Challenges in Modeling of Large Synchronous Machines

Machine simulation models allow the use of embedded numerical computation techniques. The finite element model accuracy is usually gauged by comparing its performance with experimental measurements. In this paper, a large hydrogenerator is modeled and the sensitivity of various model parameters is investigated by comparing the simulated results with experimental measurements. Four hydrogenerator units with the same design drawings are considered in the analysis. The effect of the model parameters was studied by considering the response of the open-circuit voltage in comparison with the measured open-circuit voltage, of the different units. Parameters considered included the effective model depth for a two-dimensional simulation, effective air gap, and effective material permeability of the B-H curve. A 20% variation in operational air gap between two machines of the same design resulted in over 18% difference in open-circuit voltage, particularly beyond the knee point. Reduction in permeability of the soft magnetic materials resulted in agreement between the simulation and measured results at saturation. Consequently, for large machines, the B-H curves from the Epstein measurements are insufficient and should be adjusted accordingly.