Repeated Strike Process During Disconnector Operation in Ultra-High Voltage Gas-Insulated Switchgear

Very fast transient over-voltage （VFTO）, induced by disconnector operations in gas- insulated switchgears, has become the limiting dielectric stress at ultra-high voltage levels. Much work has been done to investigate single-strike waveforms of VFTO. However, little study has been carried out investigating the repeated strike process, which would influence VFTO significantly. In this paper, we carried out 450 effective experiments in an ultra-high voltage test circuit, and conducted calculations through the Monte Carlo simulation method, to investigate the repeated strike process. Firstly, the mechanism of the repeated strike process is proposed, based on the ex- perimentai results. Afterwards, statistical breakdown characteristics of disconnectors are obtained and analyzed. Finally, simulations of the repeated strike process are conducted, which indicate that the dielectric strength recovery speed and polarity effect factor have a joint effect on VFTO. This study enhances the understanding of the nature of VFTO, and may help to optimize the disconnector designed to minimize VFTO.

A High Capacity and Working Voltage Potassium-Based Dual Ion Batteries

Potassium-based dual ion batteries(KDIBs)have attracted significant attention owing to high working voltage,high safety,low processing cost,and environmental friendliness.Nevertheless,one great challenge for practical KDIBs is to develop suitable anode materials with high specific capacity.Herein,we report an architecture of hierarchically porous antimony nanoparticles/carbon nanofibers(HPSb CNFs)as flexible,free-standing anode for high-performance KDIBs.The HPSb CNFs with hierarchically porous structure,and high-content nitrogen doping,not only offer sufficient free space to tolerate the repetitive volume expansion of Sb nanoparticles during long-term cycling,but also greatly facilitate the transport of electrons and ions within electrode,ensuring high material utilization ratio.Thus,the KDIBs,constituted by HPSb CNFs-700(calcined at 700°C)anode and graphite cathode,exhibited a high reversible capacity of 440 m Ah g^(-1)with high discharge medium voltage of 4.5 V at a specific current of 200 m A g^(-1)(the highest capacity for all KDIBs normalized by the mass of the anode),and excellent cyclic life.Outstanding electrochemical reversibility of the KDIBs was further demonstrated by ex situ XRD,ex situ Raman spectrum,and HRTEM.These results suggest the as-designed HPSb CNFs-700 with highcapacity and long-term cycling stability is a promising anode material for high-performance KDIBs.

Study on Adoption of 750 kV Voltage Class in Northwest China Power Network

This paper introduces the selection and scheme demonstration of higher voltage class in Northwest ChinaPower Network, and conclusions made by main research institutes and experts’ comments.[

Research on resonance fault caused by dc magnetic biasing in ultra high voltage transformer with parallel static var compensator

For dealing with the resonance fault of ultra-high voltage transformers(UHVTs)with the parallel thyristor controlled reactor(TCR)+the filter compensator(FC)type static var compensator(SVC)caused by dc magnetic biasing,a simulation model of UHVT with parallel SVC for the frequency analysis of the impedance characteristics and a magnetic-field coupling model for UHVT based on classic Jiles-Atherton hysteresis theories are constructed based on the MATLAB/Simulink platform.Both the theoretical and simulation results prove that the resonance fault is caused by the resonance point on the low-voltage side of the transformer,which will approach the 4th harmonic point under magnetic biasing.Based on the fault analysis,a new resonance control method is proposed by adding reactance with by-pass switches in series with FC branches.Under dc magnetic biasing,the cutoff of the by-pass switch will increase the series reactance rate of the FC branches and change the resonance point.In order to avoid the 7th harmonic increasement caused by this method,the firing angle of the TCR branches is locked between 130°and 180°.The effect of the proposed method is validated by the simulation model of a 750 kV UHVT and the results show that the mechanism analysis of the resonance fault is correct and the resonance control method is valid.

Fault simulation in ultrahigh voltage substation

Ultrahigh voltage(UHV)and extra-high voltage(EHV)have been widely used in power system,so the requirement for technology of operation management,as well as the accident analysis and disposal are more pressing.Based on the theory of Newton-Laphson power flow method,this simulation system imitates the primary device and secondary circuit,together with the normal,the abnormal and accidents in operation.Through simulation in various situations,it validates that this design is capable of simulating the complicated faults in UHV and EHV accurately and in real time.In addition,it can analyse and dispose them efficiently.

Key Technologies of Ultra-high Voltage Hybrid LCC-VSC MTDC Systems

The key technologies of ultra-high voltage hybrid LCC-VSC MTDC systems are investigated,focusing on the design of system configurations,converter topologies and the control and protection system.A double converter per pole of VSC connection is proposed along with the design of a 5000 MW VSC valve to develop a±800 kV/5000 MW large-capacity power transmission.The hybrid MMC topology capable of clearing the DC faults and the control strategy are developed to effectively improve the reliability in case of overhead line faults.The control and protection system of the LCC-VSC MTDC system is introduced to offer flexible operations under both normal and abnormal conditions,which includes voltage/current margin-based coordination,converter switch-in and switch-out,re-connection and drop-off of a third station.Simulations of an LCC-VSC MTDC system based on the LCC-VSC MTDC project are performed.

Requirement of ultra-high voltage GIS arrester to voltage gradient of metal-oxide varistor

The surge arrester with excellent protection characteristics would decrease the overvoltage level ap- plied on the power apparatus to reduce their insulation levels and manufacturing bottleneck. The arrester for the 1000-kV ultra-high voltage ac power transmission system is designed as tank-type structure. The field-circuit combination numerical method combining the three-dimensional finite element method with circuit is proposed to analyze the potential distribution of GIS arrester. By comparing several design schemes, the most effective method to improve the potential distribution along the varistor column is to increase the voltage gradient of the ZnO varistor. Synthesizing several influential factors, the suitable voltage gradient of ZnO varistor should be controlled to 435 V/mm, and the resulted nonuniform degree of the potential distribution along the varistor column inside the GIS arrester would be controlled smaller than 10%. The result in this paper provides the fundamental technical index for the study of the high voltage gradient ZnO varistors.

Low frequency oscillation characteristics of East China Power Grid after commissioning of Huai-Hu ultra-high voltage alternating current project

The low frequency oscillation characteristics of East China Power Grid after commissioning of the first ultra-high voltage alternating current(UHVAC)project-the Huai-Hu UHVAC project are studied.Several low frequency oscillation cases occurred in East China Power Grid in the past few years are reviewed and summarized.Based on the analysis of the different typical operation modes,the main low frequency oscillation modes in East China Power Grid in the early stages of development of ultra-high voltage(UHV)are summarized,and the impacts of the significant power grid maintenance on low frequency oscillation characteristics are analyzed.Besides,the oscillation mode of UHV generators to East China Power Grid is researched,and the importance of the power system stabilizator(PSS)is emphasized.Furthermore,the comparative analysis between the time domain and the frequency domain is carried out,and the influences of the governing system on low frequency oscillation characteristics are revealed.Finally,both the focus and the direction of low frequency oscillation research are presented.

特高压输电线路直流融冰变流系统设计

随着特高压的推广,特高压输电线路的抗冰融冰正成为研究的热点。特高压输电线路由于线径粗、线路长,所需融冰电流与装置容量大,其电流融冰是一个研究的难点。针对特高压输电线路的特点提出了分段直流融冰方法,其将特高压输电线路分成若干段,选取重覆冰区的线路段设置直流融冰点与融冰短路点,可以有效减小融冰装置容量。针对特高压输电线路融冰所需直流融冰装置容量大特点,为了减小融冰装置网侧电流的谐波畸变率,采用24脉波整流变压器+多台整流器并联方式,可以有效减小直流融冰装置对网侧电源的干扰,并可减小输出直流电压纹波因数。最后对所设计的特高压输电线路直流融冰装置进行了仿真研究,采用24脉波整流变压器+多台整流器并联方式后,可以有效消除整流器引起的输入侧电流中5次和7次谐波电流,整流变压器输入侧电流低次谐波总畸变率仅为0.51%,输出直流电压脉波数为24,电压纹波因数仅为0.616。仿真结果证明了所设计特高压直流融冰装置的可行性与正确性。

Ionic liquid-wrapped MXene film with bowl-like structures for highly integrated micro-supercapacitor array with ultrahigh output voltage

Recent researches in the development of in-plane micro-supercapacitor(MSC)have been dedicated to advancing its energy density in a finite storage area.However,the low ion-accessible surface area of plane electrode material has been the bottleneck limiting the energy output of MSC.Herein,we design a stable ionic liquid(IL)-MXene microemulsion system in virtue of tween-20(TW20),gathering IL-TW20 microdroplets around MXene nanosheets.The microemulsion can adhere on the current collector and form the dense MXene-TW20-IL film.The IL as spacer and electrolyte is retained in the interlamination of MXene with the elimination of the TW20 during heat treatment,which enlarges the ion-accessible surface area of the MXene electrode.Thus,the MSC based on the flexible MXene-IL film exhibits a high areal capacitance(44.6 mF·cm^(−2)),a high areal and volumetric energy density(50.7μW·h·cm^(−2)and 39 mW·h·cm^(−3)),surpassing most of the reported MXene-based MSCs.And the MSC unit can be arbitrarily integrated in terms of various energy and voltage requirements.For instance,the integrated array with thirty MSC units in series realizes a high voltage output of 90 V,widening the range of application in next-generation microelectronic devices.

Study on NO_x Decomposition by Unbalanced Plasma Under Low Temperature and Normal Pressure

Experimental mechanism of decomposing NOx througb catalytic activation, using the high energy unbalanced plasma technology to form an activation center on the metal oxide surface under low temperature and normal pressure is introduced. The results show that the low temperature normal pressure uubalanced plasma technology has provided a new method to solve NOx pollution problem with the decomposing rate over 70%.

Evaluation of High Step-up Power Conversion Systems for Large-capacity Photovoltaic Generation Integrated into Medium Voltage DC Grids

With the increase of dc based renewable energy generation and dc loads,the medium voltage dc(MVDC)distribution network is becoming a promising option for more efficient system integration.In particular,large-capacity photovoltaic(PV)-based power generation is growing rapidly,and a corresponding power conversion system is critical to integrate these large PV systems into MVDC power grid.Different from traditional ac grid-connected converters,the converter system for dc grid interfaced PV system requires large-capacity dc conversion over a wide range of ultra-high voltage step-up ratios.This is an important issue,yet received limited research so far.In this paper,a thorough study of dc-dc conversion system for a medium-voltage dc grid-connected PV system is conducted.The required structural features for such a conversion system are first discussed.Based on these features,the conversion system is classified into four categories by series-parallel connection scheme of power modules.Then two existing conversion system configurations as well as a proposed solution are compared in terms of input/output performance,conversion efficiency,modulation method,control complexity,power density,reliability,and hardware cost.In-depth analysis is carried out to select the most suitable conversion systems in various application scenarios.

Aeolian vibration control measures for ground wires of Han River long span transmission lines

Long span ultra-high voltage(UHV) transmission lines have serious aeolian vibration problems. To control these vibrations,we improved the energy balance method in the following aspects:the wind power input,the conductor self-damping,and the damper dissipated power. Meanwhile,we built a theoretical mechanical model of β wire dampers and derived energy dissipation calculation formulae. This permits the vibration energy dissipated by β wire dampers can be considered in the energy balance method. Then,we developed a computer program based on the improved energy balance method using Matlab,and analyzed UHV long span ground wires of the Han River long span project in P. R. China. The results show that the combination of β wire dampers and Stockbridge dampers can reduce vibration of UHV long span transmission lines,which provides a reference for research and construction of UHV engineering projects.

Research on the Long-distance Transmission

In view of the imbalanced distribution of power load and resources, including the status of “electric shortage” in some cities in our country, the article discusses the long-distance transmission technology. It mainly analyzed two ways of the long-distance transmission: UHV AC transmission and UHV DC transmission. The fractional frequency transmission technology and half wavelength AC transmission technology of AC transmission are introduced. Some key technologies of long-distance transmission are described. It has a guess for long-distance transmission future direction.

Alleviating freshwater shortages with combined desert-based large-scale renewable energy and coastal desalination plants supported by Global Energy Interconnection

Under the background of sustainable energy transition and environmental protection,Global Energy Interconnection(GEI),which features an innovative combination of clean energy(e.g.,solar power) and ultra-high voltage(UHV) transmission technologies,provides a means to realize global climate governance.China is a large country with unevenly distributed water resources,energy production,and energy consumption,and the large areas of desert in northern and western China have the potential for installing large-scale solar power plants.This study analyzed the potential of using large-scale solar power from deserts to coastal seawater desalination plants,which could alleviate the freshwater crisis and control desertification in China.First,the measurement data from NASA were used to estimate the potential exploitable amount of solar energy in desert areas.A macro idea was proposed for the transmission of electrical power from inland integrated energy bases to coastal seawater desalination and pumping of freshwater to western China to combat desertification and alleviate the freshwater crisis.Based on this,the electricity demands for desalination and water redistribution were estimated.As a huge interruptible load,desalination and pumping systems could be used to suppress power fluctuations of the integrated energy bases.Finally,the fundamental support roles of UHV grids in large-scale renewable energy allocation and utilization were discussed.This analysis offers a theoretical framework to help realize efficient renewable energy generation and consumption and alleviate freshwater shortage.

Impact of Synchronous Condenser on the Dynamic Behavior of LCC-based UHVDC System Hierarchically Connected to AC System

Hierarchical connection(HC)is a very attractive mode for±800 kV line commutated converter based ultra high voltage direct current(LCC-UHVDC)system connected to different AC voltage levels because of its ability to reduce the scale factor of a converter transformer.Faults in the HC-UHVDC system can cause commutation failure(CF).In this paper,impact of synchronous condenser(SC)to mitigate CF in HC-UHVDC system is analyzed.A±800KV HC-UHVDC system along with synchronous condenser is built in PSCAD/EMTDC.Transient performance analysis of HC-UHVDC for single and three phase to ground faults is investigated.Commutation failure immunity index(CFII),commutation failure probability index(CFPI),fault recovery time(FRT),and transient overvoltage(TOV)are used as measures to evaluate the effects of SC at HC-UHVDC system design.The simulation results show that SC can make the HCUHVDC system less susceptible to CF,effectively improve fault recovery performances of the overall system,and reduce transient overvoltage when single or multiple converters are blocked.The results of this research can provide technical assistance in real world HC-UHVDC projects.

Analysis of the harmful effects to buried oil pipeline from power line short-circuit fault

Based on the transmission line theory of the buried metallic structures, the concerned harmful effects to the buried oil and natural gas pipelines caused by the power line short-circuit fault are further discussed. A closed-form expression of the induced voltage caused by the short-circuit fault of the ultra-high voltage power （UHVAC） transmission line is given. The transmission line model of the buried pipeline is set up and a set of formulas for calculating induced voltage on the pipeline and the parameters of the buried pipeline in actual environment condition are given. At last, the characteristic of the harm^hl effects on the buried pipeline from the power line short-circuit fault are discussed.

Penetrating power characteristics of half-wavelength AC transmission in point-to-grid system

With the proposed concept of global energy interconnection, half-wavelength alternating current transmission(HWACT) technology becomes of more interest.HWACT lines can be adopted to establish a pointto-grid system, in which the penetrating power(PP) is produced between receiving terminals, having a significant effect on the power flow distribution.In order to investigate this phenomenon, the PP characteristics of the HWACT system are researched in this paper.First, the mathematical relationship between the transmission power and terminal bus voltages of a single HWACT line is derived using the equations of a distributed parameter model.The research indicates that the relationship between power and terminal voltages shows ‘‘reverse characteristics"opposite to those of regular short transmission lines.Then, the concept and definition of PP in a point-to-grid system with two receiving terminals are proposed, and the corresponding relationship between PP and the terminal bus voltages is derived.Simulations are carried out to validate the theory under different conditions, so that the accuracy and adaptiveness of the theoretical analysis can be proved.In addition, the results demonstrate that selecting the location for a HWACT system has demanding requirements in order to control the value of PP.

Development and advances in modular multilevel converter based HVDC projects in China

1 Introduction To achieve the goal of carbon neutrality,promoting renewable energy integration has become an urgent priority for the green energy transition.Due to the geographical distribution of renewable energy bases,bulk electrical power is transmitted to load centers through long-distance transmission lines.The line-commutated converter(LCC)based high voltage direct current(HVDC)technology,emerging in the 1970s,is the most mature solution.