Influence Of Rise Rate Of Applied Voltage On Positive Leader Inception In Long-air-gap Discharge

The characteristics of positive leader discharge in long-air gap are the important basis for the insulation design of power facilities and the design and performance evaluation of lightning shielding terminal.Leader inception stage is the key to leader formation at the electrode(or developing leader)head,which consists of three sequential processes:first corona,primary dark period and lead inception.A large number of experimental and theoretical studies have been carried out on these three sequential processes,but there are still some shortcomings,including:i)the randomness of the inception of first corona,the characteristics and formation mechanism of current pulse waveform,upon which the influence of the increase of the rate of applied voltage;ii)the existence conditions of single primary dark period,multiple primary dark periods and the disappearance of dark period,and their characteristic parameters affected by the increase of the rate of applied voltage;iii)there are deviations between the key model criteria of leader initiation and authoritative experimental results,and the validity is challenged.The physical process mechanism of the leader inception is in urgent need of clarification.This paper focus on the three aspects:the discharge behavior of first corona,the change rule of the duration of primary dark period and the mechanism of the leader inception process during primary dark period.The rate of the rise of the applied voltage was used as control variable,carried out a systematic experimental study on the response laws of various electrical parameters in the inception phase of positive leader in long-air gap,and explored the development behavior mechanism of first corona discharge and the physical mechanism of the second corona(leader)inception based on theoretical analysis and reasoning.The experimental platform,observation scheme and experimental environment control method were designed.Impulse voltages with high rate of voltage rise varying in a wide and intensive range were applied to 10 m rod-plate gap(rod radius R=1 cm).The measurement precision and range of the signals of the experimental voltage and the discharge current as well as synchronous delay and error of the two signals were determined.According to the climatic conditions of the place where the experiment was conducted,a method of controlling the experimental humidity by seasonal temperature was proposed:in summer,the absolute humidity was 21-25 g/m3;in winter,the absolute humidity is 1-4g/m3.The absolute humidity of each group was controlled in a narrow range,which greatly restrained the influence of humidity change on discharge.At the same time,the results of electrical parameters at the initial stage of the pilot at very high humidity and very low humidity were compared.The discharge behavior of first corona at high rate of voltage rise was studied.Based on the synchronous observation data of the applied voltage and the waveform of the discharge current of first corona,the characteristic parameters of current pulse of first corona(including injected charge Qic,peak Ip,rising edge time tR and tail time tT)and the influence rule of its inception voltage UiC,the increase rate of applied voltage and humidity were obtained.Based on the authoritative theoretical research results of Gallimberti and Les Renardieres,a graphical interpretation of the discharge behavior mechanism of first corona was proposed,together with the current pulse formation mechanism:i)the potential difference(Us-Ug)between the geometric potential distribution Ug before corona discharge and the electrostatic potential distribution Us after corona formation drives the discharge at the head of the streamer and determines the velocity vs of the development of the space charge wave head and the positive ion Ns generated;ii)the current pulse shape is determined by the divergence of electric field distribution at the electrode head,that is,under the extremely uneven electric field structure,the potential difference(Us-Ug)increases rapidly at first and then decreases gradually along the direction away from the electrode,while the space charge surface(collision cross section)continues to expand,forming the fast rising front and the double exponential falling tail of the current pulse.Based on the experimental results of this study,the influence mechanisms of its inception voltage Uic,and the rate of the rise of the applied voltage were analyzed and proposed:the increase of UiC or the rate of the rise of applied voltage can enlarge the space potential difference(Us-Ug)at each position,and increase the development velocity and charge quantity of space charge wave at each position,resulting in the increase of peak Ip of current pulse,tail time tT and injected charge quantity QiC.Under the same Ip,the increase of the rate of the rise of applied voltage further enhances the development speed vs and the charge quantity of space charge wave far from the electrode region,but due to the adsorption of electrons,the increase of the charge quantity injected at the tail time weakens,resulting in a linear relationship between Qic-Ip and less affected by the increase of the rate of the rise of applied voltage.Under high humidity,the increase of the number and concentration of hydrating electronegative oxygen molecules enhances the adsorption of electrons generated by the discharge,and increases the density of negative ions retained in the streamer channel of corona discharge area,resulting in the decrease of the rising time tR,the tail time tr and the charge quantity QiC of the current pulse flowing into the electrode.The change rule of the duration of primary dark period under high rate of voltage rise was studied.Based on the synchronous waveform data of applied voltage and discharge current at the inception stage of leader,the correlations between characteristic parameters of first corona(Uic and QiC),the characteristic parameter of rise rate of the applied voltage(mean value of the rate of voltage rise during primary dark period)and the duration of primary dark period were obtained.It was found that the voltage incrementΔUd during primary dark period decreases with the increase of the rate of voltage rise in high humidity climatope.For the first time,a physical quantity which quantitatively measure the rate of voltage rise is proposed,and a method is established to obtain the quantitative relationship between the duration of primary dark period and the rate of voltage rise.The physical process mechanism of leader inception under high rate of voltage rise was studied.Based on the facts and theory analysis of three types of phenomena of primary dark period,such as single primary dark period,multiple primary dark periods(multiple corona pulses)and the diappearance of primary dark period,the mechanism for the inception of second corona was first discovered and put forward:The residual positive and negative space charges in the discharge region of first corona are dislocated by a continuous applied electric field,with negative charges flowing towards the electrode and positive ions moving away from the electrode,forming the"polarization-like" effect;in a short period of time and a short distance of space charge movement,the head field near the electrode and corona region increases simultaneously,triggers the restart of the ionization wave of corona head and the inception of second corona.The physical mechanisms for the emergency of multiple corona pulse and for the leader inception were proposed:each corona current pulse has low peak value and small development scale,and the corona region is near the electrode.The applied voltage and electric field has a strong effect on the charge movement in the corona area,that is,the space charge dislocation movement is more rapid and intense,and the electric field in front of the corona head recovers faster,so that the next corona discharge will restart and develop in a short time interval before the wave front of the previous corona charge.After the space charge wavefront radius is enlarged to a certain size by several corona pulses,the forward electric field is uniform enough to achieve the initial conditions for generating a continuous development leader.The physical mechanism for the disappearance of primary dark period was analyzed and obtained,which proved the existence of "fast mechanism" for the development of positive corona streamer.