In this chapter, some preliminaries that are common to more than one chapter are discussed. These include power quality issues, repetitive control and reference frames.
2.1 Power Quality Issues
Power quality is a set of electrical properties that may affect the proper function of electrical systems. It is used to describe the electric power that drives an electrical load. Without proper power quality, an electrical device (or load) may malfunction, fail prematurely or not operate at all. Although the term “power quality” is commonly used, it mainly refers to the quality of voltages because in conventional power systems the supplies are voltage sources instead of current sources and the quality of currents is determined by the loads.
Poor power quality can be described in different ways., e.g., the continuity of power, variations in magnitude and frequency, transient changes, harmonic contents in the waveform, low power factor, imbalance of phases etc. Some commonly used terms are shown in Table 2.1. Poor power quality can be the consequence due to different reasons, e.g. the result of shared infrastructure in power systems. A fault at one site may cause poor power quality to loads at other sites. Nowadays, more and more distributed generation and renewable energy sources, e.g. wind, solar and tidal power, are developed. They often form microgrids via power inverters (Guerrero et al. 2009; Iyer et al. 2010; Lasseter, 2002; Weiss et al. 2004; Zhong ...