Experimental study on electrical evaluation of rotor insulation system of DFIG for wind power generations

The failure rate of doubly-fed induction generators (DFIG) for wind turbine generations was relatively higher than that of direct-driven generators, according to statistical data from some wind power industries. The rotor insulation was a relatively weak part of the insulation system for DFIG because it was normally subjected to a PWM impulse voltage during the long-time operation. In this paper, this research focuses on the electrical evaluation based on accelerated electrical aging tests for rotor insulation of DFIG. The accelerated aging tests under the bipolar repetitive impulse with the voltage of 5.3, 6.4 and 7.3 kV were carried out. Some of non-destructive characteristic parameters, polarization and depolarization current were measured periodically during the each interval of cycled aging periods. In addition, the appearance features of the insulation layers at different distances away from the conductor on the aged or failure specimen were observed by microscope. The feasibility and effectiveness of these non-destructive characteristic parameters on the aging status assessments of the rotor insulation system of DFIG are analyzed. The failure times or lifetime of the electrical aging from some specimens were got. Finally, the appropriate selections of the applied voltage levels for the accelerated aging tests were also discussed.