Measured transformer derating and comparison with harmonic loss factor (F_HL) approach

The eddy-current loss coefficient of a transformer can be determined by using the harmonic amplitudes of input current and the corresponding harmonic copper losses when the transformer supplies nonlinear loads. Measured data show that eddy-current loss is a linear function of frequency with a power of 2. Apparent power (kVA) derating of single-phase transformers operating with nonsinusoidal currents and voltages is measured by adjusting the total losses to the rated losses at rated temperature, whereby hot-spot considerations are ignored. Computed results of derating using the harmonic loss factor (F_HL), eddy-current loss coefficient P_{EC-Rnonlinear} and iron-core losses match well with measured data. However, the real power derating of transformer is much greater than apparent power derating for a given THD_i value, and it is independent of the frequency spectra of the harmonics. Experiments show that the other stray losses (P_OSL) in nearby conductive solid components are proportional to the power of 0.8 at low frequencies and are inversely proportional to the power of 0.9 at high frequencies.