On wind turbine main shaft bearing currents

Licentiate thesis, 2024

This study explores the main shaft bearing currents in wind turbines, examining currents both internally generated from the electrical system and externally induced. The research investigates their origins and transmission paths, starting with an overview of general bearing current phenomena, drawing insights from electrical and mechanical system studies. A downscale laboratory wind turbine, abstracted from various wind turbine subsystems, is developed, and employed.
Potential current sources are explored through both laboratory setups and real wind turbine tests, discussing transmission paths, and coupling mechanisms in simplified configurations. Laboratory experiments with the downsized wind turbine validate proposed paths, emphasizing the existence of two common mode voltage sources in modern turbines. The common mode voltage effect exhibits a periodic current pattern, with its frequency determined by converter and grid frequencies. Additionally, the study reveals Electrostatic Discharge via turbine rotor blades as another major source of main shaft bearing current. The often-unnoticed rotor blades' electrostatic discharge effect is further examined in a laboratory experiment. Placing a downsized wind turbine in an artificial environment filled with free charges, attached to the wind blades via airflow, reveals that the electrostatic discharge effect generates currents as high as hundreds of amperes in a brief instance on the main shaft, posing a considerable risk to bearing lifespan.
To protect the bearing from harmful currents, the study suggests primary strategies involving the elimination of the current sources and alteration of the current paths. The common mode voltage driven pattern bearing current can be reduced by utilizing the common mode voltage filter. Regarding the electrostatic discharge-driven bearing current, better maintaining of the main shaft ground brush is imperative to minimize the current passing through the bearing.