An on-line method to detect radial mechanical deforma- tions of power transformer winding turns is presented. First-order perturbation theory is applied to a transformer winding surrounded by the transformer tank wall and the iron core. The transformer wind- ing is modeled as thin conducting cylindrical rings (winding segments or turns) situated within a coaxial waveguide, where the outer con- ducting cylinder represents the transformer tank wall while the inner conducting cylinder represents the iron core. Antennas which radi- ate and measure microwave flelds are proposed inside the transformer tank in order to identify and quantify the mechanical deformations of winding turns. The direct propagation problem is solved using con- ventional waveguide theory with mode-matching and cascading tech- niques. An optimization algorithm is then used to solve the inverse problem whereby a good agreement between the reconstructed and true deformations of the winding segments is obtained.