Electric Power Principles, 2nd Edition

2AC Voltage, Current, and Power

The basic quantities in electric power systems are voltage and current. Voltage is also called, suggestively, “electromotive force.” It is the pressure that forces electrons to move. Current is, of course, that flow of electrons. As with other descriptions of other types of power, electric power is that force (voltage) pushing on the flow (current). In order to understand electric power, one must first solve the circuit problems associated with flow of current in response to voltage.

Most electric power systems, including all electric utility systems, employ alternating current. Voltages and currents very closely approximate sine waves. Thus, to understand the circuit issues it is necessary to prepare to analyze systems with sinusoidal voltages and currents. Robust and relatively easy‐to‐use methods for handling sinusoidal quantities have been developed, and this is the subject material for this chapter.

In this chapter we first review sinusoidal steady‐state notation for voltage and current, and real and reactive power in single‐phase systems.

2.1 Sources and Power

2.1.1 Voltage and Current Sources

Consider the interconnection of two sources, as shown in Figure 2.1. On the right is the symbol for a voltage source. This is a circuit element that maintains a voltage at its terminals, conceptually no matter what the current. On the left is the symbol for a current source. This is the complementary element: it maintains current no matter what ...