Probabilistic modelling of current harmonics produced by an AC/DC converter under voltage unbalance

Harmonic currents generated by an AC/DC power converter are subject to random variations due partly to fluctuation of the converter´s DC load and partly to power source disturbances. The latter are also responsible for the generation of uncharacteristic harmonics. Based on an analytical power converter model and probabilistic models of voltage unbalance and converter load variations, the random behaviour of both characteristic and uncharacteristic power system harmonics produced by a six-pulse AC/DC power converter undervoltage unbalance is analyzed. The probability density functions of the magnitude and the phase angle of harmonic currents are derived and compared with the results obtained from Monte Carlo simulations. It is found that the developed probabilistic harmonic model agrees very well with the results of Monte Carlo simulations and gives significant insight into the effects of voltage unbalance and converter DC load variations upon the random properties of the power converter current spectrum