Lightning analysis and lightning protection system for high-power wind turbine blades
Phenomenon and mechanism of blade damage
The typical damage phenomena that occur at lightning strike points include the following situations:
1.1 Cracking and ashing
Cracking and ashing of composite materials on the surface of blades, as well as burning or melting of metal components at lightning strikes. Cracking belongs to mechanical damage, while ashing is the result of thermal effects.
1.2 Arc
Lightning current forms an arc inside the blade, or an internal arc is often formed between the lightning strike point at the blade tip and the conductor components. The damage to the blades of wind turbines is the most severe, and arcs in the air can exist in the voids inside the blades and on the surface of the blades, which belongs to electrical damage.
1.3 Burst
When lightning spreads between the layers of composite materials, due to some moisture between the layers, the internal arc heats up the moisture, causing pressure shock and bursting of the blade or tearing and damaging the blade surface along the front and rear edges and the blade bearing beam, ranging from cracks on the blade surface to complete fragmentation of the blade. Sometimes pressure waves can be transmitted from lightning struck blades to other blades through the hub, causing damage. Belongs to thermal effects and mechanical damage.
2 Lightning strike test
2.1 Lightning discharge electrode test
The experimental method involves conducting 30 positive and 30 negative discharges on the blade, and the result is that the positive discharge did not hit the lightning arrester 0 times, with a capture rate of 100%; The negative polarity discharge failed to hit the lightning arrester 9 times, with a capture rate of 70%.
Important conclusions can be drawn from the above experiments:
(1) Lightning may not hit the lightning arrester but directly hit the surface of the blade.
(2) The blade tip lightning arrester is more likely to attract lightning strikes.
(3) Positive polarity lightning is prone to hitting the surface of the blade, and negative polarity lightning is prone to hitting the lightning arrester.
Design of 3-blade lightning protection system
Based on the above introduction, we have designed the blade lightning protection system for high-power wind turbines as follows.
3.1 Basic design requirements
The blades are equipped with lightning arresters, down conductors, and their connecting components to form a lightning protection system, which can be part of the blade structure itself or integrated into the components of the blade. It can withstand the corresponding lightning current impact under the specified lightning protection level, ensuring that the blades have no structural damage and do not hinder their continued operation until the next maintenance; Capable of withstanding expected wear and vibration caused by wind, humidity, particulate matter, etc., without affecting the dynamic characteristics of the blades. Assess the ability of the blades of the lightning protection system to withstand mechanical stress.
3.2 Receiver in Blade
The blade receiver should be located on the surface of the blade and be able to intercept the majority of lightning strikes. The blade receiver can be repaired and replaced.
The protection range of the blade receiver cannot be calculated and determined using the protection angle method and rolling ball method. The design of the blade receiver system is determined based on strict inspection and testing.
When the number of receivers in the blade reaches or exceeds the following specified values, the initial pilot flashover test in the high-voltage lightning flashover test may not be carried out. [6] [7] [8] Blade length L<20 m: 1 blade tip receiver. Blade length of 20 m ≤ L<30 m: 1 blade tip receiver, 1 pressure side receiver, and 1 suction side receiver, at a certain distance from the blade tip. Blade length 30 m ≤
L<45 m: 1 blade tip receiver, 2 pressure side receivers, and 2 suction side receivers, distributed on rotating blades. Blade length L ≥ 45 m: 1 blade tip receiver, 3 pressure side receivers, and 3 suction side receivers, distributed on rotating blades.
3.3 Downconductors in Blades
The down conductor should be reliably connected for a long time and able to withstand the combined impact of electrical, thermal, and electrodynamic effects generated by lightning currents. The down conductor should be installed on the blade before conducting a simulated lightning strike test and should be tested together with the blade for its ability to withstand mechanical stress.
The down conductor should not exceed the allowable temperature value of the blade during the transmission of lightning current.
3.4 Lightning protection of carbon fiber blades
The protection of carbon fiber blades involves some different and more complex challenges, as carbon fiber is the opposite of glass fiber as it is a conductor. Damage to fiberglass blades often occurs at the leading and trailing edges, but for unprotected carbon fiber blades, damage often occurs at the beam cap, as this is where conductive carbon is located. Lightning protection for carbon fiber blades often uses lightning strips. An early commonly used method was to embed a layer of metal mesh in contact with a carbon fiber layer under the adhesive layer of the blade. However, the lightning current hitting this layer of the grid can cause damage to the watch case, which must be inspected and repaired,
This flashing strip belongs to the aircraft radar antenna fairing flashing strip - multi section type, rather than continuous type. A series of thin conductive components are placed on a resistive material, controlling the gap to tighten into a thin composite strip, and sticking to the surface to be protected. The multi section lightning arrester does not provide a metal channel for conducting lightning current, but rather provides many small air gaps that ionize when a high-voltage electric field occurs.
