Multiple discharges before leader inception in long air gaps under positive switching impulses

There are multiple corona bursts before leader inception when the rising rate of the applied voltage or electric field is not sufficiently high enough in long positive sparks.In existing studies,no attention has been paid to whether these corona bursts occur in the same location,and they are mostly considered directly as belonging to the same discharge.However,this paper presents that in a typical rod-plate long air gap,the multiple corona bursts before leader inception are distributed in at least two different locations,and the highest probability of three discharges occurs.Also,the discharge occurs with the highest probability in the time sequence‘tip-tip-side-tip-other side’of the electrode in the first five corona bursts.For each discharge,the first corona current is a single,double exponential pulse,while the following corona currents are mostly a superposition of multiple pulses.The above findings are mainly based on experiments in a 1.4 m air gap under positive switching impulses,in which the voltage,current,and high-speed images were recorded simultaneously.Finally,based on the experimental results,this paper discusses the effects brought by ignoring the multiple discharges on key parameters of leader inception and makes some suggestions to optimise long spark experiments.

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.

Schlieren techniques for observations of long positive sparks:Review and application

Understanding the mechanism of positive leader discharge is important in lightning protection engineering and the external insulation design in high voltage power transmission systems.During the propagation of a positive leader,some processes without light-emitting,for example,the insulation recovery process after the breakdown,cannot be observed by optical photography techniques.With the combination of the digital high-speed imaging system,the conventional Schlieren techniques offer new vistas in the long air gap discharge observation.The important features of high spatial resolution,high sensitivity,and easy arrangement make Schlieren techniques powerful and effective tools for characterising the discharge processes without light-emitting.This work presents a brief review of current Schlieren techniques and discusses the design of the Schlieren optics system for long spark observations.Several interesting phenomena discovered at different phases of long sparks with high-speed Schlieren techniques are also presented.