There has already been significant work done to aid in the understanding and quantification of the effects of harmonic emissions from controlled and uncontrolled rectifiers, as well as pulse width modulated (PWM) rectifiers, in electrical networks. Models of each type of rectifier have been developed specifically for power system harmonic studies in order to accurately evaluate voltage and current distortion levels and assess potential side effects in electricity supply networks. Mitigation equipment has been extensively developed to counteract the negative effects of harmonics. Effective mitigation is heavily reliant on the accuracy of harmonic studies and the same of existing rectifier models that are incorporated in the harmonic studies. Though the work introduced in rectifier modelling for the purpose of harmonic analysis may appear to be sufficient, the majority of modelling effort for each type of rectifier has been expended while the rectifier of concern is isolated assuming no background supply distortion, abandoning the ability to quantify the influence of already existing supply distortion levels on equipment harmonic emissions. The accuracy of modelling harmonic producing equipment, specifically uncontrolled and controlled rectifiers, and PWM rectifiers, under the influence of distorted supply is investigated in this research. Given the inaccuracy of existing models of uncontrolled and controlled rectifiers, as well as PWM rectifiers, the main goal of the research is to develop a mathematical relationship between supply voltage distortion and the change in the harmonic current spectrum, specifically when background distortion levels are significant, i.e., near or above the limits set by standards, so that harmonic mitigation can be achieved in a more robust manner compared to what is possible presently. The harmonic spectrum of uncontrolled and controlled rectifiers, as well as PWM rectifiers under the influence of supply distortion will be derived employing the harmonic spectrum of the same devices connected to an undistorted supply via a function reflecting the effect of background distortion for each individual harmonic order. A harmonic generic model of an uncontrolled and controlled rectifier is proposed based on a PWM rectifier model to represent the effect of supply voltage distortion on the harmonic current which a PWM rectifier will draw. A mathematical model for the resulting harmonic current is derived using the Double Fourier Integral method. A new method for controlling the PWM rectifier is presented, which replicates the harmonic current spectrum of a typical uncontrolled rectifier with greater accuracy, particularly when operating under the influence of