The Arc Characteristics And Extinguishing Method Of The EHV/UHV DC Grounding Electrode Lines

Recently,extra-high voltage(EHV)and ultra-high voltage(UHV)direct current(DC)transmission technology has been rapidly developed and widely applied in cross-regional grids interconnection and large-capacity long-distance power transmission.The grounding electrode line is an important part of the EHV/UHV DC transmission system,which provides a grounding channel for both unbalanced current and ground return current.It has the characteristics of large current and low voltage.Due to its low external insulation level,it is vulnerable to lightning and switching overvoltage.The arcing horn consists of a pair of electrodes installed in parallel at both ends of the insulator string.The air gap between the electrodes is slightly smaller than the length of the insulator string,so that the lightning impulse flashover voltage is smaller than that of the insulator string.Therefore,the arcing horn will break down before the insulator string when an overvoltage fault occurs,protecting the insulators and the grounding electrode line,therefore ensures the safety and reliability of the EHV/UHV DC transmission system.The DC arc on arcing horn is characterized of large current amplitude,long burning time and difficulty to extinguish.In recent years,there have been many accidents caused by the DC arc on arcing horn,such as conductor fracture and string falling apart,and eventually lead to EHV/UHV DC transmission system closedown with great cost of the national economy.The DC arc of grounding electrode line under outdoor open space is influenced by the electrode type,environment and other factors,and its ignition process is extremely complex.If the DC arc can be transferred to the end of the electrode in time to avoid damage to the insulators and create favorable conditions for rapid arc extinction,it can reduce the fault or outage of the grounding electrode line to ensure the stable operation and reliable power supply of the EHV/UHV DC transmission system,and prevent the large power grid accidents.Therefore,it is necessary to characterize the electrical and motion properties of the DC arc of grounding electrode line with a deep understanding of its underlying physical mechanism.A fundamental theory underlying the DC arc extinguishing would facilitate its potential improvement in reliability and cost-effectiveness.In order to study the characteristics of thermal,electrical,magnetic and force of the DC arc,a magnetohydrodynamic model(MHD)of the arc considering thermal buoyancy was established,which realized the coupling of physical fields and motion state of the DC arc.We also built a 10~2 k W experimental platform for the DC arc of grounding electrode line to capture the arc shape and measure the electrical parameters such as current and voltage.It provides a basis for studying the electrical characteristics and motion characteristics of the DC arc.The simulation results were consistent with the experimental results,which verified the effectiveness of the model.In order to study the electrical characteristics of the DC arc,we measured the voltage and the current of the whole DC arc process,the influence of the electrode,wind speed and magnetic field on the electric characteristics of the arc was studied,and the influence of the DC arc motion on electrical characteristics was studied too.From these experiments,it is found that the arc current oscillates violently during the initial and extinguishing stages.During the arc movement,the frequent short-circuit of the arc column and the jump of the arc root cause the voltage drop and the current pulse,the amplitude of which decreases with the increase of the wind speed.It is found that as the gap distance increases,the arc voltage curve shifts upward as a whole,and the arcing time also decreases.The voltage increases faster when the expansion angle of the electrode increases.Through experimental comparison,the growth rate of the arc voltage of the double arcing horn is higher than that of the single arcing horn,indicating that the double arcing horn has a better stretching effect.The DC arc volt-ampere characteristic curve is obtained,and the arc potential gradient is fitted.It is found that the arc potential gradient decreases with the increase of the current.Voltage is the key factor to maintain the arc.The maximum value of the arc voltage is greater than the power supply voltage.The arc begins to extinguish when the voltage reaches the maximum value.In order to study the motion characteristics of the DC arc,the arc trajectory was recorded by a high-speed camera,and the effects of different wind speed,magnetic fields,and the expansion angle of the electrodes on the arc were studied.It is found that the arc shift upward due to thermal buoyancy,and the arc column is closer to the upper electrode of the arcing horn,resulting in the arc root jump mostly occurring at the upper electrode.The upper arc root moves faster than the lower arc root,and the simulation results show that the cyclone formed in front of the lower arc root hinders the movement of the lower arc root.The ablation at the end of the upper electrode is more serious,because the upper arc root moves to the end of the electrode and stays for a long time.By changing the expansion angle of the electrode,it is obtained that with the increase of the expansion angle,the longitudinal stretching effect on the arc increases,and the lateral transfer effect decreases.Besides,it is also found that with the increase of the expansion angle,the arc root jumps more frequently,and the arc column is closer to the electrode,leaving high temperature in the gap,which is not conducive to arc extinguishing.As the expansion angle increases,the thermal buoyancy effect increases,which increases the volume force of the upper arc root and makes the arc root move more rapidly.It is found that there is an expansion angle of the electrode which can give consideration to both the extension and transfer of the arc.The effect of the wind speed on the arc was studied.It is found that the effect of the wind on the arc column is greater than that on the arc root.The wind strengthens the diffusion and cooling of the arc and promotes the arc extinction.The number of the arc root jump and the arc column short circuiting decreases with the increase of the wind speed.The influence of magnetic field on arc was studied.It is found that the effect of magnetic field on arc root is greater than that on arc column.The angle between the arc root and the electrode decreases with the increase of the magnetic field.The arc column is driven by the electromagnetic force to produce a spiral,and the space size of the spiral motion increases with the increase of the magnetic field.To solve the difficulty of extinguishing the DC arc,an active arc extinguishing method based on variable resistance for grounding electrode line was proposed here,and the arcing horn with active arc-extinguishing function was designed.The electromagnetic transient simulation model of the HVDC grounding electrode line was established,which included the modules of DC current source,overhead conductor,tower,earth electrode,lightning,arcing horn and the arc extinguishing unit.It is found through simulation calculations that when struck by lightning,the voltage and current of the arcing horn decrease along the electrode line from the converter station to the earth electrode.The impact of the lightning strikes close to the converter station is more serious than those close to the earth electrode.The absorbing energy of the arc extinguishing unit determines the arc extinguishing ability of the arcing horn,and the minimum absorbing energy that the arc extinguishing unit needs to meet is determined by the electrical characteristics of the arcing horn when struck by lightning.The grounding current has little effect on the absorbed energy of the zinc oxide valve.The valves can quickly extinguish the arc of the arcing horn.The absorbed energy is mainly the energy of the lightning.According to the simulation calculation results,the parameters of the zinc oxide valves were designed,and a new type arcing horn with active arc extinguishing function was developed.The tests showed that the preformance of the developed arcing horn meet the requirements and it can be applied to practical engineering.This work unveils the electrical and motion characteristics of the DC arc of grounding electrode line,which provide a theoretical basis for further research on DC arc.The DC arc extinguishing method proposed in this work would provide important guidance for solving the important arc extinction problem of the EHV/UHV DC grounding electrode lines.