Dynamic Rotor Speed Protection For DFIG Based Wind Turbines With Virtual Inertia Control

China's wind energy resources are very rich,and has a greater value of development and utilization.In recent years,with the expanding scale of wind power plant in China,it has been in the forefront of the world.However,due to large-scale wind power access,the equivalent moment of inertia of the power network is reduced and time-varying,and there is no spare capacity to participate in the frequency regulation of the power grid.Therefore,wind power grid connection is of great importance to the power system.Both the frequency stability of the system after disturbance and the control method of stability pose new challenges.Frequency is an important factor to measure the power quality of grid.When the power system is disturbed,the active power generated by the generator in the power system is not equal to the active power by the load.The resulting power shortage may lead to decrease of the system frequency.For the traditional synchronous machine,the speed regulation system can adjust the active power of the system according to the frequency change of the system,and realize the primary frequency modulation of the system.Doubly-fed wind turbine(DFIG)are widely used in wind farms at present.The rotor speed is not related to the frequency of the power grid,and can't be used as a synchronous machine.In recent years,the proposed virtual inertia control strategy can temporarily reduce or raise the speed of the fan in the event of disturbance.So as to release or absorb some rotor kinetic energy to participate in the frequency adjustment of the system.However,the kinetic energy is excessively released,It may have an adverse effect on stability of the wind turbine and speed up the process of the wind turbine out of the frequency modulation,thus causing a secondary impact on the frequency of the power grid.The paper first analyzes the current situation of the wind power frequency modulation and frequency secondary drop,introduces the operation principle and maximum frequency tracking principle of DFIG,and simulates the maximum power operating curve under different wind speeds.Several main advantages of the DFIG are obtained,and the virtual inertia control principle of DFIG is analyzed.Secondly,according to the three operation modes of the rotor speed of the DFIG in the process of virtual inertia frequency modulation:the rotor speed is stable and does not withdraw from the frequency modulation,the rotor speed is stable but exists the frequency modulation and the rotor speed is unstable and exists the frequency modulation.Three key running points in the operation of DFIG are put forward.According to this,the dynamic rotational speed protection strategy is designed.By adjusting the power reference value of the wind turbine,the operation state of the wind turbine in the process of frequency modulation is changed,and the condition of the excessive decrease of the speed is avoided,which is not conducive to the stable operation of the wind turbine.Ensure the normal operation of the wind turbine,improve power grid frequency response at the same time,and guarantee the system frequency will not lead to secondary for wind turbine exit of frequency modulation.Then the virtual inertia control under variable wind speed is analyzed,and the results of frequency modulation when the frequency of the system rise and fall under variable wind speed are simulated.The simulation results show that if use the dynamic rotational speed protection strategy,the active output of the wind turbine will return to the MPPT curve smoothly.It will not cause a large secondary shock,and can automatically restore the wind turbine to the optimal operational point,which improves the effect of the virtual moment of inertia control.This strategy has a very high value of practical application.