Modeling of auto-transformers for load flow calculations

A new approach to modeling of auto-transformers is presented in this paper. An auto-transformer, compared to a separate-winding transformer, is characterized by small impedances, which can even be very small at the neutral tap position. Especially for auto-transformers which have identical voltage levels at both sides, this feature cannot be modeled for load flow calculations using Newton-Raphson or Current injection algorithm. Actual approaches for modeling of autotransformer using small impedances, risk that the loss of numerical stability has a negative impact on the convergence. This proposal uses additional injections for the correct compensation of error in the modeled impedance.