1. Damage of lightning to wind turbine generator;

2. Damage form of lightning;

3. Internal lightning protection measures;

4. Lightning protection equipotential connection;

5. Shielding measures;

6. Surge protection.

With the increase of the capacity of wind turbines and the scale of wind farms, the safe operation of wind farms has become increasingly important.

Among many factors that affect the safe operation of wind farms, lightning strike is an important aspect. Based on the research results of lightning

protection for wind turbines, this paper describes the lightning process, damage mechanism and lightning protection measures of wind turbines.

Due to the rapid development of modern science and technology, the single capacity of wind turbines is becoming larger and larger. In order to

absorb more energy, the hub height and impeller diameter are increasing. The height and installation position of the wind turbine determine that

it is the preferred channel for lightning strikes. In addition, a large number of sensitive electrical and electronic equipment are concentrated inside

the wind turbine. The damage caused by a lightning strike will be very large. Therefore, a complete lightning protection system must be installed

for the electrical and electronic equipment in the fan.

1. Damage of lightning to wind turbines

The hazard of lightning to wind turbine generator is usually located in an open area and very high, so the whole wind turbine is exposed to the threat

of direct lightning strike, and the probability of being directly hit by lightning is proportional to the square value of the height of the object. The blade

height of the megawatt wind turbine reaches more than 150m, so the blade part of the wind turbine is particularly vulnerable to lightning. A large

number of electrical and electronic equipment are integrated inside the fan. It can be said that almost every kind of electronic components and electrical

equipment we normally use can be found in a wind turbine generator set, such as switch cabinet, motor, drive device, frequency converter, sensor,

actuator, and corresponding bus system. These devices are concentrated in a small area. There is no doubt that power surges can cause considerable

damage to wind turbines.

The following data of wind turbines are provided by several European countries, including data of more than 4000 wind turbines. Table 1 is a summary

of these accidents in Germany, Denmark and Sweden. The number of wind turbine damage caused by lightning strikes is 3.9 to 8 times per 100 units per

year. According to statistical data, 4-8 wind turbines in Northern Europe are damaged by lightning every year for every 100 wind turbines. It is worth

noting that although the damaged components are different, the lightning damage of control system components accounts for 40-50%.

2. Damage form of lightning

There are usually four cases of equipment damage caused by lightning stroke. First, the equipment is directly damaged by lightning stroke; The second is

that the lightning pulse intrudes into the equipment along the signal line, power line or other metal pipelines connected with the equipment, causing

damage to the equipment; The third is that the equipment grounding body is damaged due to the “counterattack” of the ground potential caused

by the instantaneous high potential generated during the lightning stroke; Fourth, the equipment is damaged due to improper installation method

or installation position, and is affected by the electric field and magnetic field distributed by lightning in space.

3. Internal lightning protection measures

The concept of lightning protection zone is the basis for planning comprehensive lightning protection of wind turbines. It is a design method for structural

space to create a stable electromagnetic compatibility environment in the structure. The anti-electromagnetic interference ability of different electrical

equipment in the structure determines the requirements for this space electromagnetic environment.

As a protection measure, the concept of lightning protection zone of course includes that electromagnetic interference (conductive interference and

radiation interference) should be reduced to an acceptable range at the boundary of the lightning protection zone. Therefore, different parts of the

protected structure are subdivided into different lightning protection zones. The specific division of the lightning protection zone is related to the

structure of the wind turbine,and the structural building form and materials should also be considered. By setting shielding devices and installing

surge protectors, the impact of lightning in Zone 0A of the lightning protection zone is greatly reduced when entering Zone 1, and the electrical and

electronic equipment in the wind turbine can work normally without interference.

The internal lightning protection system is composed of all facilities to reduce the lightning electromagnetic effect in the area. It mainly includes lightning

protection equipotential connection, shielding measures and surge protection.

4. Lightning protection equipotential connection

Lightning protection equipotential connection is an important part of the internal lightning protection system. Equipotential bonding can effectively

suppress the potential difference caused by lightning. In the lightning protection equipotential bonding system, all conductive parts are interconnected

to reduce the potential difference. In the design of equipotential bonding, the minimum connection cross-sectional area shall be considered according

to the standard. A complete equipotential connection network also includes equipotential connection of metal pipelines and power and signal lines,

which shall be connected to the main grounding busbar through lightning current protector.

5. Shielding measures

Shielding device can reduce electromagnetic interference. Due to the particularity of the wind turbine structure, if the shielding measures can be

considered at the design stage, the shielding device can be realized at a lower cost. The engine room shall be made into a closed metal shell, and

the relevant electrical and electronic components shall be installed in the switch cabinet. The cabinet body of the switch cabinet and control

cabinet shall have good shielding effect. Cables between different equipment in tower base and engine room shall be provided with external metal

shielding layer. For interference suppression, the shielding layer is effective only when both ends of the cable shield are connected to the

equipotential bonding belt.

6. Surge protection

In addition to using shielding measures to suppress radiation interference sources, corresponding protective measures are also required for

conductive interference at the boundary of lightning protection zone, so that electrical and electronic equipment can work reliably. Lightning

arrester must be used at the boundary of lightning protection zone 0A → 1, which can lead a large amount of lightning current without damaging

the equipment. This type of lightning protector is also called lightning current protector (Class I lightning protector). They can limit the high

potential difference caused by lightning between the grounded metal facilities and power and signal lines, and limit it to a safe range. The most

important characteristic of lightning current protector is: according to 10/350 μ S pulse waveform test, can withstand lightning current. For

wind turbines, lightning protection at the boundary of power line 0A → 1 is completed at the 400/690V power supply side.

In the lightning protection area and the subsequent lightning protection area, only pulse current with small energy exists. This kind of pulse current

is generated by the external induced overvoltage or the surge generated from the system. The protection equipment for this kind of impulse current

is called surge protector (Class II lightning protector). Use 8/20 μ S pulse current waveform. From the perspective of energy coordination, the surge

protector needs to be installed downstream of the lightning current protector.

Considering the current flow, for example, for a telephone line, the lightning current on the conductor should be estimated at 5%. For Class III/IV

lightning protection system, it is 5kA (10/350 μ s)。

7. Conclusion

The lightning energy is very huge, and the lightning strike mode is complex. Reasonable and appropriate lightning protection measures can only reduce

the loss. Only the breakthrough and application of more new technologies can fully protect and utilize the lightning. The lightning protection scheme

analysis and discussion of wind power system should mainly consider the grounding system design of wind power. Since wind power in China is

involved in various geological landforms, the grounding system of wind power in different geology can be designed by classification, and different

methods can be adopted to meet the grounding resistance requirements.