Lightning interaction with tall objects
Project Name: Investigation of Lightning Interaction with Tall Objects.
About the project
Nowadays, more and more tall objects are built all over world, such as power tower and windmills. They are very prone to lightning strikes. Once lightning strikes to these objects, it probably leads to outage of power supply and damage of the windmill structure. Consequently, the objective of our research is to find out why tall objects are subject to lightning strikes, that is, the lightning attachment to tall objects.
What is lightning? Lightning is a transient, high-current electric discharge phenomenon, which is divided into cloud-to-cloud (CC) lightning and cloud-to-ground (CG) lightning. CG lightning is most harmful, and it is divided into four types by initial leader propagation direction and charge polarity, namely downward negative lightning, downward positive lightning, upward negative lightning, upward positive lightning.
Recently, I work on the lightning interaction with will turbine blades. Lightning is a major cause of severe damages to wind mills. According to the damage records, the blades of the turbine are the most vulnerable components. They have the highest frequency of damage by direct strikes and the highest repair cost due to the structural failure. The high strike probability of wind turbine is usually explained by the height of wind turbine blades which is much larger than the surroundings. Recently, it has also been suggested that stable upward connecting leaders initiate more easily from wind turbines than from static objects due to the rotation of turbine blade. It is thought that the tip of moving blade runs out of the sheath of corona discharge region where the electric field is significantly shielded.
How is the lightning attachment to wind turbine blades?
A simplified case is used here to illustrate the mechanism of lightning attachment to a wind turbine blade. In a case of downward negative lightning, a step leader first descends from the base of thunder cloud above the will mill, as it is shown in Figure 1. a. Then, upward connecting leaders initiate from the receptors on blades, which propagate towards the descending downward leader. The lightning attaches to the receptor which the tip of streamer corona zone at the tip of the upward connecting leader initiated from first reaches the descending stepped leader. When this process takes place, a highly conductive path is created between the thundercloud and the ground such that the first return stroke peak current pulse takes place. After this, there could be a time interval until a subsequent dart leader starts propagating from the thundercloud towards the ground. During this time interval, the blade rotates clockwise with a considerable angle θ and changes the position, as it shows in Figure 1. b. The dart leader descends with a higher velocity than the preceding stepped leader along the pre-heated channel of first return stroke and may connect with an upward connecting leader initiated from the first striking point or a point different to the first struck point.
How we model this attachment process?
The lightning attachment process due to stepped and dart leaders is analyzed here with the Self-consistent Leader Inception and Propagation Model -SLIM-.
SLIM simulates a discharge process locating in front of ground objects in chronological sequence, considering the influence of downward lightning leader propagating dynamically. This discharge process includes four main stages, namely the first streamer inception, the unstable upward leader initiation and the stable upward leader propagation and the connection of both leaders (if the lightning attachment finally completes). This chronological sequence starts at the first streamer inception when the local electric field (due to both the thundercloud and the downward leader) at the surface of the analyzed object fulfills the streamer criterion. The first unstable upward leader segment initiates if the charge of any secondary streamer is equal to or larger than 1µC. Once the unstable upward leader segment initiates, its propagation is evaluated through the charge of the streamer at the tip of the upward leader. When the velocity of unstable upward leader reaches a threshold value, the unstable upward leader turns into a stable upward leader. Simultaneously, the physical parameters of the leader (potential gradient, channel radius, injected current and propagation velocity) are evaluated based on the charge of the streamer at the tip of the leader with the thermo-hydrodynamic leader channel model of Gallimberti.