Disconnector operations in GIS may generate very fast transient overvoltage (VFTO), which is characterized by its high peak value, short rise time, and high frequencies. Mostly, researches on VFTO are focusing on the apparatus involving in the active circuit. However, VFTO may pass through open devices by electromagnetic coupling. To measure VFTO under this situation, lots of field experiments have been carried out in a 750kV substation. VFTO is measured at the test tap of the high voltage reactor bushing, using the sensor we developed. In one of the experiments, the breaker is kept open, VFTO is generated by disconnector operations (in AIS)on one side of the breaker and measured on the other side. Results show that under this situation, VFTO could reach the peak values of about 55kV and70kV, the main oscillation frequencies are about 236kHz, 535kHz and 1.59MHz. To verify this case further, computer simulation has also been carried out in the laboratory with the help of ATP. The results agree with each other well.
The corona discharge effect is one of important problems in the design and operation of transmission lines. The transmission lines pass by the high altitude region with low air pressure, which brings many special problems to the design and operation of the transmission lines. Temperature and humidity also have obvious effect on the corona characteristics of the transmission lines. Consequently, it is signiticant to study the impact of air pressure, temperature and humidity on the corona characteristics and discharge mechanism of transmission lines. A computation model is adopted for evaluating the corona inception voltage on conductors, which is established according to the gas discharge theory. The charge simulation method is introduced to the computation model to calculate the electric field intensity. Then the criterion of corona inception voltage which considers whether the number of positive ions in the secondary avalanches is greater or equal to that in the primary avalanche is used to calculate the corona inception voltage of conductors. The influence of air pressure, humidity and temperature on the corona inception voltage of conductors is analysed systematically. Corona inception voltage decreases as air pressure is lowered or temperature is raised. It is mainly due to the enlargement of ionization zone as a result of the increase of effective ionization coefficient. Simultaneity, corona inception voltage decreases with humidity increasing for strengthening of collisional ionization intensity.
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