Power transformer resonance-measurements and prediction

The authors describe and summarize the results of impedance measurements of 12 converter transformers and 12 nonconverter transformers in the 50 Hz to 1 MHz frequency range. Impedance characteristics associated with various parameters are discussed. Studies of transformer winding first resonant frequencies and prediction equations are presented. Many conditions are represented: single and three-phase transformers, different core and winding designs, high voltage and low voltage windings, and various terminations. From measured driving point impedance, it can be clearly seen that the profile of impedance changes both in terms of magnitude and phase angle as a function of frequency. The first resonant frequency is found at the first zero crossing of the phase angle. The highest value found was 80 kHz