PWM rectifiers receive significant attention due to their flexible control and perceived low harmonic currents, and as a consequence are replacing controlled and uncontrolled rectifiers as the preferred technology in industrial applications. However, low harmonic currents from PWM rectifiers are generally limited to applications where supply voltages have very low background distortion. An investigation using derived mathematical equations, simulation and laboratory measurement is presented to assess the influence of higher levels of background voltage distortion on the harmonic current distortion of PWM rectifiers. The mathematical expression for the harmonic current is derived using the Double Fourier Integral method. A simulation model is developed to quantity the effect of increased supply voltage distortion on harmonic currents from PWM rectifiers. Simulation results and laboratory measurements are presented for the simple case of a PWM rectifier to validate the methodology, which will ultimately be used for future work to quantity the influence of supply voltage distortion. As expected the increase in harmonic current of a PWM rectifier is heavily impacted by the severity of the voltage distortion appearing at the PWM rectifier terminals.