Transient current distribution among elements in metallized film capacitor for converter valves

The metalized film capacitors (MFC) in converter valves are subjected to high-frequency currents under the action of power electronic switching, which will cause large differences in currents among capacitor elements inside MFC and seriously affect the stability of MFC. In this paper, experiments were conducted to measure the current waveforms among 90 elements under impulse, short-circuit discharge and sine waves at different frequencies. The equivalent circuit topology was simplified by using decoupling and controlled source equivalence, and parameters were extracted by electromagnetic calculations combined with experiments. Finally, an equivalent circuit model was established and the current distribution among elements was calculated. Influenced by the reactance of the parasitic inductor and the capacitor element, the frequency range can be divided into 3 regions based on the current distribution characteristics: in the high frequency range (>100 kHz), the current is mainly concentrated on several capacitor elements near the connection terminals. In the middle frequency range (about 10 kHz-100 kHz), the current oscillates between elements and the inductor, resulting in a wavy distribution of current amplitude, strongly influenced by frequency. In the low frequency range (<10 kHz), the current distribution varies very little among 90 elements. These results can support the study of performance evolution, fault analysis and design optimization in MFC.