**Reduce the construction capacity of wind farm grid-connected channels**

China has a vast land area and complex terrain conditions. The status and distribution characteristics of wind energy resources vary with the terrain and geographical location. The areas rich in wind energy resources are mainly distributed in the southeast coast and nearby islands and northern areas. The Three-North (Northwest, North, Northeast) region is rich in wind energy resources. The wind farm has flat terrain, convenient transportation and no destructive wind speed. It is the largest continuous wind energy resource area in China, which is conducive to the development of large-scale wind farms. However, the capacity of the local power grid is small, which limits the scale of wind power, and it is far from the load center, requiring long-distance power transmission. This makes the long-distance transmission of large-capacity wind power an important task for China’s wind power industry. Moreover, the larger the scale of the wind farm, the higher the voltage level corresponding to the access grid, and the higher the proportion of the construction cost of the transmission channel to the investment cost of the wind farm.

The power output of a wind farm group is random and fluctuating, and the time for a single wind turbine to reach the rated power output generally accounts for 1% to 5% of the total time. , the proportion of time for wind farms to reach rated output will further decrease. For example, the output of Nordic wind farms rarely reaches 75% of the installed capacity level. Therefore, the optimal transmission capacity of the wind farm group is often smaller than the installed capacity of the wind farm group. The specific optimal transmission capacity should be determined according to the actual situation of the local wind farm group.

If the grid-connected channel capacity of the wind farm is planned according to the maximum value of the typical daily output, the energy storage system will stabilize the output fluctuation of the wind farm and make the output curve of the wind farm smoother. This reduces the capacity of the grid-connected channel that needs to be constructed, thereby saving a corresponding part of the investment. The cost saving mainly depends on the difference between the peak power before and after the output curve is smoothed, which can be expressed as (1):

In the formula, ko – the unit cost of the grid-connected channel (10,000 yuan/MW).

- Investment cost analysis

The investment cost of energy storage device mainly includes the investment and construction cost of energy storage device (including site construction cost, grid-connected equipment cost and battery system cost), see formula (2):

The annual operation and maintenance cost of the energy storage device is shown in formula (3):

where Ci-site construction cost;

Kp, cmp – unit cost and annual maintenance cost related to rated power (10,000 yuan/MW);

Kw, Cmw – unit cost and annual maintenance cost related to rated capacity [ten thousand yuan / (MW h);

Wmax- the rated energy storage capacity of the energy storage system (Mw h).

- Total Value Evaluation Model
- Annual net income objective function

The economic factor of building an energy storage system in a wind farm is one of the most important goals, so we aim to maximize its annual net income. Combining the benefits and costs of the energy storage system analyzed above, we can obtain its annual net profit. The revenue objective function is (4):

In the formula, λ is the fixed asset depreciation coefficient of the equipment, and the vanadium battery has a long cycle life and can be used for 20 years.

- Power fluctuation measurement index objective function

The research shows that the battery energy storage system has fast power throughput and flexible four-quadrant adjustment ability, which not only suppresses the voltage fluctuation generated by the wind farm, but also effectively reduces and shortens the amplitude and time of the active power oscillation. The output is smooth. The goal of the energy storage system to adjust the power output of the wind farm is to minimize the sum of the squares of the changes in the output of the wind farm between two adjacent time periods. The objective function can be expressed as (5): - Constraints

In the process of smoothing the output of the wind farm, the vanadium battery energy storage system mainly considers the charge and discharge power constraints of the energy storage system in Equation (6); 3 and 4 in Equation (6) consider the power constraints of the energy storage system ( In order to protect the battery, its minimum power is not less than 5%wmax, assuming that the energy storage system always stores 5%wmax power), the cumulative discharge power of the energy storage device at any time does not exceed the cumulative charging power, and the cumulative remaining power does not exceed the rated power 95% of capacity. At the same time, under different load conditions and operation modes of the power grid, its capacity to accept wind power is also different, so it is necessary to consider the constraints of the system on the capacity of wind power, as shown in Equation (6) 5. Then the constraints of the optimization model are as follows (i = 1, 2..288):

In the formula, η–the energy storage efficiency of the charge-discharge cycle of the energy storage device, including the loss of the grid-connected equipment and the charge-discharge loss of the battery; – the conversion efficiency of charge/discharge.

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