Analytical solution to harmonic characteristics of traction PWM converters

The paper presents an analytical technique for calculating the AC input current harmonic spectrum of single-phase PWM converter drives used in railway traction. The technique is based on a 3-D modulation model and the double Fourier series (DFS) method. Formulas in the form of Bessel functions representing the harmonic spectra for several combinations of interlaced PWM converters are derived. These give a useful insight into the harmonic generation and cancellation process which occurs with interlaced configurations. They are also used in a theoretical explanation of the interlacing angle, system parameter symmetry and load balance between interlaced converters. The accuracy of this analytical technique is confirmed by comparing the results with those obtained from a frequency domain model (FDM) of the PWM converter drive.