Study On Interactive Relationships Between Wind Turbines & Power Grid

This thesis analyzes interactive relationship between wind turbines and power grid, mainly focusing on the practical problems in wind farm in recent years. A variety of analytical tools and algorithms are developed and used to analyze measured data from wind farms& wind turbines and to find out their internal co-relations.The major contributions of this thesis work are listed as following:1. A three-dimensional space of harmonic amplitude, frequency and active power is established by analyzing lot of voltage and current measurement samples from various types of wind turbines. Then a novel rule of analyzing the harmonics generated from variable speed wind turbines (VSWT) is proposed. The concept of variable frequency harmonics is used to calculate the random positions of maxima of the harmonics and inter-harmonics. A novel, unified, deterministic three-dimensional (harmonic amplitude, frequency and active power) model for wind turbine is developed. The accuracy of this model is validated using the power quality data obtained from three different wind turbines of various power ratings. An experimental method of determining the parameters of this model is also provided. This model is also applicable for fixed speed wind turbines.2. A statistical method for analyzing the phase of the harmonics is proposed based on measured data and power quality standard "IEC 61400-21" for wind power. This method is then applied to analyze the measurement data from wind farms and the simulation data from Electro Magnetic Transients Program (EMTP). The characteristic harmonics of wind farm is estimated statistically by superposition of harmonics of each wind turbines. Through the control of harmonic phase, the harmonics’superposition of each wind turbine can be effectively reduced, thereby improving the power quality of wind farm.3. Harmonic and DC bus overvoltage may occur in the process of high voltage ride through (HVRT) of wind turbine. A control and test algorithm to modify the hardware equipment of existing low voltage ride-through (LVRT) is proposed. The controlling and testing method of HVRT are realized.4. Characteristic harmonic frequency of wind turbine depends on frequency of rotor current and power grid. The characteristic inter-harmonics with maximum amplitudes can be found out through theoretical analyses and real-tests. A variable-step method to detect characteristic harmonics based on rotor speed is proposed. The frequency, amplitude and phase of the characteristics inter-harmonics can be accurately obtained. Method to differentiate different types of wind turbine flickers is analyzed. Thereafter magnitudes of different types of flickers are calculated. Comparative study of the relationship between flicker and low-order harmonics is also done.5. The impact of abnormal operation of power grid for wind turbines is analyzed using real-test data of wind farm. The harmonic and converter overvoltage under unbalanced voltage and overvoltage events in power grid are studied. The third-order harmonic and inter-harmonic under unbalanced grid voltage are calculated and simulated. It can be concluded from the analysis that third-order harmonic under unbalanced grid voltage is distributed in three-phase positive sequence voltage. These harmonics are fed into wind farm and power grid through the transformer along with the fundamental current. An improved control algorithm is proposed to suppress these third-order harmonic contents.6. Sub-synchronous resonance (SSR) caused by the transient process of wind turbine is analyzed using real-test resonance data of wind farm. Damages caused to the mechanical and electrical system due to SSR are analyzed. A detection and inhibition method for SSR is proposed. Based on theoretical analysis of the transient processes, such as connecting and switching of wind turbine to the power grid; real-test data and numerical simulation results; possible negative effects on the wind turbine operations are summarized.7. The characteristics of the main types of wind turbines are summarized. The theoretical analysis and modeling using EMTP and MATLAB tools according to IEC61400-27 standards are conducted for three different commonly used models. A data table of wind turbine aerodynamic power input is established using the standard experimental model of wind resources to simulate the operating condition of the actual wind farm.