To calculate the mutual impedance, a current is injected to bus 1 and the voltage at bus 2 is calculated, while all other sources in the system are deactivated (voltage sources short-circuited and current sources are open-circuited). In general, the absolute values of the impedance's (Z12 and Z21) will be the same.

Only the complex angle will be different, if a transformer with a specific vector group is used.

In the attached files, the calculation is illustrated by a simple example. It can be verified by a harmonic load flow calculation too.