Investigation of thermal stress in the rotor of Doubly-Fed Induction Generators at synchronous operating point

Doubly-Fed Induction Generators (DFIG) are commonly used in wind power plants. Combined with a rotor-side frequency converter, they allow the turbine to rotate at variable speed for maximum aerodynamic efficiency. The underlying speed-power relation contains the synchronous operation point, which deserves special attention as rotor currents become direct. Due to the three phase system, current magnitudes differ and therefore cause diverging ohmic losses within the rotor windings. This paper investigates the rotor temperature profile caused by unsymmetrical losses at synchronous operation on the example of a 1.5 MW DFIG. Thermal analysis is based on a reconstruction of the cooling conditions, that were quantified by measurements in asynchronous operation. Those data is used to tune a thermal model, which is composed of an axial analytical part and a radial finite-element structure. Subsequently, synchronous operation is simulated by changing the load distribution within the model. Results exhibit an overtemperature rise of 86% at the winding heads, which confines the maximum rotor current at synchronism.