Analysis of the DC-Link Capacitor Current of Power Cells in Cascaded H-Bridge Inverters for High-Voltage Drives

Cascaded H-bridge (CHB) inverters used in high-voltage variable frequency drives are usually composed of several power cells. Each power cell consists of a three-phase uncontrolled rectifier as its input stage and a single-phase inverter as its output stage. It is difficult to obtain the analytical expression of the dc-link capacitor current because of both the nonlinear characteristics of the uncontrolled rectifier and the low-frequency harmonics caused by the single-phase inverter. On the other hand, exact analysis of the capacitor current is important for dc-link capacitor selection. This paper proposes an analytical method for calculating the capacitor current. Considering both the input current of the inverter and the output current of the rectifier, an approximately linear equivalent circuit model is established. According to the proposed equivalent circuit model, the analytical expressions of the harmonic spectrum and the root mean square value of the capacitor current are derived. The capacitor ripple voltage is further estimated. Applicable conditions of the proposed method are also presented. This method may be useful when designing dc-link capacitors of power cells in CHB inverters. Simulation and experimental results show that the proposed method can achieve good accuracy in the continuous conduction mode.