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The variation of negative corona-generated space charge related to the humidity has been systematically calculated and analysed by computational approaches. A negative fluid model in which the humidity could be adjusted was established to calculate the charged-particle density and electric field under different applied voltages. The continuity equations were solved by a flux-corrected transport algorithm and the Poisson equation was computed by the successive over relaxation method. The variations of important parameters due to humidity were discussed including ionisation coefficient, attachment coefficient, effective ionisation coefficient and electron generation rate. Each of them increases with the rise in relative humidity (RH). The particles densities of electrons, positive ions and negative ions when RH is 50, 80 and 90% were also calculated, respectively, which decreased as RH increased. [ABSTRACT FROM AUTHOR]

The usage of medium‐voltage switchgear is more and more common in high altitude areas. Few relative standards about the selection of switchgear air gap in low air pressure conditions are currently available for referring. In this study, four different types of switchgear air gaps, as well as a typical rod‐plane gap, are taken as the samples, and their positive and negative lightning strike discharge characteristics under low air pressure are tested in the artificial climate chamber. The effects of the electrode structure, polarity, air pressure, humidity, and gap distance on the discharge performances are analysed as well. Research results indicate that the four switchgear air gaps and typical rod‐plane gap differs greatly in lightning impulse discharge performances and polarity effects. In low air pressure conditions, the lightning impulse breakdown voltage of either positive or negative polarity can be corrected by U50 = U50,0 × (P/P0)n. The discharge voltage under positive lightning impulse is more easily affected by air pressure than that under negative lightning impulse. Atmosphere humidity will cause dispersiveness of air‐gap lightning impulse discharge voltage, which performs to be non‐linear with a gap distance lower under 400 mm. Research results are referable for the design, selection and installation of medium‐voltage switchgear in high altitude areas. [ABSTRACT FROM AUTHOR]