Capacitor Voltage Switching Ripple in Three-Phase Three-Level Neutral Point Clamped Inverters with Sinusoidal Carrier-Based PWM

This paper provides an extensive theoretical analysis of the capacitors voltage ripple for three-phase three-level neutral point clamped inverter topologies. The capacitor voltage ripple amplitudes are theoretically estimated as a function of both amplitude and phase angle of output current, and the modulation index. Each capacitor voltage basically consists of switching frequency and the third harmonic components, thereby both components are considered in the analysis. In particular, peak-to-peak distribution and maximum amplitudes of the capacitor voltage switching ripple over the fundamental period are obtained. As a first simplified approach, reference is made to sinusoidal carrier-based PWM with sinusoidal load currents, representing either grid connection or passive load with a negligible current ripple. Based on the ripple requirements of capacitors voltage, a simple and effective original equation for designing capacitors has been proposed. Numerical simulations have been carried out by Matlab/Simulink in order to verify the analytical developments.