Analysis And Suppression Of Bearing Currents And Common Mode Currents In Permanent Magnet Synchronous Wind Generators

Permanent magnet synchronous generators(PMSGs)are widely used in wind power generation systems.The common mode voltage caused by the converter can induce the bearing voltage and the bearing current through coupling of the parasitic capacitance.The bearing currents could cause the premature failure of bearing,and it also will induce the wind turbine shut down,burn out and cause huge losses.At the same time,the common mode current generated by the high frequency common mode voltage flows into the earth through the stator frame,which will induce harmonic interference.Therefore,it is important to study the problem of the bearing current in PMSG and put forward the mitigation techniques of the common mode current.Firstly,this thesis calculates the parasitic capacitances of the PMSG.The analytical formula for parasitic capacitances of the surface and built-in mounted PMSG are presented.Based on the partial capacitance theory of multi-conductor systems,parasitic capacitances are calculated through the finite element method.Using above two methods to the capacitance of the 2.1 MW surface and built-in mounted PMSG are calculated,the results show that the analytical calculation of the built-in mounted PMSG are consistent with the finite element calculation.But there is great difference in the surface mounted PMSG,then the causes of the error are analyzed.Secondly,this thesis carries out the modeling and simulation analysis on the bearing current of the PMSG.Bearing voltage ratios(BVR)of the surface and built-in mounted PMSG are calculated,and it shows BVR of the surface mounted PMSG is same as the built-in mounted PMSG.The value of BVR of PMSG is between the induction motor and the doubly-fed induction generator.The simplified model of the surface mounted PMSG is surveyed,and it is proved that the simplification is effective.The sensitivity of the bearing voltage ratio in the built-in mounted PMSG is analyzed.The bearing current simulation model of converter-PMSG system is built in MATLAB,which can forecast the common mode voltage,common mode current,bearing voltage and bearing current.At the same time,two kinds of bearing current suppression factors are analyzed.The results show that the suppression method on bearing currents cannot effectively mitigate the common mode current.Thirdly,aiming at the common mode current suppression,the inductance parameter calculation,design scheme and the suppression effect of the common mode chock effect are studied.The design can consider the common mode voltage,the circuit parameters,the cable diameter,the common mode current peak and the material properties of the common mode chock.The design of the common mode chock for the surface mounted PMSG is carried out.,and the simulation results show that the common mode chock can effectively restrain the common mode current.Finally,this thesis studies the electrical characteristics of the bearing.The experimental scheme for measuring the electrical parameters of the bearing is designed,and the relationship between the equivalent bearing capacitance and resistance and the motor speed and bearing temperature is analyzed.The results show that the equivalent capacitance of the bearing is inversely proportional to the motor speed and proportional to the bearing temperature,the equivalent resistance of the bearing is directly proportional to the motor speed and inversely proportional to the bearing temperature.