra‐high voltage alternating current Test Base.The maximal information coeffi-cient analysis,linear regression fitting and other statistical methods were used to discuss the dispersion characteristics of very fast transient overvoltage(VFTO)and transient enclosure voltage(TEV)from large‐scale experimental data.The results show that the dispersion of VFTO and TEV in amplitude aspects both increase gradually as the breakdown voltage increases,with the maximum difference of five times of TEV amplitude at normalised breakdown voltage.In terms of frequency characteristics,the dispersion of different fre-quency components in TEV is always greater than that of VFTO,especially for the high‐frequency components.Moreover,by comparing the frequency characteristics of TEV at different positions,we notice that the dispersion is almost independent of the spatial po-sition but only determined by the randomness of spark discharges.These discoveries reveal the importance of the dispersion in switching transients and make up for the lack of theoretical understanding of the correlation among them.

Discharge Characteristics of Long SF6 Gas Gap with and Without Insulator in GIS Under VFTO and LI

Gas insulator switchgears(GISs),widely used in electric power systems for decades,have many advantages due to their compactness,minimal environmental impact,and long maintenance cycles.However,very fast transient overvoltage(VFTO)increases caused by a rise in voltage levels can lead to GIS insulation failures.In this paper,a generating system of VFTO and standard lightning impulse(LI)is established.The insulation characteristics of SF6 gas with and without insulators under VFTO and standard LI are investigated.Experimental results show that the 50%breakdown voltages of the inhomogeneous electric field rod-plane gap under positive VFTO and standard LI are higher than that under negative VFTO and standard LI.The research shows that the 50%breakdown voltage under VFTO could be lower than that under standard LI at 0.5 MPa for the negative polarity.Moreover,the polarity effect of the insulator without defect is different from that with defect.Similarly,the breakdown voltage of the defective insulator under VFTO could be lower than that under standard LI by 8%.The flashover channel under VFTO is seen as more than that under standard LI.Based on the analysis of discharge images and experimental results,it is concluded that the polarity effect is related to the distortion effect of ion clusters formed by SF6 on the electric field.Additionally,the steepness and front time of impulse plays an important role in the initiation and further development of discharge on insulator surface.Finally,the research shows that different discharge characteristics between VFTO and standard LI may be caused by different wave fronts and oscillation on the tails of the impulses.

Foreword for the Special Section on AC and DC Ultra High Voltage Technologies

A LONG with the rapid development of its economy,China is facing serious challenges in energy resources.Coal,hydro,inland wind and solar power are mainly dis-tributed in the West and North of China.However,power demand centers locate in Eastern and Central China.Distance between energy bases and load centers ranges from 800 km to 3000 km.China needs to develop remote and bulk power transmission to optimize power allocation.

Characteristics of Long Air Gap Discharge Current Subjected to Switching Impulse

Measuring the pre-breakdown current of long sparks in air is important for investigating the discharge mechanism.Since the breakdown of long air gaps is conducted by a series of streamer-leader processes,the corresponding current signals cover a bandwidth of 0 to more than 20 MHz.Measurement accuracy of the current from the high voltage side is affected by the displacement current and impulse electromagnetic interference.In this paper,a coaxial current sensor with a DC bandwidth of 74.45 MHz is developed.A displacement current-restrained electrode structure is proposed to reduce the equivalent capacitance between the current sensor and the ground over 30 times.Combined with the digital optical fiber synchronous acquisition unit,a current measurement system for long air gap discharge is established.For the purpose of the UHV system’s external insulation optimization design,the discharge current waveform of a 6 m rod-plane air gap under positive switching impulse voltage with 250µs and 1000µs time to crest is obtained.Discharge images and stressed voltage are combined to analyze the continuous feature of a current waveform under critical time to crest impulse and discontinuous feature under long front duration impulse.For the purposes of a lightning protection study,the current waveform of a 10 m rod-plane air gap is subjected to negative switching impulse.Finally,the pulse characteristics of the current corresponding to the single channel and branching stepped negative leader are discussed.

Research and application of UHV power transmission in China

The power demand increases rapidly in China;however,the areas of huge power demands are of long distance from most areas of abundant energy resource in the country.Therefore,China put in great effort to develop ultrahigh voltage(UHV)power transmission systems to optimise its energy allocation.This includes(i)systematically developing key technologies such as overvoltage suppression,external insulation design,and electromagnetic environment control,and(ii)developing key equipment such as UHV alternative current(UHVAC)transformers,circuit breakers,and series compensated equipment,and UHV direct current(UHVDC)converter transformers,smoothing reactors,converter valves,and DC transmission control and protection systems.Eight AC UHV projects and 11 DC UHV projects have been built in China,which play an important role in the optimal allocation of energy.Plus there are one more UHVAC and three more UHVDC transmission projects in construction,while UHVAC gas-insulated lines will be applied in the UHVAC line crossover the Yangtze River and the±1100 kV UHVDC power transmission technology is in development.Here,the development and application of UHV power transmission technologies in China are described,some main challenges the UHV projects encountered are discussed,and experiences obtained from the operation of UHV systems are introduced.It is concluded that China obtained mature experience in developing,constructing,and operating UHV systems and successfully realised long-distance,large-capacity power transmission,and the UHV power transmission technology has become an important measure for energy allocation in large areas.

Development and prospect of direct‐current circuit breaker in China

The direct‐current circuit breaker(DCCB)is the most ideal choice for DC fault isolation in DC grids.Despite a late start,China's research and development on the DCCB have made outstanding achievements.This article provides a brief glance of current China's DCCB development status.It begins by sorting out the technical route according to the topology of DCCB.Then it systematically summarises both mechanical and hybrid DCCBs with focussing on the aspects of topology structure and principle,key technology and characteristics,prototype development and application.It is apparent that Chinese scientists and engineers confronted the worldwide problem of large capacity DC breaking,and put forward a comprehensive solution which consists of an innovative topology structure based on coupled negative voltage circuit,breaking throughs on the key technologies such as highly controllable and reliable fault current commutation,millisecond‐level ultra‐fast and efficient electromagnetic repulsion mechanism,high tolerance and high stability power electronic switch,low residual voltage and fast response energy consumption device,etc.The article states that the world's first set of hybrid high‐voltage(HV)DCCB,and the first set of mechanical HV DCCB have been developed.These DCCBs will soon be deployed to the DC grids which have the highest voltage levels therefore require the strongest breaking capacity.These achievements are leading the world in the development and application of DCCB.The article also discusses the overall development trends of DCCB in the areas of new topologies,key techno-logical breakthroughs and application scenarios,etc.These discussions serve as references for DCCB's future technological advancement and its ever‐expanding applications.

Metal particle movement and distribution characteristics under AC voltage and ballplane electrodes

Metal particles,difficult to be eliminated in gas-insulated metal-enclosed switchgear(GIS),can cause GIS discharge and breakdown between electrodes,or flashover on the insulator surface.It influences the development of ultra-high voltage(UHV)projects.Therefore,the work optimised the model of metal particle movement under AC voltage,studying the metal particle movement and distribution characteristics between ball-plane electrodes through experiment and simulation.Under AC voltage,the particle jumps on a small scale on the plane surface.With the increase of voltage,the jump amplitude increases.However,the collision frequency decreases until the particle collides with the ball electrode.When the initial phase angle of power changes,the particle-moving pattern is symmetrical in the angle ranging from 0 to 180°,and from 180 to 360°.The collision frequency changes slightly with the increase of jump amplitude when the angel ranges from 0 to 120°.

Simulation of positive streamer propagation in an air gap with a GFRP composite barrier

The puncture of glass fibre reinforced polymer(GFRP)laminate is a primary damage pattern of wind turbine blades due to lightning strikes.A numerical simulation model of positive streamer propagation in a needle‐to‐plate air gap with a GFRP laminate is established to investigate the breakdown mechanism of GFRP laminate.The model not only considers the dynamics of charged particles in the air and the composite laminate,but also the current continuity at gas-solid interfaces.The simulated streamer discharge pattern and the surface streamer length are in good agreement with the observation results.The distributions and evolutions of the electron number density,electric field,and surface charge densities during streamer propagation are obtained.It is found that the enhancement of the electric field on the GFRP laminate is caused by the rapid deposition of positive and negative space charges on the GFRP laminate after a secondary streamer incepts on the lower surface of the GFRP laminate.The effects of the applied voltage,relative permittivity,and thickness of the GFRP laminate on the electric field on the GFRP laminate are investigated.The obtained results could assist in further under-standing of the mechanism of GFRP wind blade breakdown due to lightning strikes.

Methods for alleviation of impacts of axial diffusion on decomposition products monitoring in gas‐insulated transmission lines

Considering the long length of gas‐insulated transmission lines(GILs),novel methods are required to alleviate the impacts of axial diffusion on decomposition products'moni-toring.The diffusion process of two typical decomposition products,SO2 and SO2F2,was studied.During the diffusion experiments,SO2 and SO2F2 were generated by partial discharges.Based on Fick’s law,an axial diffusion model of SO2 and SO2F2 was proposed.By combining the experiment and simulation results,it was found that the diffusion coefficient of SO2 was greater than that of SO2F2.In addition,the concentration variation at different positions would experience some delay and decay in axial direction.The delay and decay were inversely related to the diffusion coefficient and increased with prolongation of the distance.Based on the diffusion characteristics determined,methods for optimising the monitoring positions,correcting concentration,and using the concentration ratio and generation rate were proposed to alleviate the impacts of diffusion.The methods could allow the decomposition products to be monitored in 1 month and reduce the monitoring error to 10%or less.The diffusion model and the methods could be useful for monitoring decomposition products in GILs.

Charging models for airborne suspended particles around HVDC lines

Ground total electric field(GTEF)is a vital parameter in the field of electromagnetic environment of high voltage direct current(HVDC)lines.Some experimental works re-ported that airborne suspended particles can be charged in the corona ion flow of the lines and thus affect GTEF.This paper aims at studying the charging models for particles.Equivalent unipolar charging models(EUCMs)and bipolar charging models(BCMs)for describing field and diffusion charging mechanisms in bipolar ion environments are introduced and the Maxwell-Wagner relaxation is proposed to be incorporated.Besides,the EUCMs and BCMs are analysed and validated through comparisons with experimental data.The results show that BCMs are applicable to describing the particle charging in a bipolar ion environment,while EUCMs are not.The predictions made by BCM that takes into account the Maxwell-Wagner relaxation are in good agreement with experimental results.Moreover,the BCMs are proposed to analyse the experimental results about GTEF in the presence of particles,which are not understood before.It is demonstrated that the BCMs are able to qualitatively explain these results.Finally yet importantly,a new charging model combining field and diffusion charging mechanism is proposed for airborne suspended particles around HVDC lines and validated by experiments.The results show that the proposed hybrid field-diffusion charging model is able to give predictions consistent with experiments.