An investigation of zero order harmonics in power transformers

An investigation was done on the effects of zero order harmonics produced in a 7.5 kVA single phase transformer using DC produced by injection on the primary from an external source and also by half wave rectification on the secondary with a resistive load. The accuracy of the discrete Fourier transform (DFT) for determining the DC component was also looked at, along with the ability of current transformers to detect a low frequency, or quasi-DC, component superimposed on an AC signal. Polynomial functions were developed to predict harmonic production, line current amplitude, input power, power factor, core losses, etc. as functions of the DC current value for both cases. Results show a strong similarity in produced current harmonics for both eases. The DFT is an accurate tool for determining the value of a DC component. Passive current transformers are not able to accurately detect a DC, or slowly varying AC, component.