Research On The Measurement Method For Temperature Field Of Positive Leader Discharge And Its Thermal Characteristics

Insulation characteristics of long air gap are key subjects for the external insulation design of high voltage transmission system. Leader discharge is the dominant process of the long air gap discharge and its main characteristic is thermal ionization. Due to the large randomness and small spatial scale, the thermal characteristic parameters of the leader discharge lack accurate measurement and profound study. This paper focuses on the process of long air gap leader discharge under positive impulse voltages, the measurement method for the leader temperature field is performed, and the characteristics of inception and development of leader discharge are studied in depth.Firstly, the principle of gas temperature measurement method was described according to the light propagation characteristic in non-uniform flow field. The main lenses in the schlieren system were designed aimed at eliminating the spherical aberration in order to satisfy the high spatial resolution demand. The imaging system was designed based on the high speed camera and high spatial and time resolution observation was acquired. The optimization design of the schlieren system was verified by analyzing the recorded gray image under different knife edge positions.Secondly, the elliptic cross section of the leader inclined development was discussed, and a new Abel inversion considering the elliptic cross section was proposed. The relationship of the temperature and refractive index was established in consideration of all the particle components in the leader channel. In order to decrease the error of the Abel inversion, the image interpolation was introduced to increase the pixel resolution. The temperature fields produced by the alcohol burner were measured by the schlieren system and its accuracy was verified in contrast to the measurement results of the precise thermocouple.Next, the synchronous observation system consisting of temperature measurement, discharge current measurement, spatial transient electric field measurement, high speed optical image observation and synchronous trigger system for leader discharge under positive impulse voltages was established. The1m rod-plane leader inception experiments were conducted, the spatial and temporal distributions of the stem temperature during the streamer to leader transition were obtained and the existing temperature criterion of the leader inception was verified using the synchronous observation system. The existing leader inception model was updated by considering the distribution of the current density injected into the stem and the validity of the model was verified in contrast to the results of experiments.At last, the1m rod-plane leader development thermal characteristics experiments under impulse voltages were carried out using the synchronous observation system. The spatial distributions of temperature in the leader channel were obtained and analyzed. The influences of time and inject charge on the thermal diameter were counted and the thermal expansion under different voltage variation rate were obtained and discussed. The mechanism of thermal expansion of the leader channel was studied based on the thermodynamics theories, and the model was validated by the measured results. The influences of development length on the thermal diameter were obtained, and the relationship between the thermal diameter and development length was studied.