When a transformer is subjected to FRA testing, the leads are configured in such a manner that four terminals are used. These four terminals can be divided into two unique pairs [14], one pair for the input and the other pair for the output. These terminals can be modeled in a two-terminal pair or a two-port network configuration. Figure 1 illustrates a two-port network where z11, z22, z12 and z21 are the open-circuit impedance parameters.