Optimal Termination Time Of Inertia Control From DFIG Wind Turbines

Wind power generation has been growing rapidly all over the world,since wind energy is clean,renewable and plentiful.Wind turbine is usually connected to power system though electronic devices,so its output power is isolated from the system frequency and it cannot react to frequency fluctuations,and contributes nothing to system inertia.Hence high penetration of wind power decreases the system inertia and worsens the system's ability to react to disturbances.Inertia control of wind turbine is an effective solution to improve the inertia of power systems with high penetration of wind power.With inertia control,the kinetic energy stored in the rotor is released by emulating the inertia of synchronous machine,and wind turtbines can contribute to frequency regulation.The rotor speed keeps decreasing during the energy releasing process,and the inertia control must be terminated before hitting its lower limit in order to reaccelerate the rotor and guarantee the safe operation of wind turbine.The termination of inertia control causes sudden decrease in output power of wind turbine and results in secondary frequency dips.This paper analyzes and studies the secondary frequency dips caused by termination of stepwise inertia control(SIC).Model of SIC is constructed and simplified to analyze the influence of termination time on the secondary frequency dip and derive the optimal termination time.The numerical and analytical solutions of the optimal termination time and the fixed termination time(FTT)are derived and analyzed.The FTT derived from the simplified model is independent of the operating conditions of the wind turbine and has clear physical meanings,which makes it practical for field application.Research status of frequency regulation of wind turbine and secondary frequency dip is analyzed and summarized in this paper.Principles of wind turbine's operation and inertia control as well as system frequency response model are introduced,and the influence of wind turbine on system inertia and frequency response is analyzed.The process of SIC is modeled and the system frequency response is derived based on the model.Optimal functions for termination of SIC is constructed in order to minimize the secondary frequency dip by setting the partial derivative of frequency nadir to zero,and the optimal termination time is derived by numerically solving the optimal functions.The numerical solution of the optimal termination time can minimize the secondary frequency dips but its derivation needs multiple iterations and is inconvenient for analysis.Taylor expansion is conducted on the frequency response and optimal equations to obtain analytical solutions of optimal equations.After the Taylor expansion,optimal equations are simplified to linear functions and are easy to solve.The solutions of the linear functions are have clear expressions and can be used to analyze the influence factors of the optimal termination time.Analysis shows that the optimal termination time is affected by the system parameters,wind speed,output power of the wind turbine and the magnitude of the initial load disturbance.The SIC process is approximated by a simplified model for the further simplification of calculation.The simplified model physically neglects the coupling between the termination power decrease and the termination time,as well as the recovery process of SIC.The FTT that is independent on the operation status of wind turbine and the magnitude of initial disturbance is derived by applying optimal functions to the simplified model.Analysis and simulations show that both the optimal termination time and FTT can effectively reduce the secondary frequency dip,and there is no significant difference between their frequency responses.Hence the secondary frequency dip can be minimized by the FTT that only depends on the system parameters and is irrelevant to the operating condition of WT.Furthermore,FTT has clear physical meanings.By terminating SIC at FTT,the frequency nadir caused by SIC termination and the frequency overshoot resulting from the initial disturbance will appear at the same time,which can reduce the secondary frequency dip to the maximum extent;and the mechanical power of synchronous generator is near to its maximum value at FTT,which can best compensate the power drop caused by the SIC termination and realize the coordination control of wind turbine and synchronous generator.