Characteristics of streamer discharge path under impulse voltage

Author： GOZ Electric Time：2024-09-01 09:26:16 Read：11

In recent years, my country's ultra-high voltage direct current transmission projects have been continuously developed, greatly improving the transmission capacity of the lines. At the same time, the insulation requirements for equipment have also been greatly increased. As the most widely used insulating medium, the insulation characteristics of air gaps have gradually become one of the key issues in the insulation design of ultra-high voltage transmission projects. Since the 1970s, domestic and foreign scholars have conducted a large number of experimental studies and simulations, and obtained the physical mechanism, characteristic parameters, and random characteristics of gap discharge, such as discharge start delay, development direction, and discharge path dispersion.

Streamer is the initial stage of discharge in a non-uniform electric field. It occurs before the leader and flashover stages and has an important influence on the gap breakdown process. At present, the experimental research on streamer discharge characteristics is mainly carried out with streamer electrical parameters and morphological parameters as the research objects. Related literature uses a pressure tank to change the air pressure and humidity during the discharge test, and it is found that the degree of ionization of the streamer head decreases with the decrease of air pressure. The related literature uses ICCD camera to observe the optical morphology of streamer development in the wire-plate gap, and it is found that under the action of pulse voltage, the external voltage rise rate increases from 1.5×10^3kV/μs to 2.0×10^3kV/μs, and the streamer development speed increases from 0.5×10^6m/s to 2.5×10^6m/s. In order to further study the common branching phenomenon in the development of streamer discharge, the related literature uses the axial-radial observation method to build a long streamer discharge development observation platform, and uses the pseudo-color restoration algorithm to process the streamer development image to obtain a more recognizable streamer development morphology. The streamer branches are dendritic structures. The related literature built a short air gap discharge experimental platform, changed the external voltage and electrode structure to conduct experiments, and found that when the external voltage increases, the number of streamer branches increases accordingly. The related literature experimentally studied the streamer initiation characteristics under impulse voltage, and found that the streamer initiation delay obeys the Rayleigh distribution. Through experimental research, the theoretical basis of streamer discharge was established, the influencing factors of discharge characteristics were explored, and it was gradually recognized that the discharge has the typical characteristics of dispersion and randomness.

In order to explain the dispersion phenomenon of streamer discharge, the relevant literature is based on the experimental data of the streamer start delay of the rod-plate gap under the positive polarity lightning impulse voltage, considering the influence of the voltage change rate on the discharge start, and fitting the streamer start field strength criterion under the positive polarity impulse voltage suitable for engineering practical application. The dispersion of the streamer start is reflected by the free electron formation delay, and the streamer start delay is divided into the boost delay and the statistical delay of free electron formation. In order to analyze the randomness of the discharge, the relevant literature established a three-dimensional Monte Carlo particle collision numerical model for the branching phenomenon in the streamer discharge process, and explored the relationship between the streamer branch development and the discharge parameters at the millimeter scale. The calculation accuracy is high and can reflect the real discharge morphology and structural characteristics, but the model calculation is too large and it is difficult to expand to the numerical simulation of m-level long air gaps. Based on the fractal dielectric breakdown model, the WZ model of the discharge channel voltage drop and the discharge threshold field strength parameters is introduced. The discharge threshold field strength parameters are used to suppress the growth of the discharge channel to the left and right sides. The numerical simulation results tend to the experimental observations, and the calculation amount is small. On this basis, the relevant literature proposed two simulation calculation methods for streamer development: single path and multi-path. For a single path, only one path grows within a single-step development time step, and the time step is the development time of the path; for multi-path, the time step is fixed, and all paths with a development time less than the step can develop. Considering the mutual influence and competition between discharge branches, the relevant literature selects single-path fractal development. Based on the fractal model, the relevant literature further considers the influence of space charge on discharge simulation, and determines the time constant in the discharge process by solving the electric field and accumulated charge. However, the above simulation models are mainly aimed at the random development of the streamer discharge path, ignoring the streamer start delay, gap structure, and environmental factors such as air pressure and humidity.

In view of the above problems, based on the existing fractal dielectric breakdown model, this paper considers the dispersion of streamer start delay and the influence of environmental factors on discharge, introduces the positive streamer start criterion at different altitudes and temperatures, and establishes a dynamic streamer random development calculation model for rod-plate gap under space charge shielding. Based on this model, the discharge development process of different gap structures is numerically simulated to obtain characteristic parameters such as streamer start delay, development morphology, development speed and space charge amount. An experimental observation platform is built to carry out comparative tests of operational impulse discharge. The model is verified by combining the experimental results, and the relationship between the streamer axial development speed and the axial space electric field distribution is analyzed to explore the influence of gap structure and external voltage rise rate on discharge randomness.

Conclusion

Based on the positive streamer initiation model and fractal theory, this paper establishes a dynamic development model of air gap streamer discharge, studies the characteristic parameters of the streamer discharge process, such as the initiation delay, development morphology, development speed, and discharge channel space charge, and builds a 5 cm air gap streamer discharge test observation platform under the action of operating impulse voltage. Combined with the test results, the correctness of the model is verified, and the following conclusions are obtained:

(1) The initial streamer axial development speed first increases and then slows down to stop, with an average axial development speed of 5.39×10^5m/s; in the process of the secondary streamer developing toward the plate electrode, the development speed first increases and then gradually slows down. When the streamer develops to the vicinity of the plate electrode, the development speed increases again, with an average axial speed of 5.70×10^5m/s. The development speed is positively correlated with the change trend of the field strength distribution of the streamer head.

(2) The streamer development speed and discharge development morphology are affected by the external voltage rise rate. The rate of increase of applied voltage increased from 0.45 kV/μs to 11.15 kV/μs, the average axial development speed of the streamer in the rod-plate gap increased from 5.70×10^5m/s to 8.32×10^6m/s, the fractal dimension of the discharge channel increased from 1.19 to 1.28, and the randomness of the discharge path increased.

(3) Under the same waveform applied voltage, as the electrode curvature radius increases, the electric field distribution at the electrode head becomes more uniform, effective free electrons become more difficult to appear, the critical starting voltage of the streamer becomes higher, the starting delay increases, and the dispersion of the discharge start increases.