Abstract
Transformers can be treated as a time-invariant system which can be characterized by its response to the Dirac's delta function. This impulse response is completely unique and should remain so over the system life. Therefore, it can be used as a kind of indicator to check if the internal compartments, elements or parameters have physically changed during service life. The system response is the convolution of the system impulse response with the excitation signal. This time-domain response has its equivalent response in the frequency domain. Thus, the frequency response of a time-invariant system should also remain unchanged if system parameters are unchanged. This concept can be utilized to evaluate the mechanical structure integrity as well as diagnosis of transformers and rotating machines. It is called frequency response analysis. This chapter is specifically focused on frequency response measurement and analysis of transformers.