Research On Transient Frequency Response Of Power System Containing High Penetration Of Wind Power

With the rapid development of wind technology, wind power penetration in the system is increasing. The contact and impact between wind turbines and power grid are strengthened. Large-scale wind power replacing conventional units will inevitably reduce system inertia, and the resulting frequency fluctuation problem is increasingly concerned by people. Many power companies require grid-connected wind farm can participate into frequency regulation just like conventional units. It will change the original frequency response characteristic, and make the set of under-frequency load shedding and over-frequency generator tripping complicated. Therefore, It has theoretical and practical significance to analysis frequency response characteristic of wind turbines and assess transient frequency of power system containing high penetration of wind power. The main research results are as follows:(1)A simplified model of wind turbine is proposed for the complexity and high coupling characteristics of DFIG electromagnetic transient model. The operating principle and control strategy are analyzed. The detailed mathematical model of wind, turbine and control system, shaft, generator and converter are established. In the study of system frequency response, attentions can be paid on active power-frequency, ignoring the impact of reactive power-voltage dynamics. Main steps and method of the simplified model for DFIG are proposed. The detailed electromagnetic model of generator, shaft, converter can be simplified as singular-order motional model, singular-mass block model and simplified current source model. Simulations are performed on standalone case system, and show that, compared with the electromagnetic transient model, simplified model has low amount of calculation, can simulate wind farm active power output accurately.(2)Research on the effect of wind power connected into grid on system frequency response is conducted. Factor affecting the dynamic characteristics of power system is analyzed. Effect on inertial response and primary frequency regulation are analyzed in mechanism level. DFIG uses decoupling control strategy, makes its own inertia "hidden", cannot provide inertial response; FSIG can provide short-term inertial response, but it does not have the capability of primary frequency regulation. Simulations show the frequency response characteristics of DFIG, FSIG and conventional unit. According to the "hidden inertia" feature, stand alone system equivalent model after DFIG connected to grid is established. Analytical solution of the model is derived. The effect of wind power penetration on system frequency response is quantitatively analyzed in the aspect of the initial frequency decreasing the lowest point frequency( and the steady stat(3)Assess the transient frequency response considering wind farm participating into frequency regulation. It is an inevitable requirement of "grid-friendly" power to make wind turbines participate into frequency regulation. The principle of kinetic control and power reserve control is summarized. Original frequency characteristics will be changed when DFIG involved in frequency regulation. Based on average system frequency method, simplified DFIG model is introduced for assessing system dynamic process. Frequency feedback to additional control link, output power of wind farm is added to the equivalent concentrated load. When disturbances occur, frequency response process can be calculated. Simulations are performed on IEEE 30 bus system and show that, compared with full-time domain simulation, the proposed method can get accurate frequency response, can assess the transient frequency response for setting the configure of under-frequency load shedding and over-frequency generator tripping.