Rotor Interturn Short Circuit Impact on Large Hydrogenerator Magnetic Quantities

In this paper, a two-dimensional time-stepping finite-element model of a large hydrogenerator considering field interturn short circuits (ITSCs) default is developed and presented. The study quantified the impact of the degree of rotor ITSC on the radial magnetic flux density, radial force density, unbalanced magnetic pull, and electromagnetic torque of a 74-MVA industrial large hydrogenerator with 76 poles. The proposed numerical model was validated by comparing computed and experimentally measured magnetic flux density of a chosen healthy machine; good agreement between measured and computed results is obtained. Moreover, a detection method using two search coils installed on stator teeth is used. This method allows by inspection of the peak value the quantification of the degree of ITSC. The average value of the magnetic flux density of the defective pole decreases linearly (more than 5%) with the increase of the number of ITSCs; as well, the unbalanced magnetic pull increases with a polynomial of degree 2 with the increase of the degree of ITSC.