Nanosecond photography observation of the discharge process across the 220 kV/500 kV insulators in lightning impulse

Long air gap discharge is the basis for the study of lightning protection and external insulation design of high voltage lines. However, there is a lack of detailed and comprehensive observation of the long air gap discharge process at the ns-level, as well as higher voltage insulator discharge observations. In this paper, the lightning impulse discharge/flashover tests are conducted on 220 kV glass insulators and 500 kV glass/composite insulators. Firstly, the U50% test is performed to determine the U50% value of each insulator under standard lightning impulse, and 1.1U50%, slightly higher than U50%, is applied to ensure the breakdown of the insulator. Subsequently, emICCD is utilized with a very short exposure time (minimum 10 ns) to obtain repeated single shots of discharge images under different delays, exposure times and light intensity gain. These images were spliced together based on time sequence to create a continuous discharge process. The discharge processes of 220 kV glass insulator and 500 kV glass/composite insulator are compared and analyzed. The results show that in the early stage of discharge, the discharge processes at the low voltage end of both 220 kV and 500 kV glass insulators are more intense. Additionally, the downward leader is faster than the upward leader in the discharge process of 500 kV composite insulators.


Introduction
Long air gap discharge serves as the foundation for lightning protection and external insulation design of high voltage lines.Current research primarily focuses on the test of long air gap discharge characteristics, long air gap discharge mechanism and the simulation model optimization for long air gap discharge [1].Among them, scholars both domestically and abroad have conducted numerous studies using discharge images to investigate the discharge mechanism of insulators with different materials, lengths and polarities.However, different exposure times, shooting intervals and shooting discharge stages have different degrees of influence on the subsequent discharge process analysis.The length of exposure time can result in variations in discharge morphology.In the initial stage of insulator discharge observation research, CCD cameras with an exposure time of microseconds range [2][3][4] were generally used.However, the extended exposure time does not accurately depict the transient spatiotemporal distribution during discharge, leading to deficiencies in the study of microscopic parameter characteristics.In recent years, the ICCD cameras have made up for this deficiency.With nanosecond exposure times, ICCD cameras can capture the transient changes in discharge patterns and weak discharge stages induced by lightning impulse voltage with rapid voltage rise and short breakdown time.[5][6][7][8] Zhang et al, used ICCD to capture a single shot of the positive and negative standard wave impulse discharge process of a 110 kV insulator.The shortest exposure time was set as 10 ns, and by sequentially splicing the images, the complete breakdown and non-breakdown discharge processes were observed [9].At present, the study of the long air gap discharge process lacks the nanosecond single image observation test of insulators with higher voltage levels under lightning impulse discharge.
Therefore, in this paper, positive standard wave impulse tests are carried out for 220 kV glass insulators and 500 kV composite/glass insulators.Using extremely short exposure time (minimum 10 ns), emICCD is employed to capture single shots of the discharge images with different delays and light intensity gains during the discharge process.These images are then pieced together to form a continuous discharge process, allowing for the analysis of the spatio-temporal distribution characteristics of breakdown discharges in 220 kV/500 kV insulators made of different materials under the influence of positive standard waves.

Experimental condition
The discharge test was carried out in the National Engineering Laboratory of UHV Engineering Technology (Guangzhou).The test device mainly includes an impulse voltage generator, a voltage divider, a console, an emICCD, a high-speed camera, a 220 kV glass insulator and a 500 kV composite/glass insulator.The dry arc distance and test waveform of the test sample are shown in Figure 1.
The test contents mainly include the U50% test and discharge observation test of 220 kV glass insulator and 500 kV composite/glass insulator under positive standard lightning impulse.Firstly, the U50% test is carried out to determine the U50% value of each insulator under standard lightning impulse, and then a voltage value slightly higher than U50% is applied to ensure the breakdown of the insulator.The emICCD is used to shoot the discharge image under different delays and light intensity gains during the discharge process under a very short exposure time (10 ns).At the same time, a large number of repeated tests are carried out to reduce the dispersion of the discharge image.

U50% test
The measurement methods and steps of insulator and rod-plate gap U50% are as follows: ∂ Step 1: The test sample is installed, and the capacitance resistance parameters of the impulse voltage generator are adjusted so that the test waveform generated by the circuit is within the error range specified by the standard.∂ Step 2: Impulse voltage is applied to the sample according to the test conditions.In order to improve the accuracy of U50% determination, the amplitude of each stage voltage rise and fall is 1 kV.Until the first critical breakdown voltage is found, the test data are recorded from this critical breakdown voltage, at least 30 times of pressurization, and the wave tail time, breakdown time and breakdown voltage value are recorded at the same time.∂ Step 3: The test conditions are changed, Step 2 is repeated, and the test data and meteorological information are recorded.The U50% value is calculated based on the measured voltage data.The calculation method is as shown in Formula (1): where Ui is the measured value of the i-th destructive discharge voltage, kV.ni is the number of tests under the same applied voltage Ui. n is the total number of effective tests.

Discharge observation test
The peak voltage of 1.1U50% is applied to the discharge image to ensure the insulators are broken down during each discharge test.At the moment of the ball gap breakdown of the impulse voltage generator, the trigger signal generated by the bottom trigger box is transmitted to the console of the control room through the fiber optic cable.The waveform recording software begins to record the test waveform.At the same time, the trigger signal of the shutter is transmitted to the ICCD camera through the output signal interface of the console.The ICCD trigger level is set to 0.5 V.After receiving the external trigger signal, the ICCD takes a single shot of the discharge process according to the delay, exposure time and intensity gain set by the software.The length of the fiber optic cable from the trigger box to the console is about 20 m, and the distance from the console to the ICCD is about 45 m.Due to the large dispersion of discharge time, the discharge images taken at the same time have different degrees of difference.Therefore, a large number of repetitive tests are carried out to reduce the difference in discharge images taken at each delay time.Thousands of discharge images are taken in the discharge test, and the light intensity gain of each shot is between 1 and 1000 according to the light intensity of the discharge stage.

Discharge observation test
The impulse discharge test was carried out according to the experimental step, and the U50% test data under different working conditions were obtained, as shown in Table 1.Based on the U50% value obtained in the experiment, 1.1U50% is used as the peak voltage applied in the subsequent discharge image shooting experiment to ensure that the insulator can stabilize the breakdown and realize the complete process of insulator breakdown and discharge shooting.The applied voltage waveform and breakdown point are shown in Figure 2.

Discharge images of 220 kV glass insulator
The discharge process images of the 220 kV glass insulator are shown in Figure 3.It can be seen from Figure 3 that under the impact of a positive standard lightning wave, the 220 kV glass insulator begins to streamer discharge at the high voltage end at t=26 ns to produce a large number of electrons.As the voltage action time increases, the streamer continues to develop to the ground end.After t=1800 ns, the streamer has penetrated the space between the two ends of the glass insulator.At this time, the applied voltage still maintains a high peak value, and the degree of ionization at both ends is further strengthened.The first insulator at both ends of the test sample shows a bright cluster arc light, and the arc light at the grounding end is brighter.As the degree of ionization continues to increase, the arc at both ends begins to develop towards the middle.It can be seen from the discharge image of t=2100 ~ 4200 ns that the arc at the low voltage end develops faster, the arc at both ends flashovers along the air in the first few insulators, and the discharge path in the middle is flashover along the surface of the insulator.

Discharge images of 500 kV insulator
The discharge images of 500 kV composite/glass insulators under positive standard lightning impulse are shown in Figures 4~5.The dry arc distance of an insulator with a 500 kV voltage level is long, up to 5~6 m, so the statistical delay of discharge is much longer than that of an insulator with a low voltage level.Obvious streamer discharge will occur after t=4000 ns, so the trigger delay of discharge shooting is set to t ≥ 4000 ns so that the discharge process can be observed more clearly.
It can be seen from Figure 4 that when t=4000 ns, the streamer has penetrated between the two grading rings of the insulator, and the degree of spatial ionization on the surface of the insulator is very large.At this time, as the voltage continues to impulse, the downward leader appears first at the lowvoltage grounding end of the upper end than the high-voltage end.After the downward leader develops a certain length (t=4300 ns), the upward leader begins to appear at the high-voltage end.The upward leader and the downward leader develop to the middle of the insulator at the same time, and the downward leader is faster and presents a spiral development.When t = 7000 ns, the two pilot channels are relatively close, and the gap has completed the breakdown at t=10000 ns.The discharge channel of the final breakdown of the 500 kV composite insulator is connected from the left side of the lower equalizing ring to the right side of the upper equalizing ring.It can be seen from Figure 5 that for the discharge process of a 500 kV glass insulator under the impact of a positive standard lightning wave, after the streamer penetrates the whole channel, the upper glass insulator begins to discharge along the surface of the first few insulators, resulting in a light arc of cluster discharge.Because the discharge path is along the surface of the insulator, the first discharge at the low voltage end does not start from the installed grading rings.From the discharge images of t = 6500 ~ 8600 ns, it can be seen that the discharge of the glass insulator does not develop from both ends to the middle, but the discharge along the surface of the upper end gradually extends to the lower end, and finally breaks down along the air gap between the last few insulators.
The differences in discharge paths of insulators of different materials can be obtained more clearly through the ns-level exposure shooting of emICCD, which is consistent with the simulation results in Zhang et al.'s work [10], in which the electric field intensity at both ends of the composite insulator is the largest, and the electric field intensity in the middle is similar and changes smoothly, resulting in the discharge path being away from the air gap on the surface of the insulator.While glass insulators have many of the maximum electric field strengths, discharge occurs along the surface of the insulator, which will affect the discharge time.At the same time, regarding the development speed of the discharge path, the formula for calculating the lead development speed proposed by CIGRE and its coefficient has been widely recognized [11], in which the k value is the coefficient of the lead development speed, and the larger the k value is, the faster the lead development speed will be.
Under positive lightning impact, the leading speed development coefficient k recommended by CIGRE and the velocity of the leader development calculated by the discharge photograph are shown in Table 2.
Table 2. k value recommended by CIGRE and velocity of leader development.

Air gap construction
Composite insulator 0.8×10 −6 0.78~0.81 Glass insulator 1.2×10 −6 1.62~2.27 In this paper, it is determined that the breakdown times of 500 kV glass insulator and composite insulator under the impact voltage of 1.1U50% are about 8.7 λs and 10 λs, respectively.The velocity of the leader development is calculated according to the process from leader initiation to breakdown, indicating that the discharge development speed of the glass insulator is faster than that of the composite insulator, which is consistent with the size of the CIGRE recommended k value.

Conclusions
To investigate the characteristics of insulator breakdown discharge development, this paper has conducted observational experiments on 220 kV and 500 kV insulators under positive standard lightning impulses.The emICCD camera captures a clearer discharge path and breakdown time.Additionally, comparisons of discharge paths and characteristics are made between different insulators under 1.1 times U50% impulse.The results are as follows.Different materials of insulators cause different discharge paths due to different electric field distributions: composite insulators discharge along the air, while glass insulators discharge along the glass.At the same time, the discharge path affects the discharge velocity.According to the photographs according to the process from leader initiation to breakdown, the breakdown times of glass insulator and composite insulator are 8.7 λs and 10 λs respectively, and the velocities of the leader development are calculated as 0.78~0.81m/λs and 1.62~2.27m/λs, which corresponds to the lead development coefficient k of glass insulator, recommended by CIGRE, indicating that the discharge development rate of glass insulator is faster than composite insulator.