Research On Frequency Response And Control Strategy Of Wind Farm Integrated Power Systems

Frequency stability is the important component of the stability of power systems. Frequency is a significant index for the security of AC power systems, which reflects the balance between active power and load. The active power reserve is usually sufficient for the traditional power systems, the grid can ensure the frequency stability with powerful disturbance resisting ability. For the power systems becoming more and more complex, especially with the new renewable power integration, the nonlinear degree and uncertainty of system have increased significantly. It is possible to induce frequency event, it seriously threatens the safe and stable operation of power systems.With the development of wind power technology, wind power as a new renewable energy has been paid more and more attention worldwide. Wind power has become the most mature and realistic renewable energy except the hydropower. However, the high fluctuation and randomness features of wind power bring a challenge for the system frequency with the continuously increased integrated power capacity. At present, the researches mainly focus on the wind power forecast and voltage stability problems, study on the frequency stability of wind integrated power systems is comparatively weak. In order to better utilize the wind energy, this paper presented the method for analyzing the frequency stability of wind farm integrated power systems, and the related frequency control strategies have been studied.The randomness and volatility of wind power is the key factor of wind power integration problems. It is significant important to master the frequency changing trend of wind power integrated power systems for frequency controlling. This paper develops an improved elastic back-propagation neural network method to assess system frequency. The proposed method is applicable for different types of wind farms, the forecast results can be used to design appropriate frequency control strategies for the system operators.For the dynamic operating characteristics of wind power integrated systems, the simplified frequency response model based on the direct current load flow has been modeled to analysis the frequency dynamic performance. Based on the proposed frequency response model, this paper introduces a fuzzy logic system to analyze the frequency response of wind farms composed of variable speed wind turbine generators. When a network disturbance occurs, the fuzzy logic system can determine the frequency response of the wind farm according to real-time input variables. The simulation results indicate that the designed system is effective and can contribute to frequency stability analysis.Along with the ever-increased wind power capacities, it is an inexorable trend for wind farm to participate the system frequency control. The frequency control of integrated wind farm is different from the conventional generators, the traditional fixed value control strategy is not suitable for the wind power generators. According to the output model of wind turbine generator, the droop-control coefficient probabilistic model based on the variable wind speed is presented, and its probability density function is calculated through the method on semi-invariant and Gram-Charlier progression. The calculation of IEEE 14-bus system is carried out, the result indicates that the probabilistic model could reflect the stochastic characteristic correctly, the frequency regulation capability of wind farm is influenced by the random output, the model presented in this paper is feasible which could provide useful theory reference for wind farm to participate in frequency control.The influence of wind farm inertial constant on the system frequency response can not be ingnored, when the wind power penetration rate is high. Owing to improve the reliability of frequency control for grid-connected wind farm, a new control strategy for a wind farm and energy storage combined system, which is based on the time-variable frequency response characteristic of wind power integrated system, is proposed. The results of calculation examples indicate that the presented strategy could effectively level the fluctuation of wind farm output power with fast dynamic response, provide active power support for the wind farm to participate into the grid frequency control in accordance with the real-time frequency response characteristic. The control strategy could effectively improve the stability of wind farm participating into frequency regulation.