Advantages And Disadvantages Of Multilevel Inverters

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3.1 CLASSIFICATION OF MODULATION STRATEGIES The modulation methods used in multilevel inverters can be classified according to the switching frequency was discussed in Rodriguez et al (2002), Celanovic and Boroyevic (2001) and Rodriguez et al (2001). Modulation techniques that work with high switching frequencies have many commutations for the power semiconductors in a cycle of the fundamental output voltage. Multilevel inverters generate sinusoidal voltages from discrete voltage levels, and Pulse Width-Modulation (PWM) strategies accomplish this task of generating sinusoids of variable voltages and frequencies. Several techniques for the implementation of PWM for multilevel inverters have been developed. The well-known high switching…show more content…
3.2.1 Fig. 3.2.2
Fig 3.2 Multi-Carrier Control,
Fig. 3.2.1 Control Signal and Carrier signals Fig.3.2.2 Output Voltage.
This result has been obtained for the multi-cell inverter in a seven-level configuration, which uses three series-connected cells in each phase. The smallest distortion is obtained when the carriers are shifted by an angle of 120°.
A very common practice in industrial applications for the multilevel inverter is the injection of a third harmonic in each cell to increase the output voltage was discussed in Hammond (1997) and Hill and Harbourt (1999). Another advantageous feature of multilevel SPWM is that the effective switching frequency of the load voltage is much higher than the switching frequency of each cell, as determined by its carrier signal. This property allows a reduction in the switching frequency of each cell, thus reducing the switching losses.
The advantage of the method is that it is very simple. But it has two disadvantages. The first method cannot completely eliminate the low order harmonics. Therefore the low order harmonics cause loss and high filter requirements. The second method is the high switching frequency which causes high switching loss and low efficiency

Fig.3.3.1 Fig.
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