Determining negative sequence currents of turbine generator rotors

In parallel to their dynamic duty synchronous generators must be capable of bearing permanently an additional negative sequence current of up to 10% of the rated current according to the IEC 60034–1 standard. On turbine generators this does not only charge the damper winding but causes eddy currents and losses in the solid rotor iron in parallel. The complex current distribution cannot yet be satisfactorily calculated. The paper presents a new method of calculating these currents based on a lumped circuitry taking into account even the cross slits in the pole face. Target of the development is a fast routine calculation tool for determining the negative sequence current capability of a rotor already in the design stage of a generator. 3D-FEM reference calculations were performed as well as measurements on a rotor mock-up for studying complex local current distributions e.g. in the vicinity of the cross slits and the rules to be implemented in the lumped circuit network. FEM calculations themselves, affordable for research efforts, are considered too laborious and time consuming for being considered as routine design calculation tools.