Reduction of Bearing Currents in Doubly Fed Induction Generators

Bearing currents have been identified as a major cause of electrical machine failure when operating from a solid-state pulse-width modulated inverter. This is because at frequencies corresponding to the switching time of the inverter semiconductors, charges accumulating in the parasitic capacitances between the stator and rotor create voltages across the bearings of the machine that eventually discharge and damage the bearings. It is to be anticipated that generators as well as motors would be susceptible to bearing failure in this way, particularly in wind turbine applications where a frequency converter is often used to convert the power produced by the variable speed turbine to AC electrical power at a constant line frequency. This paper presents a novel approach to reducing bearing currents in doubly fed operation wound rotor induction machines where the rotor is controlled by a back-to-back rectifier/inverter system. The approach used is to constrain the inverter PWM strategy to reduce overall common mode voltages across the rectifier/inverter system, and thus significantly reduce bearing discharge currents. It is shown that the common mode voltage can be almost completely eliminated, at the price of a reduction in voltage gain of the rotor supply to 57% of the supply voltage. However, since the wind generator is never required to operate at more than plus or minus 30% above or below synchronous speed. this limitation is of little consequence in variable speed wind turbine applications.