The evaluation of direct current (DC) corona inception voltage in humid air is of importance for high-voltage equipments and high-voltage DC transmission lines, when the occurrence of DC corona in humid air is taken into account. The secondary electrons to sustain a negative DC corona discharge are produced primarily by photoemission from the electrode surface. The process of photoemission in a negative DC corona discharge can be described through the Townsend second ionization coefficient due to photoemission (γp). In this work an approach for the calculations of the Townsend second ionization coefficient γp at different relative humidities has been proposed. Using the presented approach, γp as a function of relative humidity has been calculated and compared with the experiments. The good agreement between the calculated results and the experiments are obtained. The present method is an efficient one for the calculation of γp at different relative humidities, especially for the humidities considered practically, and can be further used for the evaluations of negative DC corona inception electric fields in humid air.

The corona on conductors is of importance for ultra-high voltage transmission. Humidity in air is an important factor of affecting corona inception electric field. Due to long distances of the ultra-high voltage direct current (dc) transmission, the transmission line crossing a high humidity area is inevitable, and thus the humidity should be taken into account in evaluation of the corona inception electric field of positive dc conductor. In this work a modification of Peek formula of calculating the corona inception electric field of positive dc conductor has been performed for taking into account the effect of air humidity. Using the modified Peek formula, the corona inception electric fields for the conductors with different radii under different humidity 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 dc conductor under different humidity.

The corona characteristic of HVDC tube bus is the main factor taken into account for choosing tube bus of HVDC converter substation. The intension of corona could affect the electromagnetic environment of converter station and generate audible noise. By establishing the calculation models of electric field strength along the tube bus surface and the electric field strength on ground, the paper computes the corona onset voltage gradient and gives the relation curve of high and electrode spacing of tube bus. If the tube bus in HVDC converter substation is installed at the high and electrode spacing, got by the relation curve, it will not generate corona and its electric field strength will meet the requirements. The control indexes of tube bus corona characteristic parameters for HVDC converter substation are confirmed in combination with calculation results and related standards. Based on the control indexes, the paper gives the collocations of different tube bus types.

The corona characteristic of HVDC tube bus is the main factor taken into account for choosing tube bus of HVDC converter substation. The intension of corona could affect the electromagnetic environment of converter station and generate audible noise. For the HVDC transmission line has long distance, it is inevitable for converter station to built in high humidity area. It is necessary for studying the corona characteristics of converter station bus and choosing the best type of the bus to consider the humidity. By establishing the calculation models of electric field strength along the tube bus surface, the paper computes the influence of humidity on corona inception electric field, high and electrode spacing of tube bus. And gives the high and electrode spacing, at which the tube bus in ±660kV HVDC converter substation will not generate corona.

The HVDC busbar corona characteristic is the main factor taken into account for choosing busbar of HVDC converter substation. By establishing the calculation models of electric field strength along the busbar surface, ground total electric field strength and audible noise, the paper computes and analyses the corona starting gradient, corona starting voltage, total electric field strength and audible noise level of tubular busbar in ±660 kV HVDC converter substation. The control indexes of tubular busbar corona characteristic parameters for ±660 kV HVDC converter substation are confirmed in combination with calculation results and related standards. Based on the control indexes, the paper assesses the corona characteristic parameters with different busbar types and collocations, and the tubular busbar type and collocation for ±660 kV Qindao HVDC converter substation are obtained. The tubular busbar with diameter of 250 mm is chosen, under the environment height above 12 m.