Capacity Configuration and Selection of Emergency Power Supply for Wind Farms under Typhoon Conditions

China's coastal areas are rich in wind energy resources, and there is a huge market demand for wind power generators. However, due to the impact of typhoons every summer, the wind farms are cut off, causing serious damage to the wind turbines and causing significant economic losses. Based on optimizing the yaw system under typhoon conditions, this paper configures and selects the capacity of diesel generators. First, this article introduces the structure and main functions of the yaw control system. Secondly, through the study of the under-stage wind field model, the algorithm of wind speed extreme value and the calculation method of yaw angle and yaw direction are proposed, and hill climbing algorithm is introduced to optimize the work of the yaw control system. Then, a method for calculating the actual power of a single yaw control system is introduced. On this basis, the capacity configuration plan of the standby diesel generator set was determined, so that it can meet the normal yaw work of the whole field yaw system in the event of a power outage in the field to ensure the safety of the wind turbine.


Introduction
The coastal areas are rich in wind energy resources, but the operation and maintenance of wind farms are very difficult due to the frequent typhoons. If the wind farm is in the state of power failure, how to ensure the reliable operation of the yaw system of wind turbines in coastal areas, realize the protection of wind turbines, and thus reduce the economic loss of wind farms is the key to the construction of wind farms in coastal areas under extreme working conditions.
Only some domestic wind farms use diesel generators as emergency backup power. In literature [1], in order to study the dynamic characteristics and failure mechanism of wind turbines in the case of typhoons, the coordinated superposition method is adopted to calculate the fluctuation time history of typhoons, and the load profile of wind turbines is simulated through simulation. Literature [2] adopts time domain simulation method to estimate the inrush current of the wind generator transformer during the isolated island operation of a large offshore wind farm, so as to allocate auxiliary diesel driven synchronous generator to provide power for the whole wind farm. In literature [3], an orderly wind abandon strategy for offshore wind power with grid elasticity in mind is proposed, which can effectively make use of the regulatory capacity of the system and avoid the impact of short wind power slope events and severe shortage on the balance of supply and demand. In literature [4], by analyzing the failure principle of wind turbines under typhoon conditions, it is concluded that the adoption of force reduction system in terms of mechanics can improve the load capacity of wind turbines to some extent. Literature [5] first estimated the load of wind turbines under typhoons, and on this basis adopted the normal pitch -flying plume grounding strategy and wind turbine stopping strategy to ensure the safety of blade structure.
In summary, the existing literature only estimates the damage of typhoon conditions, and only adopts the method of mechanical reinforcement for prevention and control measures, without considering the damage of wind turbine generators through yaw system. To this end, this paper firstly optimizes the yaw system of wind turbines, realizes the optimal control of yaw system in the face of different wind direction and wind speed, and then realizes the optimal type and capacity configuration of diesel generator set according to the optimized yaw system.

Overview of yaw control system
The principle diagram of yaw control system is shown in Figure 1.

Optimization strategy of yaw control system
In this paper, in order to ensure that the wind turbine is not damaged under typhoon conditions, the minimum output value of the wind turbine is calculated based on the improved mountain-climbing algorithm, so as to minimize the load borne by the wind turbine. First, assume that the wind speed is unchanged. When the wind direction changes, the output power of wind turbine will change. Set , the yaw movement after haven't meet the requirements of system setting, also need to continue to drift action, by the method of recursive calls to detect the output power of wind turbines, until after 1 e P P   and 2 e P P   , argues that the yaw motion to set target system, stop the yaw motion, at the same time at the moment the latest wind turbine power output value now P to min P , more value for the next time the yaw motion. If appears during yaw action, the yaw direction is wrong.
After returning to the initial position, change the yaw direction.
When the wind speed changes, it will have an intuitive impact on the power captured by the wind turbine. The direct manifestation is that the output power of the wind turbine generator changes significantly. After the wind speed and wind direction are changed, according to the previous description, if the condition now is met, the wind turbine yaw system will start and rotate according to the pre-set direction. After the rotation, it will be judged whether

. Calculation of maximum typhoon wind speed
Under typhoon conditions, the maximum wind speed of the typhoon needs to be calculated, and the typhoon wind field model is obtained based on the basic theory of the relationship between the atmospheric pressure field and the wind speed. This paper adopts the wind field model proposed by Batts, which is a relatively mature and simpler wind field model. The model includes the sea surface typhoon wind field model and the attenuation model considering the typhoon registration.
In this model, the maximum gradient wind speed gx V is: where K is the empirical constant, taking 6.72; max R is the maximum wind speed radius (km); P  is the typhoon center pressure difference (hPa); f is the Coriolis coefficient, and its expression is as follows: where  is the rotation speed of the earth (rad/s);  is the latitude of the air cluster.

Maximum power measurement for yaw system
According to the operating principle of the wind turbine yaw system, it can be seen that the operation of the wind turbine yaw system is mainly affected by the following forces: z M is the brake damping moment applied by the brake damper to prevent the fluctuation of yaw system during operation, and it shall not be considered when the yaw motor is in motion state; f M is the frictional resistance moment on the slewing support device. w M is the rotational resistance torque caused by the wind pressure acting on the nacelle. Since the yaw speed is relatively low, the yaw inertia torque is temporarily not considered. To simplify the model, it is necessary to convert the resistance moment of nacelle into the motor shaft. For yaw system of wind turbine, yaw resistance moment is: where 0 R is the length of the wind turbine blade; max F is the maximum wind pressure suffered by the engine room. Its calculation formula is as follows: where  is the air density ( A is the projected area of the blade; B is the number of blades.
Then the resistance moment m T converted to the motor shaft is: where  is the transmission efficiency from engine room to yaw motor shaft, taking 80%.
The angular velocity m  of the motor is calculated as follows: where 1 2 i i ， are yaw gear box transmission ratio and small big gear transmission ratio respectively; r  is the yaw angular velocity, which is generally set as 0.5 s  .
From the product of motor torque and angular velocity, the power of yaw motor can be obtained as follows: where m P is the actual power consumed by the calculated single yaw motor.

Configuration and selection of diesel generator
Under typhoon conditions, it is assumed that the rated selfuse electric power of a single wind generator set is 0 P , the power of yaw motor is 1 P , rate of yaw motor is 1 K ( 1 K is 1.0 if the working rate of yaw system of full-court fan is 100%)..At this time, the power ratio of yaw motor of a single wind turbine generator set is 1 0 P P , and the ratio ratio is 1 1 0 P K P . Then the simultaneous rate of active power of a single wind field is k I : where N means that there are N wind turbines in the whole wind farm, and the power consumed by the whole wind farm can be expressed as: (13) According to the actual power consumed by the whole wind farm, the optimal configuration and selection of diesel generator set can be carried out.

Case study
Putuo 6 Offshore Wind Farm located in the southeast of Liuheng Island, Zhoushan City, Zhejiang Province. The Wind farm is equipped with 63 wind turbines of 4.0MW. Putuo 6 Offshore wind Farm is located in the typhoonprone sea area, with the maximum wind speed of 49.6m/s and maximum wind speed of 64.48m/s at a height of 90m within 50 years. Therefore, Siemens SWT4.0-130 units are selected. Siemens SWT4.0-130 is a three-blade wind turbine with 6 yaw motors per unit. The specific parameters are as follows: the mass of the wind turbine is 120T, the mass of the engine room is 167T, the diameter of the yaw bearing is 3.035m, the friction coefficient of the yaw bearing is 0.08, the transmission ratio of the yaw gear box is 975, the transmission ratio of small and large gears is 154/15, and the rated power of the yaw motor is 5.5kW. The impeller has a diameter of 130 meters, a blade length of 63.5m, and a maximum chord length of 3m.
Take the maximum wind speed of 49.6m/s as an example, and the air density is

Conclusion
Wind energy as a renewable energy is more and more favored by countries all over the world. Therefore, it is timely to pay attention to and make efforts to promote the rapid development of wind power generation technology in China, and its social and economic benefits are selfevident. In this paper, the working mode of yaw control system under typhoon condition is optimized. By calculating the power of a single yaw motor, the maximum power of the yaw system is calculated. On this basis, the standby diesel generator set is selected and its capacity configuration is optimized, so that it can meet the requirements of the yaw system of the whole wind farm.