# Adjustable Capacity Evaluation Method Based on Step-by-Step Power Mapping of Offshore Wind Farms
## 1 Introduction  
Building the power system with renewable energy as the main energy source is the important measure to reduce carbon emissions. The development cost of wind energy is lower than that of solar energy, and the safety of wind power generation is better than that of nuclear power generation. In view of the advantages of wind energy in the field of clean energy, wind power has developed rapidly in recent years, especially offshore wind power. In 2021, China's new installed capacity of offshore wind power reaches 1.8 times the previous cumulative installed capacity. Despite this, onshore wind power still occupies more than 90% of the share, and offshore wind power still has huge development potential. The application of long-distance offshore transmission lines, power electronics, and large transformers has caused significant power quality problems and challenges in voltage and frequency stability to the grids connected to offshore wind farms [1, 2, 3, 4]. To overcome the power quality problem, the combination of high-frequency switchgear and reactive power compensation equipment can change the parameters of the grid to alleviate the problem [5]. The establishment of combined power plants can reduce the impact of power changes on the grid and promote the consumption of renewable energy. The comprehensive utilization of offshore wind energy and wave energy to establish a combined power plant can reduce short-term power changes, which is conducive to the integration of the power plant into the grid [6]. Reference [7] proposed a system for incorporating multiple distributed offshore wind farms into an onshore grid. A system control strategy was established for the circulating current converter. At the same time, a new black-start method and a comprehensive primary frequency control scheme were designed. Reference [8] designed a DC transmission system for offshore wind farms, and applieda new modular multi-channel DC converter. The control strategy of the grid-connected scheme can maintain the current-voltage balance of the DC transformer while achieving maximum power control. Reference [9] proposed a coordinated control strategy that can provide inertia support for the power grid, which realized cascade control by means of the communication between the offshore power grid and the onshore power grid. References [10; 11; 12; 13; 14; 15] analyzed the reliability and economy of off grid hybrid energy system using wind power, and pointed out that this system is an effective solution to solve the energy supply in remote areas. There are some other studies on grid-connected system construction and control strategy establishment for offshore wind farms [16; 17; 18; 19; 20; 21]. At present, most of the researches on grid connection of offshore wind farms focus on these two aspects, and the estimation of adjustable capacity is rarely mentioned. The offshore wind farm has two independent monitoring systems. One is the SCADA (Supervisory Control And Data Acquisition) system of the wind turbine, and the other is the monitoring system responsible for collecting other data. The wind farm inputs the collected data into the AGC (Automatic Generation Control) system for analysis and calculation, and then reports the calculated adjustable capacity to the dispatching department of power grid. The dispatching department of power grid issues orders to offshore wind farms according to the adjustable capacity. Therefore, the accuracy of the adjustable capacity calculation affects the control effect. The method currently used in offshore wind farms is to directly multiply the power of the wind turbine by the empirical coefficient to obtain the adjustable capacity, which has a large error. Since the wind turbine is far from the land, various losses will be generated when transmitting electricity. When calculating the loss, the physical model requires too many parameters, and the calculation process is complicated. Moreover, the marine environment is changeable, so the parameters measured by experiments are not accurate.