# Adjustable Capacity Evaluation Method Based on Step-by-Step Power Mapping of Offshore Wind Farms
## 2 Division of Power Transmission Links in Offshore Wind Farms  
The power of the offshore wind farm is generated by the wind turbines. After collecting electricity and boosting, it is transmitted to the onshore part through a long-distance high-voltage submarine cable for grid connection. During the transmission process, losses will occur in various processes. Therefore, the adjustable capacity reported by the AGC system of the wind farm to the power grid dispatching department needs to be corrected on the basis of the maximum theoretical power of the wind turbine. However, the current calculation method of the adjustable capacity used by wind farms is not accurate enough to meet the requirements of the power grid. This paper will sort out the power transmission processes of offshore wind farms, and establish a power map of wind farms considering the losses in each transmission process, so as to estimate the adjustable capacity of offshore wind farms based on the maximum theoretical generating power of wind turbines.  
The offshore wind farm can be divided into two parts: offshore part and onshore part. The connection between these two parts is realized by transmitting power through high-voltage submarine cables. Figure 1 shows the basic structure of a typical offshore wind farm.  
The collector system collects and boosts the power generated by wind turbines, and then transmits it to the onshore part through the high-voltage submarine cable. After deducting the loss of onshore part, the power delivered to the land that finally flows into the grid is the grid-connected power of the wind farm. The power of the grid connection point corresponds to the real-time power of the wind turbine, while the adjustable capacity of the wind farm corresponds to the maximum theoretical power of the wind turbine. To achieve adjustable capacity evaluation, it is necessary to first establish a mapping between the power of wind turbines and the power of the grid connection point.  
Based on the offshore wind farm structure shown in Figure 1, the power transmission of the wind farm can be divided into three main processes:  
1. Power transmission from wind turbines to the head of the high-voltage submarine cable. This process includes power collection and boosting. The power loss includes the loss generated by the collector system and the station power in the offshore part.
2. High-voltage submarine cable transmission section. In this process, the submarine cable transmits power from the offshore to the onshore part. The main reason for power loss is that during the power transmission process, each component of the submarine cable will generate power loss due to the rise in temperature and the establishment of electric and magnetic fields.
3. Power transmission from the end of the high-voltage submarine cable to the grid connection point. In this process, the power is transmitted to the onshore part of the wind farm and then fed into the power grid. The power loss comes from the station power in the onshore part and the consumption of other compensation equipment.