Experimental Study on Branch and Diffuse Type of Streamers in Leader Restrike of Long Air Gap Discharge

One of the main problems in the Ultra High Voltage （UHV） transmission project is to choose the external insulation distance, which requires a deep understanding of the long air gap discharge mechanism. The leader-streamer propagation is one of most important stages in long air gap discharge. In the conductor-tower lattice configuration, we have measured the voltage, the current on the high voltage side and the electric field in the gap. While the streamer in the leader-streamer system presented a conical or hyperboloid diffuse shape, the clear branch structure streamer in front of the leader was firstly observed by a high speed camera in： the experiment. Besides, it is found that the leader velocity, width and injected charge for the branch type streamer are greater than those of a diffuse type. We propose that the phenomenon results from the high humidity, which was 15.5-16.5 g/m3 in our experiment.

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.

Observation and analysis of positive leader re-illumination in a 10 m ultra-high voltage transmission line gap under switching impulse voltages

The leader propagation is one of the most important stages in long air gap discharge.The mechanism behind leader re-illumination remains unclear.In high humidity conditions(20.0–30.1 g/m^(3)),we have conducted experiments of long sparks in a 10 m ultra-high voltage(UHV)transmission line gap under switching impulse voltages.The positive leaders predominantly propagate discontinuously,with almost no significantly continuous propagation occurring.The leader channels are intensely luminous and each elongation segment is straight,with streamers resembling the“branch type”which differs from the“diffuse type”streamers at the front of continuous propagation leaders.The distribution of the propagation velocities is highly random(3.7–18.4 cm/μs),and the average velocity(9.2 cm/μs)significantly exceeds that of continuous propagation(1.5–2.0 cm/μs).Analysis suggests that the current-velocity models suitable for continuous leader propagation do not align well with the experimental data in re-illumination mode.Based on the discharge current waveforms and optical images,it is speculated that the newly elongated leader in re-illumination mode does not evolve gradually from the stem(about 1 cm)but rather evolves overall from a thermal channel much longer than stem.

Experiment on Leader Propagation Characteristics of Air Gaps in UHVDC Transmission Towers Under Positive Switching Impulse Voltages

Rapid developments in EHV/UHV transmission systems require a deeper understanding of the mechanism of long air gap discharge.Leader propagation is one of the main processes in long gap breakdown.In this paper,the leader propagation characteristics of real size±800 kV UHVDC transmission tower gaps under positive switching impulse voltages(185/2290μs)are investigated.An integrated observation platform consisting of an impulse voltage divider,a coaxial shunt,a high-speed video camera,and a set of integrated optical electric field sensors(IOES),is established.The waveforms of impulse voltage,discharge current,electric field variation at specific positions,and time-resolved photographs of discharge morphology are recorded.Axial leader velocity and the relationship between leader advancements and injected charge are obtained.The typical value of leader stable propagation velocity is 1.7–2.2 cm/μs,which varies slightly with the gap length and applied voltage amplitude.The leader velocity in the re-illumination process is much higher,and is seen as varying from 5 cm/μs to 30 cm/μs,with an average value around 10 cm/μs.The charge in leader channel per unit length is 20–40μC/m,which illustrates a near-direct proportion relationship between discharge current and leader velocity.The observed parameters are important for further simulation of the tower gap breakdown processes.