Estimation of Stator Vibration in DFIM Under Various Types of Rotor Excitation

Variable speed power generation using Doubly Fed Induction Machine requires an adjustable frequency supply at the rotor terminals which can be achieved using power converters. Ideally, a variable frequency sinusoidal voltage should be applied, but it is not possible without the help of power converters. For a typical 250 MW doubly fed induction machine (DFIM) with a speed variation of −10.73% to +8.33%, excitation power of 25 MW at 3.3 kV and 11600 Amps is required. With this much high rotor current at low fundamental rotor frequency (3.75 Hz) electromagnetic forces in the air gap contain harmonic force components. Also, any time harmonic of radial force density in the rotor will be induced in the stator as a distinct time harmonic, modified by the influence of space harmonics in DFIM with regular three phase integral slot winding on both stator and rotor and vice versa. So, if the frequency and order of electromagnetic force components conflicts with mode shape and natural frequencies of the machine stator strong magnification in vibration will occur or more severely develops mechanical resonance. The stator vibration of electrical machine is generated by the electromagnetic forces acting on the surface of the stator tooths. This paper analyzes stator vibration for 250 MW DFIM in multiphysics simulation with its rotor winding excited through pure sinusoidal excitation at fundamental rotor frequency of 3.75 Hz and with two-Level voltage source converter at 3.75 Hz fundamental rotor frequency. Hardware results with supported multiphysics simulation on a laboratory prototype of 2.2 kW rating DFIM.