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

The Doubly-fed induction generator (DFIG) is one of the widely used forms in wind power generation system. The common mode voltage caused by the converter can induce bearing voltage though common mode path formed by the parasitic capacitance and result in the bearing currents, which could shorten bearings lifetime and seriously affect the safe and stable operation of wind power. It is extremely important for analyzing bearing currents and its mitigation techniques to DFIGFirstly, the calculation and measurement of parasitic capacitances in DFIG are performed. In 1.5MW DFIG case, the analytical formulas of the parasitic capacitance are derived and part capacitances in induction generator are solved with finite element calculation. Three methods on measurement of capacitance are proposed. The analytic calculation results and the finite element calculation results are compared with the measured parasitic capacitances. It is shown that the finite element calculation results are close to the measured values.Secondly, doubly-fed induction generator bearing currents model is investigated. So far, the effects of stator windings on bearing currents had never been analyzed. In this thesis a complete bearing current model considered stator windings is set up, which is used for comparing with the simplified model without stator windings. Through comparing bearing voltage ratio of the two models, the effects of the parasitic capacitance between the stator windings and the other parts can be neglected, and simplified model can be used to analyze bearing current problems.For the mitigation techniques on bearing currents, a method of adding a metal shielding layer in the rotor slot is proposed in this thesis. The influence of thickness and segment structure of the shield layer on the mitigation are analyzed based on finite element calculation. Then eddy current losses caused by the shield are calculated. The results show that shield layer can inhibit the bearing currents dramatically and the thickness of shield layer has no influence on the mitigation, and shield layer segments will not decrease the mitigation. Rotor speed and power load will impact eddy current loss of the shielding layer.Finally, three different insulation bearing on the suppressing of bearing currents are analyzed, including the insulated cover, insulated bearing housing and insulated bearing. The insulation layer radius and the thickness have important effects on bearing voltage ratio for the first two methods. The effects of mitigation for the bearing coated with different ceramic material are also analyzed. The analysis shows that the effect of inhibiting high frequency current with the three methods are very limited.