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Experimental evaluation of discharge characteristics in inhomogeneous fields under air flow

The impact of high velocity air flow on discharge characteristics generated in strongly inhomogeneous electrical fields has been investigated under DC and combined DC + impulse voltages. In the laboratory, a needle plane electrode configuration with a gap distance of 20 cm was exposed to high voltages and a laminar air flow up to 22 m/s. In the first setup, the gap was exposed to a variable DC potential of up to 100 kV in order to create space charges in the vicinity of the electrode. The impact of the air flow on partial discharges and the dynamic behavior of the space charges is evaluated by means of partial discharge measurement and ultraviolet photography. The results show that the air flow increases the frequency of partial discharges in the gap due to an increased rate of space charge removal in the high field area around the tip of the electrode. The partial discharge behavior shows higher dependency on air flow at positive tip polarity as compared to the negative polarity. In the second setup, the standard impulse voltage created by a multistage impulse voltage generator was superimposed to a DC voltage, which continuously created corona and space charges around the tip of the electrode. Breakdown of the gap was triggered by means of the impulse voltage. The DC offset was varied from 0 kV to 100 kV in 20 kV steps and the 50 % flashover voltage was determined for both polarities. The results show that the impact of the air flow is highest at negative DC potential with negative impulse voltage, with an increase in flashover voltage of 40 % when air flow is applied. At positive polarity, the impact of air flow is only observable at 100 kV when the wind extinguishes the stable streamer stem.