Time-Frequency Analysis of Excitation Signals Used to Determine the Transfer Function of the Power Transformers Windings

In the diagnostic tests of power transformers windings condition, the method of the transfer function (TF) analysis has been used for several decades. Its gradual development included introducing the various types of excitation, pulse-type and sinusoidal signals into the measurement practice, as well as various digital signal processing (DSP) methods to improve the signal-to-noise ratio (SNR) and reduce the measurement uncertainty. In each case, used excitation signals must enable estimation of the transfer function over a wide frequency range, up to not less than 2 MHz. The paper presents the measurements results and analysis of time-frequency properties of selected excitation signals that are or can be used to determine the winding transfer function in off-line or on-line diagnostic tests. The discussion of results and conclusions based on analyzed signal properties are presented.