Pseudorandom carrier based subharmonic PWM for cascaded multilevel inverters

Multilevel inverters are replacing conventional two level inverters due to reduced dv/dt stresses, common mode voltages, and lower harmonic distortion. They are finding applications in motor drives, active filters and power conditioning. Cascaded Multilevel inverter is the most used topology among Multilevel Inverters, due to reduced number of components at higher voltage levels. Constant frequency PWM is normally employed for cascaded multilevel inverter. However, to overcome EMI and higher harmonic distortion, a random modulation technique, adopting pseudorandom carrier modulation is employed. This improves the spread spectra effect; where in the signal generated with a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with wider bandwidth. The pseudorandom carrier involves random selection of two triangular carriers of same frequency but opposite in phase depending on the output states of pseudorandom generator. Pseudorandom carrier modulation is employed on various kinds of sub harmonic PWM techniques. Total Harmonic Distortion (THD) and Harmonic Spread Factor (HSF) are considered for evaluating the performance of these methods. HSF was found to be 5% less in pseudorandom carrier scheme compared to constant frequency scheme. Current THD was found to be less than 4% in all schemes.