Electrical Power System Essentials

4.1 INTRODUCTION

The line that has been chosen in this book, generation – transmission – distribution – utilization, stems from the pyramid structure of the power system: a relatively small number of large power stations supply the transmission network, which in turn supplies the distribution networks that fan out to provide the individual loads with the demanded energy. But the causality is in fact the other way around! The power system is and must be designed and organized in such a way that the demand can be fulfilled: the consumers are supplied with the requested amount of active and reactive power at constant frequency and with a constant voltage. Consumer demand is not constant but varies from hour to hour each day, from day to day with in a week and from season to season. An example of a daily load curve is shown in Figure 4.1. The minimum load of the day, which is called the valley load, is about 40–60 ≈ of the system peak load and usually occurs between 4 and 5 a.m. The generation should be able to fulfill the fluctuating demand, and the transmission and distribution systems have to be able to facilitate the flow of energy.

Most power systems are vertically operated, which means that the grid consists of large power plants feeding bulk power into the high-voltage transmission network that in turn supplies the distribution substations. A distribution substation serves several feeder circuits and a feeder circuit supplies numerous loads of all types. A light to medium industrial ...