AC Circuits and Power Systems in Practice

6Power Flows in AC Networks

In this chapter we explore the flow of active and reactive power in AC networks, in particular between network nodes interconnected with transmission lines. These may operate at either HV, MV or LV potentials, and we will see that a line’s X/R ratio influences to a large degree the use to which it can be put.

Lines that are largely inductive in nature (i.e. having high X/R ratios) are suitable for the transmission of active power, often over great distances. However, if these are used to transmit substantial quantities of reactive power, the voltage drop created quickly becomes excessive and severely reduces the useful distance over which transmission is possible.

High voltage transmission falls into this category, and these lines are generally used for the transmission of active power between generation assets and load centres. Since reactive power can be more easily generated near to a load centre, it seldom appears in large quantities on the transmission network. This choice is also an economic one. High voltage transmission assets are expensive and are generally dimensioned with a priority given to active power transfer capability, without significant coincident reactive power flows.

We shall see that high X/R lines also have the advantage that active and reactive power flows can be controlled independently of one another; they are said to be decoupled. In contrast, long MV and LV lines have considerably lower X/R ratios and, as a result, the ...