An investigation of the harmonic characteristics of transformer excitationcurrent under nonsinusoidal supply voltage

This paper investigates the effect of supply voltage harmonics on the excit ation current of a typical 25 kVA single-phase distribution transformer. A complete analytical model is developed to calculate the time domain wavefor m and the harmonic components of the excitation current. The key findings a re The phase angles of the supply voltage harmonics determine whet-her or not those voltage harmonics increase or decrease the distortion of the input cu rrent (with respect to the sinusoidal supply voltage case). A peaked input voltage wave distorted by 3rd, 7th, 11th, ..., harmonic components, or a fl attened input voltage wave distorted by 5th, 9th, 13th, ..., harmonics, cre ates a flattened current (i.e., decreases the current harmonics and distort ion). A flattened input voltage wave distorted by 3rd, 7th, 11th, ..., harm onic components, or a peaked input voltage wave distorted by 5th, 9th, 13th , ..., harmonics, creates a peaked current (i.e., increases the current har monics and distortion). The 3rd harmonic component in the supply voltage has the highest effect on the current harmonics and distortion. The effect of higher voltage harmonic s gradually diminishes with frequency. The magnitudes of the individual cur rent harmonics follow the same decreasing/increasing pattern of changes as does current distortion variation. The corresponding variations in harmonic current phase angles is negligible.