Introduction

A nonlinear load may be defined as a load which, having a sinusoidal voltage applied to it, passes a nonsinusoidal current. Many everyday loads are nonlinear but the nonlinearity is often unimportant; an unloaded transformer is nonlinear owing to magnetic saturation. Significant nonlinear loads associated with UPS equipment include rectifiers and switched mode power supplies.

Any regular nonsinusoidal waveform may be regarded as a compound wave made up from a fundamental component and harmonic components; the composition of such waveforms may be established mathematically by applying Fourier s analysis. Even harmonic currents and voltages indicate a lack of symmetry between positive and negative half cycles.

The phase sequence of three-phase harmonics is in accordance with the following pattern:

The concept of harmonic phase sequence may not be familiar to readers and Table 4.1 may offer an explanation.