Charge-discharge current distribution among elements in metallized film capacitors used for MMC

At the instant of power electronic switching action, the charging and discharging current distribution on elements inside a metallized film capacitor (MFC) in a converter will affect the stability of the capacitor. In this paper, the current distribution among 90 capacitor elements inside a MFC was calculated using an equivalent circuit. When the charging current rises rapidly from 0 to 1000 A after $1 \mu \mathrm{s}$, the current on each element will oscillate and stabilize to a steady current after hundreds of $\mu \mathrm{s}$. At the moment of stopping charging, the current on elements will be reversed oscillation, and a large discharge current will appear. In a column of elements, peak currents on the two end elements are large, and peak currents on the middle elements are small. The closer the element is to side metal strips, the earlier the peak current appears. The maximum oscillation current can be up to 9.38 times of the steady state charging current. The longer the current rise time, the smaller the peak oscillating current on element 1. When the charging time becomes shorter and the charging stops during the oscillating charging period, the amplitude of oscillating currents on elements becomes larger and the peak discharge current may even exceed the peak charging current.