The power transformers could face situations when a high frequency voltage of up to several MHz is superposed on the rated 50 Hz three-phase voltages. Examples could be Power On-Line Communications, lightning strokes, or switching surges. The high frequency voltages are identical voltages (common mode) on all the three phases of the power transformer, i.e., they form a zero-sequence three-phase voltages system that is superimposed on the 50 Hz directsequence three-phase rated voltages system. The paper presents experimental results regarding the behavior of a three-phase transformer in a common mode connection and excited by zero sequence voltages in the frequency range 100 Hz-100 kHz. A principal conclusion is that both the 'magnetizing' and leakage inductances are due to air magnetic fields. As a result, the 'magnetizing' inductance and 'core' losses are much lower than those of a three-phase transformer in a conventional connection and exited by three-phase direct sequence voltages.