Power and Harmonics in Nonsinusoidal Systems

Abstract

Rectification used to be a much simpler topic. A textbook could cover the topic simply by discussing the various circuits, such as the peak-detection and inductor-input rectifiers, the phase-controlled bridge, polyphase transformer connections, and perhaps multiplier circuits. But recently, rectifiers have become much more sophisticated, and are now systems rather than mere circuits. They often include pulse-width modulated converters such as the boost converter, with control systems that regulate the ac input current waveform. So modern rectifier technology now incorporates many of the de-de converter fundamentals. The reason for this is the undesirable ac line current harmonics, and low power factors, of conventional peak-detection and phase-controlled rectifiers. The adverse effects of power system harmonics are well recognized. These effects include: unsafe neutral current magnitudes in three-phase systems, heating and reduction of life in transformers and induction motors, degradation of system voltage wave-forms, unsafe currents in power-factor-correction capacitors, and malfunctioning of certain power system protection elements. In a real sense, conventional rectifiers are harmonic polluters of the ac power distribution system. With the widespread deployment of electronic equipment in our society, rectifier harmonics have become a significant and measurable problem. Thus there is a need for high-quality rec-tifiers, which operate with high power factor, high efficiency, and reduced generation of harmonics. Several international standards now exist that specifically limit the magnitudes of harmonic currents, for both high-power equipment such as industrial motor drives, and low-power equipment such as electronic ballasts for fluorescent lamps and power supplies for office equipment. This chapter treats the flow of energy in power systems containing nonsinusoidal waveforms. Average power, rms values, and power factor are expressed in terms of the Fourier series of the voltage and current waveforms. Harmonic currents in three-phase systems are discussed, and present-day standards are listed. The following chapters treat harmonics and harmonic mitigation in conventional line-commutated rectifiers, high-quality rectifier circuits and their models, and control of high-quality rectifi-ers. R. W. Erickson et al., Fundamentals of Power Electronics