Observations of corona point discharges from grounded rods under thunderstorms

Abstract

We investigated features of corona point discharges when a grounded conductive rod is subject to a high background electric field. In the laboratory, high voltage is applied to an upper plate in a plane-to-point experiment, and the discharges are observed from a grounded rod using an ultraviolet camera and a photosensor. The current is measured using a shunt resistor, a current transformer, and a high impedance ammeter. We have found that corona current pulses have fast rise time (tens of nanoseconds) and slow decay (hundreds of nanoseconds). For the initial stage of the discharges, above a certain threshold, the frequency of pulses increases with the voltage applied. In the field, one current sensor coupled to a grounded rod, 1.5 m above a roof, was installed in a site located at an altitude of 2525 m above sea level. The same pulses observed in the laboratory are recorded during periods of electric field enhancement caused by the movement of charged clouds or lightning activity near the sensor. For that specific installation, the frequency of pulses is correlated with the ambient electric field measured 250 meters away. To the best knowledge of the authors, this is the first time in which such correlation is made for outdoor measurements, seeing that other works correlate the electric field with the average current flow. Pulses of positive corona were no longer observed when the electric field magnitude was lower than 1.8 kV/m and pulses of negative corona were more atypical and presented a higher threshold, of about 3.8 kV/m. This study is relevant for understanding the production of corona and space charges in high structures, such as wind turbine blades, towers, and buildings in general.

Optical Signatures Associated With Streamers and Leaders of Laboratory Discharges

“…We measured blue emissions associated with negative streamers during the voltage rise in the rod‐to‐rod experiment. They are more difficult to see due to their lower amplitude and shape (Arcanjo et al., 2020). We observed a large pulse in the blue associated with the negative leader inception and step (see Figure 3a).…”

“…The blue irradiance presented some pulses of low amplitude during the voltage rise, likely related to the negative streamers that are incepted from the upper rod. Negative coronas in the air usually have less current amplitude and are fainter than positive coronas for similar high voltage levels (Arcanjo et al., 2020). We did not observe evident pulses of red irradiance before the last 5 µs of the discharges.…”

On the Induced Currents to Wind Turbines by the Earth’s Atmospheric Electric Potential: Experiments With Drones

“…As it has already been observed [23], tall structures under fairweather with relatively low potential gradients induce low currents due to point discharge phenomena, which is also dependent on the sharpness of the tip. Additionally, for wind turbines, a second source of current induction is present because of the rotation of the blades, as revealed in these experiments.…”

“…This is important for the model simulation analysis performed later in the article since point/corona discharge is expected to occur at the tip. The use of different electrodes for the tip and selection of their sizes was informed by observations in [23], where sharp needles were found to start point/corona discharge at lower threshold voltages than rounded ones. For the flights reported in this paper, the electrode used in each flight is indicated in Table 1.…”

DC-driven positive streamer coronas in airflow

“…For streamer coronas driven by DC voltage (so-called onset streamers or transient coronas), the effect of wind is of particular interest since space charge transport is thought to be the key factor driving the self-pulsating behavior of the discharge [12,13]. The influence of airflow in self-pulsating discharges driven by DC-voltage has implications in a wide range of problems ranging from atmospheric electricity [14] to plasma sources for chemical treatment or bio-engineering [15,16,17].…”