The paper presents an in situ parameter estimation method to determine the equivalent circuit parameters of the three-winding

transformer (TWT). The suggested method also estimates geometrically a complex parameter that is mutual leakage between

secondary and tertiary windings, which would be useful in transient studies. Beside the saturation effect of the transformer is taken

into account by estimating a highly nonlinear parameter that is magnetizing circuit reactance. Different search based optimization

tools are applied for parameter estimation among which the results obtained using genetic algorithm is found to be encouraging.

Load test data at one particular operating point namely supply voltage, load currents, input power, and load impedance is suffi cient

to estimate the parameters.