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This paper proposes an analytical algorithm for the computation of the magnetization and loss curves for a transformer from standard no-load test report data. This is the input to standard transformer models for power system simulation. In the case of delta-coupled windings, the conversion from test-report root-mean-square line quantities to peak phase values requires processing of the triplen harmonics in the current. This is handled analytically in the paper with utilization of the concept of incremental inertance and conductance. The analysis shows that only the third harmonic needs to be treated to reach sufficient accuracy within the measurement errors. The test case with a 290-MVA generator stepup transformer shows an increase in the calculated current of above 10% at rated voltage with proper triplen harmonic handling. This difference will increase considerably with excitation voltage.