A Review of Multilevel Inverter Topology and Control Techniques <s> III. MULTILEVEL INVERTER MODULATION TECHIQUES <s> Cascaded multilevel inverters synthesize a medium-voltage output based on a series connection of power cells which use standard low-voltage component configurations. This characteristic allows one to achieve high-quality output voltages and input currents and also outstanding availability due to their intrinsic component redundancy. Due to these features, the cascaded multilevel inverter has been recognized as an important alternative in the medium-voltage inverter market. This paper presents a survey of different topologies, control strategies and modulation techniques used by these inverters. Regenerative and advanced topologies are also discussed. Applications where the mentioned features play a key role are shown. Finally, future developments are addressed. <s> BIB001 </s> A Review of Multilevel Inverter Topology and Control Techniques <s> III. MULTILEVEL INVERTER MODULATION TECHIQUES <s> In this study, the most common multilevel inverter topologies and control schemes have been reviewed. Multilevel inverter topologies (MLIs) are increasingly being used in medium and high power applications due to their many advantages such as low power dissipation on power switches, low harmonic contents and low electromagnetic interference (EMI) outputs. The selected switching technique to control the inverter will also have an effective role on harmonic elimination while generating the ideal output voltage. Intensive studies have been performed on carrier-based, sinusoidal, space vector and sigma delta PWM methods in open loop control of inverters. The selection of topology and control techniques may vary according to power demands of inverter. This paper and review results constitute a useful basis for matching of inverter topology and the best control scheme according to various application areas. <s> BIB002 </s> A Review of Multilevel Inverter Topology and Control Techniques <s> III. MULTILEVEL INVERTER MODULATION TECHIQUES <s> This paper deals with the voltage unbalance problem in multilevel inverters. Multilevel voltage source converters are the new trend in the power converter option for high-power applications. One of the major limitations of the multilevel converters is the voltage unbalance between different levels. The elimination of voltage unbalance in a single-phase five-level inverter which uses coupled inductors and feedback control, which can produce a five level output voltage with only one dc source. With this topology, splitting of the dc voltage is avoided, which eliminates the problem of voltage balancing in the circuit which was a prominent problem in the conventional topologies. The feedback control of the inverter is implemented using a PI controller. The six power switches used here have the same voltage stress and only one set of coupled inductors are adopted. <s> BIB003
Sequel to the increased number of level, higher level of complexity is experienced while controlling MLI. However, this complexity could be used to add additional capabilities to the modulation technique, namely; reducing the switching frequency, minimizing the common-mode voltage, or reducing the DC link voltage imbalance BIB003 , BIB001 . Several modulation techniques have been proposed for cascaded multilevel inverters which are usually an extension of the two-level modulations. These techniques can be classified based on switching frequencies, as shown in Fig. 8 . Figure 8 . Classification of multilevel inverter modulation  A. Sinusoidal Pulse Width Modualtion. SPWM involves the superimposing a sinusoidal modulating signal (reference) on to a high frequency triangular carrier signal (the most common and easiest technique) per phase. (k-1) carrier signals are required klevel inverter. The instantaneous intersections between these two signals switch the switching device 'ON' if the modulating signal is greater than carrier signal assigned to that switch. The ratio of these signals' amplitude and frequency called amplitude modulation index, m a and frequency modulation ratio, m f defined in (1) and BIB002 . where A sin , A tri , f sin and f tri are amplitudes and frequencies of the modulating signal and carrier signal respectively. Table I summarizes the implication of this modulation indices to the output of the inverter. Other interesting carrier-based multilevel PWM are sub harmonic PWM (SHPWM) and switching frequency optimal PWM (SFO-PWM)  . By super-imposing the carrier signal on one another as shown in Fig. 10 , level shift PWM and phase shift PMW are achieved. This method is further classified into three as shown in Fig. 3 based on the carrier signal orientation. 