Godson, Ikhazuangbe Ivbuobe
(2021)
Multibody dynamic analysis of lightning protection systems for modern wind turbines.
PhD thesis, University of Nottingham.
Abstract
Modern wind turbines are increasing in size due to higher megawatt, reaching height of about 220 m with blade as long as 80 m moving with a tip velocity of 60-80 m/s. They are sometimes located offshore for better wind condition, bringing them closer to lightning, and the blade is mostly affected when hit. When lightning attaches to the blade surface instead of the air-terminal, the blade and sometimes the entire wind turbine may be destroyed resulting in downtimes, loss of wind turbine, expensive cost of repair and can cause some power companies to shut down and dismantle. Damage incidences has been recorded as well as increased insurance claims. In the past decades, lightning protection issues were well addressed for static objects and Lightning Protection Systems (LPS) are usually installed on modern wind turbines by applying the Electro Geometric Model (EGM) methods. Recently, these methods are invalidated and according to IEC 61400-24, the EGMs are no longer appropriate for large wind turbine blades. Lightning can either be downward initiated or upward initiated. In the presence of a thundercloud, tall wind turbines are increasingly subjected to upward lightning attachment triggered by the wind turbine itself. Lightning strike frequency, point of lightning attachment and lightning protection systems has been evaluated based on downward initiated lightning. However, these might not be effective for upward lightning. Research has shown that the maximum electric field strength distributed on the surface of the wind turbine and the surrounding air is very important in determining the point of inception of upward leader and would help in improving lightning protection systems. In contrast to static objects, maximum electric field strength required for the inception of upward leader from wind turbine changes majorly due to blade rotation and certain blade conditions such as; polluted blade condition, varying receptor sizes, receptor positions and type of protection method.
Maximum electric field variations due to these blade conditions has not been considered in literature. Analyzing the maximum electric field strength and conducting experimental test on full scale wind turbine is presently very difficult due to height constraint and lack of suitable equipment.
The work presented in this thesis aims at investigating the variations in maximum electric field strength required for the inception of upward leader from wind turbine due to varying blade conditions. The work extends the vertical tri-pole cloud charge distribution model made from two positive charges and a negative charge representing the idealized gross charge structure of a thundercloud to create an ambient field representing uniform electric field due to cloud charge distribution at 200 m above ground. The numerical model is developed with finite element analysis; COMSOL Multiphysics. The extended model is applied to a 3D electrostatics model of a full-scale wind turbine; Vestas V100 with 2 MW rated power, 164 m rotor diameter and 49 m long blade, and a model of a 19.1 m blade length of 600 kW wind turbine. As the blade is rotated, the variations in maximum electric field strength required for the initiation of upward leader is obtained. The results obtained for the point of initiation of upward leader is in agreement with high voltage strike attachment test experiment.
Initially, the model is utilized to evaluate and compare various receptor sizes. The receptor size is evaluated by comparing the electric field strength as the receptor size is changed and the blade is rotated.
Secondly, the model is utilized to evaluate and compare discrete receptor position from the blade tip. A more proficient receptor size and position is proposed again, the model is used to evaluate the effect of pollution on lightning protection systems. It is found that pollution dampens the performance of the receptor by 93% and on average, the general performance of the receptor in a polluted blade condition is found to be less than 10%.
Finally, two lightning protection systems (metallic cap and receptor methods) commonly in use are applied on a full scale polluted offshore wind turbine and evaluated. A new protection system with higher lightning protection efficiency is developed. This is found to influence the electric field distribution immensely.
This work forms a platform for further investigation on modern wind turbines with regard to protecting long blades from lightning. Manufacturers may look at the finding of this work when dealing with the design aspects of very large future wind turbine blades of practical importance.
Actions (Archive Staff Only)
 |
Edit View |
Tools
Tools
![[img]](/style/images/fileicons/application_pdf.png)