Preliminaries - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 62

2
Preliminaries
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
2.1.1 Introduction
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 formmicrogrids via power
inverters (Guerrero et al. 2009; Iyer et al. 2010; Lasseter, 2002; Weiss et al. 2004; Zhong and
Weiss 2011), which may or may not be connected to the grid. A major power quality problem
in these applications is the harmonics in the voltage provided by the inverters. This is the main
power quality issue to be addressed in this topic.
There are two sources of harmonics: one is from the inverters (e.g. because of the pulse-
width-modulation and the switching) and the other is from the loads or the grid. The majority