AC Circuits and Power Systems in Practice

4Magnetic Circuits, Inductors and Transformers

In this chapter we examine the theory of transformers, since they comprise a major part of the power system and were, to a large extent, the reason that alternating current became the generally accepted medium for the transmission of electrical energy. The ability to easily change from one AC potential to another, makes it possible to transmit large amounts of energy across considerable distances at HV potentials with minimal transmission loss. We begin with an analysis of magnetic circuits, inductors and single‐phase transformers and explore the apparent difference in behaviour between current transformers and voltage transformers. In Chapter 7 we consider the characteristics of three‐phase transformers.

In this chapter we will also introduce the concept of the per‐unit system of expressing electrical quantities. This is a convenient way to perform calculations across transformer boundaries because per‐unit quantities do not change when moving from one potential to another.

We begin our study of transformers with a discussion of magnetic circuits and their similarities to electric circuits.

4.1 Magnetic Circuits

Just as in an electric circuit where the current flows around a closed path, so lines of magnetic flux in a magnetic circuit also follow a closed path. Indeed, there is more than a passing similarity between these two quantities. Electric circuits consist of materials capable of supporting the passage of an electric ...