Analysis And Mitigation Of Bearing Currents In Doubly-fed Induction Generator For Wind Power Turbines

Doubly-fed induction generator(DFIG)is the main type of wind power generation in the world at present.The security and stability of DFIG have drawn widespread attention from the scholars in this field.In recent years,with the increase of single unit capacity of doubly-fed wind turbine generator system,the bearing current of DFIG is particularly serious.Bearing fault caused by bearing current will shorten the bearing life and affect the safe and stable operation of doubly-fed wind turbine generator system severely.The bearing current of DFIG is more complex because of the difference of the internal structure of DFIG from the general variable frequency motor.Therefore,it is necessary to deeply analyze the bearing current of DFIG and research the method of bearing current suppression.Firstly,the bearing current analysis model of doubly-fed wind turbine generator system is investigated.The parasitic capacitance inside the DFIG is calculated by the finite element numerical analysis method,and the common mode equivalent circuit is established according to the internal structure of the DFIG.Through the analysis of the bearing voltage ratio of the generator-side converter and the grid-side in the doubly-fed wind turbine generator system,it can be seen that the influence of the generator-side converter on the bearing current is far greater than that on the grid-side converter.The impact of grid-side converter for bearing current can be ignored.Secondly,the common mode voltage of three-level doubly-fed wind power converter is analyzed based on the generation mechanism of bearing current of DFIG.Based on the traditional three-level neutral-point-clamped topology,an H14 topology which can suppress the common mode voltage is proposed.The working principle of H14 converter is introduced.The common mode voltage characteristics of H14 converter under four operation patterns are derived.The control method of H14 converter based on switching function is designed.The common mode voltage suppression effect of H14 converter is tested by simulation.In addition,the system model of doubly fed wind turbine system is established based on the proposed H14 topology.In the control system of the generator-side converter,a compound controller is designed,which consists of the traditional PI control and the repetitive control.The repetitive controller based on the bandwidth coefficient(BRC)is used in the inner loop control and the traditional PI controller is used in the outer loop control.Through the stability analysis,the compensation term of BRC is determined.The simulation results fully verify that under the compound control,the H14 topology can effectively suppress the bearing current of the DFIG,and the harmonic characteristics of the DFIG are well.Starting from the practical problems of bearing current in wind power engineering,the analysis and suppression of bearing current of DFIG are investigated in this paper,which provides a new idea for the design of wind power system.