Study On Influences Of Air Humidity On Direct Current Corona Discharge

In order to implement the energy strategy of transmitting the power from west to east in china, many transmission lines have been built. For high voltage direct current (HVDC) transmission, corona discharge should be taken into account. The corona discharge can generate not only power loss, but also audible noise and radio noise, which may lead to a poor environment around the transmission lines. With the development of economy, environment problems have attracted much attention and are restricted by the rules of environmental protection. Thus, the corona becomes an important factor for considering the structure of the HVDC transmission line as well as its construction costs. Due to its long distance, the HVDC transmission will travel the area with the complicated landforms and high humidity. By reason of this, it is of significance, theoretically and practically, to study the effects of humidity on the corona.In this thesis, the studies on the influences of humidity on direct current (DC) corona are presented. The effects of humidity on the parameters of dominating the corona have been systematically calculated and analyzed. These parameters include ionization coefficient, attachment coefficient, photon adsorption coefficient, the drift velocities of the electrons and positive ions, and the radius of the corona layer. Based on both the division for humid air and the analyses for the compositions of the corona inception electric field of DC conductor of taking into account the humidity effect, and by means of the numerical simulations of the corona inception electric field, the modification for Peek formula of calculating the corona inception electric field of DC conductor has been performed. Using the modified Peek formula, the influences of humidity on the different corona effects have been systematically calculated. The main contents and results are summarized as follows:(1) The introductions for the compositions and characters of ambient air as well as the physical processes in corona discharge including ionization and attachment have been given. Based on these, the mechanisms of the corona discharges for the conductors with positive polarity and negative polarity, respectively, have been described. The one-dimensional models for the distributions of space charge number densities in positive and negative corona discharges have also been presented.(2) The characteristics of corona discharge are related to the following parameters, i.e., ionization coefficient, attachment coefficient, photon adsorption coefficient, the drift velocities of the electrons and positive ions. Therefore, the influences of air humidity on corona discharge are governed by the humidity dependence of these parameters. Based on a division of humid air into dry air and water vapor, the systematical calculations of the effects of air humidity on these parameters have been performed, and the relationships between these parameters and air humidity have also been obtained. These relationships are as follows. The ionization coefficient increases with increasing humidity, and this effect is enhanced by higher electric filed. With the increase of humidity, the attachment coefficient increases for lower electric filed, and reverse is true for higher electric filed. The photon adsorption coefficient increases with increasing humidity. The drift velocities of electrons increase and those of positive ions decrease, when increasing humidity. These effects are weakened for higher electric filed. In addition, the humidity effect of the corona layer has been calculated, showing that the radius of the corona layer decreases with increasing humidity.(3) Based on a one-dimensional model for the distributions of space charge number densities in the corona layer, the effect of humidity on the current density has been calculated and analyzed, and the conclusions are given below. For the positive polarity conductor with large radius, the current density decreases with increasing humidity, and the increase of the electric filed at the surface of conductor enhances this effect. For the positive polarity conductor with small radius, the current density presents the variety from decrease to increase with humidity. In this process, the increase of the electric filed at the surface of conductor makes this effect weak when the current density decreases, and the reverse is true when the current density increases. For the negative polarity conductor, the current density increases with increasing humidity, and this effect is enhanced by the electric filed at the surface of conductor.(4) The Townsend second ionization coefficient is an important parameter for describing negative DC corona discharge. The investigation on the influence of the film, which is formed due to the water molecules adsorbed on the surface of conductor, on the work function has been done under different air humidities. On the basis of this, an approach for the calculations of the Townsend second ionization coefficient yp at different humidities has been proposed. Using the proposed approach, γp as a function of humidity has been calculated and compared with the experiments. The good agreement between the calculated results and the experiments are obtained. The proposed approach is of importance for the study on the conductor discharge when taking into account the water molecules adsorption on the conductor surface.(5) Peek formula has been widely applied for the past decades because of its convenient use, but it does not take into account the influence of humidity on the corona inception electric field. So far, there is no an analytical expression for estimating preferably the corona inception electric field on conductor, when taking into account air humidity around conductor. Based on a division of humid air into dry air and water vapor, by analyzing the compositions of the corona inception electric field of DC conductor of taking into account the humidity effect, and by means of the numerical simulations of the corona inception electric field with different air humidities, the investigation on the modification for Peek formula has been performed, and the modified Peek formulas of calculating the corona inception electric field of positive and negative DC conductors have been given for taking into account the effect of air humidity. Using the modified Peek formula, the corona inception electric fields for the conductors with different radiuses under different humidities are calculated and compared with the reported experimental results. Reasonable agreement between calculated results and the experiments is obtained, which shows that the modified Peek formula is a good one for the effective and easy calculations of the corona inception electric field of positive and negative DC conductors under different humidities.(6) Using the modified Peek formulas given in this thesis, the systematical calculations and analyses for the effects of air humidity on the corona audible noise, the corona loss, and the corona radio interference have been performed. The following results are obtained.For positive polarity conductors, the A-weighted acoustic power level (PWL) decreases for large radiuses and increases for small radiuses with increasing humidity. For negative polarity conductors, PWL increases generally with increasing humidity, and this effect becomes evident for small radiuses, compared to large ones. For the conductors with different radiuses and polarities, L50sound level (SLA) increase with increasing humidity.For positive polarity conductors, the corona loss decreases for large radiuses and increases for small radiuses with increasing humidity. For negative polarity conductors, the corona loss presents the variety from decrease to increase with humidity for large radiuses, and increases with increasing humidity for small radiuses.The corona radio interference from positive polarity conductor decreases with increasing humidity. For the conductors with large radiuses this effect becomes evident under the small electric field at the surface of conductor. For the conductors with small radiuses this effect is no obvious and is hardly related to the electric field at the surface of conductor.