Evaluation of the low-frequency neutral-point voltage oscillations in the three-level inverter

The nearest vectors to the reference vector are commonly used in space-vector modulation strategies. The main advantages of these modulation strategies are the low switching frequencies of the devices, the good output voltage spectra and the low EMI. However, when these techniques are applied to the three-level neutral-point-clamped inverter, low-frequency oscillations appear in the neutral-point voltage for some operating conditions. As a result, the value of the DC-link capacitors must be increased in order to attenuate such oscillations. In this paper, these amplitudes are quantified for two modulation strategies that use nearest vectors to the reference vector. Owing to the nondimensional variables used in the analysis, the information provided will help for the calculation of the DC-link capacitors in a given specific application. Simulated and experimental examples are presented.