Modal transformation decomposes three-phase power system voltages or currents into three sets of de-coupled quantities. The symmetrical components transformation is a well-known example of modal transformation. In this paper, the applications and performances of three different modal transformations for determining power system harmonic impedances are investigated. The results show that the symmetrical components transformation is not the best candidate for harmonic impedance calculation. The Clarke and 012 transformations can provide more reliable results. Computer simulation and field measurement results are used to demonstrate the pros and cons of different transformation methods.