AC Circuits and Power Systems in Practice

1Power Systems: A General Overview

In this chapter we present an overview of the structure of a modern power system, from the low voltage distribution networks with which we are partly familiar, to the high voltage transmission system bringing energy from remote electrical generators.

Before we begin, we need to define the different potentials which we will encounter throughout the power system. We use abbreviations to denote different voltage levels, as outlined in Table 1.1. However, as is often the case, there appears to be no universally accepted definition, so you may see slightly different definitions used elsewhere.

Table 1.1 Voltage definitions.

LV (low voltage)	<1000 V AC
MV (medium voltage)	1–35 kV AC
HV (high voltage)	35–230 kV
EHV (extra high voltage)	>230 kV

1.1 Three‐phase System of AC Voltages

The alternating voltage distributed to our homes has (ideally) a sinusoidal form. Sinusoidal waveforms are chosen because, when pure, they contain only one frequency; that is, they should contain no supply frequency harmonics (i.e. multiples of the fundamental frequency). Unfortunately, due to the increasing number of non‐linear loads connected to the power system and the non‐sinusoidal currents they consume, harmonic voltage distortion is becoming an increasing problem. This is particularly so in the LV network and, as a result, the AC voltage we receive often contains some harmonic distortion. However, despite this it is still approximately sinusoidal. ...