6.12: Thévenin Equivalent Circuits
The household power distribution system, encompassing distribution lines and transformers, serves as the primary network. Electrical appliances within a household can be represented as load impedance. To simplify this intricate distribution system, Thévenin's theorem can be applied to create a Thévenin equivalent circuit. If an AC circuit is partitioned into two parts (circuit A and circuit B), connected by a single pair of terminals as shown in Figure 1.
Figure 1: Circuit portioned into two parts
Figure 2:Circuit A replaced by its Thévenin equivalent circuit
Replacing circuit A with its Thévenin equivalent circuit (a voltage source in series with an impedance) does not alter the current or voltage of any element in circuit B (shown in Figure 2). The values of the currents and voltages of all the circuit elements in circuit B will be the same irrespective of whether circuit B is connected to circuit A or its Thévenin equivalent. Two parameters are required to find the Thévenin equivalent circuit: the Thévenin voltage and the Thévenin impedance. Figure 3 shows an open circuit connected across the terminals of circuit A to determine the open-circuit voltage Voc , while Figure 4 indicates that the Thévenin impedance Zt is the equivalent impedance of circuit A*.
Figure 3:Thévenin equivalent circuit with Voc .
Figure 4:Thévenin equivalent circuit showing Zt. .
Circuit A* is formed from circuit A by replacing all independent voltage sources with short circuits and all independent current sources with open circuits. Generally, the Thévenin impedance Zt can be determined by replacing series or parallel impedances with equivalent impedances repeatedly.
