Spread spectrum modulation scheme for two-level inverter using vector quantised space vector-based pulse density modulation

A Space Vector based Pulse Density Modulation scheme for a two level Voltage Source Inverter is proposed in this paper. The proposed scheme employs a first order Sigma Delta Modulator. The principle of Vector Quantization is used for quantizing the reference voltage Space Vector in the Sigma Delta Modulator. For the spatial quantization, the inverter voltage vector space is divided into seven Voronoi regions. In this paper, a method is proposed to code these Voronoi regions using instantaneous reference phase amplitudes without using lookup table. The proposed scheme naturally selects the switching vectors in the over-modulation condition and hence results in a smooth transition from linear to over-modulation region. In Pulse Density Modulation scheme, the switching frequency varies randomly, resulting in the spreading of harmonic spectra. The minimum pulse width that can occur in the proposed scheme is equal to the sampling time period. Therefore, the occurrence of pulse width less than the trun on time of the switching devices is avoided. The proposed scheme is implemented for a 11.5 kVA two level inverter driving 2-HP three phase induction motor. Experimental results of proposed scheme are compared with s Space Vector PWM scheme and Random Space Vector PWM scheme.