Determination of Parasitic Capacitance of High-Power Medium Frequency Transformers: Case study of a high voltage DC biased transformer for wind power application

Journal article, 2023

Determination of parasitic capacitances is a necessary step in the design process of transformers to be used in power electronic devices. At high switching frequencies, these capacitances together with inductive elements of the circuit cause unwanted oscillations during fast transients such as sharp fronts of square voltage waves appearing in a dual active bridge converter. In the paper, parasitic capacitances of high-power transformers supposed to operate at frequencies up to several kHz and high DC offset voltages (100 kV and higher) are analyzed. Two methods based on the equivalent circuit approach are suggested. To validate the methods, they are used to determine the parameters of an equivalent circuit of a prototype transformer developed earlier. The capacitances are calculated using the results of 3D FEM simulations performed based on the structure of the transformer as input. Furthermore, another set of data used for validation of the proposed calculation methods was obtained from the conducted direct measurements of frequency dependent impedance of the transformer. The accuracy of the proposed methods is discussed. It is concluded that the methods can be used for quick estimations of the parameters of transformers designed for power electronic applications.