Bearing currents in doubly-fed induction generators

An increasing demand on the quality of wind power systems affects also more and more the fields of lifetime and reliability. But, problems like bearing damages because of bearing currents counteract these desired demands. Especially doubly fed induction generators, which are wide spread in use for wind turbines up to the MW-class can be limited in reliability because of the mentioned problem. Hereby, the rotor-side feeding by IGBT voltage source inverters causes an uprise of bearing currents. Unfortunately, this parasitic effect, well known from stator-side inverter-fed machines, is different and maybe even worse since the capacitive voltage divider, responsible for the transfer of the common mode voltage to the bearings, differs to a great extend. The analysis of bearing current circuits given in this paper will help to explain these differences. Moreover, some fundamental model characteristics of doubly-fed induction generators is mentioned in order to give hints for measures against bearing currents. Measured effects gained from a 1.5 MW generator support these investigations and evaluate the efficiency or the problems of measures for decreasing bearing currents