Mechanisms and Return of Experiences of Electric Discharges in Ball and Sleeve Bearings of PWM-Fed Machines with Focus on Wind Turbine Double-Fed Induction Generator

The Double Fed Induction Generator with a rotor fed by PWM converter through a slip ring is the most common architecture used for onshore windmills. Compared to motors fed by power electronic through the stator, this configuration is more severe for the electrical stresses in the ball bearings. The first part of this paper describes two variable speed systems, on one hand stator Voltage-Source-Inverter-fed motor on oil sleeve bearings, and on the other hand rotor Voltage-Source-Inverter-fed generator on ball bearings. The second part overviews the common mode voltage mechanisms from the drive inducing resonance in the electric system and high frequencies currents in the bearings. A new electro erosion indicator of electric discharges is presented and completed by two others existing indicators for antifriction bearings electric lifetime. The methods of calculating capacitances and the risks of discharges are explained for ball bearings, and for sleeve bearings with a detailed focus on the spreadsheet of the IEEE Standard 1349. The last part focuses on recommendations of rotor grounding based on experimental measurements and lifetimes prediction of ball bearings for a 3.95 MW Double Fed Induction Generator system powered by a 2 -level voltage source inverter.