One of the main problems that intermittent power sources (such as photovoltaics, wind power, etc.) bring to the power system is its limited ability to provide auxiliary services, especially the ability to participate in frequency regulation. Due to the volatility of their power generation and the rules that the grid is mandated to buy back renewable energy generation, intermittent power sources typically generate power in a maximum power tracking fashion and do not participate in frequency regulation.
So far, since the grid-connected penetration rate of wind power is still relatively low, the lack of frequency regulation capability of wind farms is not a problem. However, as the installed capacity of wind power increases, some traditional thermal power generation with frequency regulation capability will be replaced, which makes it necessary for wind power to provide auxiliary services such as frequency regulation in the future.
Wind power generation is uncontrollable, and it is less economical to allocate spare capacity for wind farms than traditional power generation, because these traditional power generation as a backup will lead to a further decrease in the system load rate.
However, as the problem of wind power threatening the security of the power system becomes more and more prominent, the participation of wind power in frequency regulation has a long way to go. From the "grid-connection guidelines" issued by some countries with particularly high wind power penetration, it can also predict the possibility of wind power participating in frequency regulation in the future, although frequency regulation control is very difficult for intermittent wind power.
Taking Denmark as an example, some advanced service functions are proposed in its wind power grid connection guidelines, including wind power participating in maintaining the stability of the system frequency. In Ireland, due to the weak connection between the regional power grids and the particularly high penetration rate of wind power, the fluctuation of wind power output power has a relatively large impact on the system frequency.
2. Frequency modulation method
At the wind farm level, it is feasible to implement wind power to participate in primary frequency regulation. When the system frequency is near the rated value, the wind farm can participate in the primary frequency regulation by reducing the power operation to reserve a certain capacity. The offshore wind farm in Horns Rev, Denmark, is equipped with a special control system that enables the wind farm to reduce power to participate in the primary frequency regulation of the power system. Of course, above or below the rated frequency, a boundary area will be set. When the system frequency is within this area, the frequency regulation requirements for the wind farm will not be implemented, and the wind turbine is in normal operation; when there is a significant deviation in the frequency, the wind farm can adjust the active power output and the deviation of the frequency to have a linear relationship.
When the wind farm is in normal operation, each wind turbine generally operates in the maximum power tracking mode. To achieve the reduced power operation of the wind farm, there are many different control methods, such as reducing the power output of a fixed amount, maintaining the actual output power and the producible power of the wind farm at a fixed value, and limiting the power ramp rate of the wind farm.
3. Constraining factors
First, for wind farm operators, any active reduction of power generation will lead to non-maximization of power generation revenue in the case of uncontrollable wind power.
In addition, since the available capacity of the wind farm is directly related to the maximum wind energy that can be obtained by each wind turbine at that time, the intermittent wind power makes the requirement of wind farm power increase not necessarily satisfied (Fortunately, however, due to the differences in power generation between multiple wind farms connected to different regions of the power system, the overall output power of the wind farm cluster can be smoothed on the time scale of primary frequency regulation).
The above facts have triggered a feasibility study on wind power combined energy storage to participate in the primary frequency regulation of the system to achieve the following goals:
1) Optimizing the economic operation of wind farms. When the wind farm is equipped with an energy storage system, the wind turbine can run close to the maximum power point, and the energy storage system adjusts the power at the grid connection point of the wind farm based on the system frequency deviation (Power regulation can be carried out in a fixed proportion of the rated power of the wind turbine in a short period of time).
2) Due to the adoption of the energy storage system, the reliability of the reserve capacity of the wind farm is ensured while the power output is increased.