Quench processes in a 72 kVA superconducting four-winding power transformer were studied experimentally by sudden short-circuiting of the secondary load. An instantaneous decrease in the transport current was observed in both of the primary and secondary main windings during the first half cycle immediately after the short circuit. The equivalent propagation velocity of normal regions along the wire during the period of instantaneous decrease in current was about two orders of magnitude larger than existing theoretical predictions. Two-dimensional propagation of the normal region was also observed after the very fast propagation. In the primary main winding, the equivalent velocity of the very fast propagation was linearly increased with the voltage applied to the normal region of the winding. This type of very fast progagation was explained by a model in which a broad part of the winding is changed into the normal state via a flux-flow state with very low resistivity, due to the excess current. In the secondary main winding, similar fast propagation was also generated by local heating of the winding above a threshold level in the region of the transport current. The experimental results show that the instantaneous transition to the normal state is attributed to a quench process peculiar to superconducting cables, due to the redistribution of the transport current among strands. It is proposed that the abnormal transitions to the normal state can be useful for self-protection of a.c. superconducting windings from burning.