INTRODUCTION
Multilevel inverter synthesizes a desired output voltage from different levels of dc voltages as inputs. As the number of levels increases the synthesized output waveform approaches the sinusoidal wave with the reduced harmonic distortion (Corzine et al., 2004). Also as steps added to the waveform increases, harmonic distortion of the output decreases gives 0 as the level increases. It plays a vital role in high power application and considered to be an active research area because of its well known advantages of low output Total Harmonic Distortion (THD), increase in the amplitude of the output voltage, low voltage stress of devices and low system electromagnetic interference (Rodriguez et al., 2002; Franquelo et al., 2008; Chiasson et al., 2003; Dixon and Moran, 2006).
The 3 basic multilevel inverter topologies available in the literature are: Diode clamped topology (neutralpointclamped), flying capacitor topology and cascaded topology (Du et al., 2006). The main drawback of diode clamped multilevel inverter topology is the requirement of excessive clamping diodes and restriction in high power range of operation. The main disadvantage of flying capacitor multilevel inverter topology is the requirement of large number of capacitors per phase and control is required to maintain the capacitors voltage balance (Adam et al., 2008; BusquetsMonge et al., 2008; McGrath and Holmes, 2002). Among the 3 the most commonly used topology is the cascaded multilevel inverter topology. Its main features are simple layout, modular structure and the use of minimum number of components. This topology also reduces the problem of unbalance capacitor voltage (Ebrahimi et al., 2010; Babaei, 2010; Khomfoi and Tolbert, 2007; Zhao et al., 2010).
In this study, the comparison shows the superiority of the modified 7level inverter.
CASCADED MULTILEVEL INVERTER
A single phase cascaded 7level inverter is shown in Fig. 1. It has 3 separate DC sources and each supply is connected to a full bridge or a Hbridge inverter. Each level can generate the output voltages of +V_{dc}, 0 and V_{dc}. By triggering the switches S_{n1} and S_{n4}, +V_{dc} and V_{dc} can be obtained by turning on S_{n3} and S_{n4}. If S_{n1} and S_{n2} or S_{n3} and S_{n4} are ON, the output voltage is 0. The AC outputs of each level are connected in series such that the overall output is the sum of the inverter outputs (Corzine et al., 2004).
The number of output voltage levels of multi bridge inverter N is defined by N = 2S+1, where S is the number of DC sources required and the number of switches required is defined by 4N. The main drawback of this topology is the requirement of separate DC source for each Hbridge. The number of semiconductor switches used in the topology is more compared to modified cascaded multilevel inverter.
MODIFIED CASCADED MULTILEVEL INVERTER
The main advantage of modified cascaded multilevel inverter uses less number of switches compared to the conventional cascaded multilevel inverters. Modified cascaded 7level inverter is shown in Fig. 2.

Fig. 1: 
Cascaded 7level inverter 

Fig. 2:  Modified cascaded 7level inverter 
This type of inverter consists of a conversion unit with a switching device connected in series with a voltage source and a diode. The number of conversion units is equal to the number of output levels. By triggering S_{11}, V_{DC1} is obtained at output which is equal to V_{dc}.

Fig. 3:  Output voltage waveform of modified cascaded 7level inverter 
Similarly, by triggering S_{11} and S_{12}, we can obtain the output voltage as the addition of V_{DC1} and V_{DC2} equal to +2V_{dc}. If all the switches (S_{11}, S_{12} and S_{13}) are ON, the +3Vdc level is obtained.
The output levels are unidirectional. By using Hbridge, the output is converted into bidirectional. At positive half cycle, switches S_{1} and S_{4} are ON and during negative half cycle S_{2} and S_{3} are ON in the Hbridge inverter. The switching table for modified 7level inverter topology is shown in Table 1. Figure 3 shows the output voltage waveform of modified 7level inverter (Murugesan et al., 2011; Corzine et al., 2003):
No. of levels (N) = (2×No. of DC sources)+1
HARMONIC REDUCION
To reduce the harmonics in the multilevel inverter, the firing angles of various stages can be selected accordingly. For a 7level inverter, 3 firing angles α_{1}, α_{2} and α_{3} can be selected at different values in order to reduce the lower order harmonics (Rashid, 2004). The conditions to select the firing angles are given by the following:
where, M is modulation index and (m1)/2 is No. of separate DC sources.
The instantaneous phase voltage is:
And the modulation index M is:
where, V_{cr }is Peak Carrier Voltage, m is No. of output levels.
For the given 7level inverter, we have chosen the M value as 0.8 for effective harmonic elimination. Hence, Eq. 2 can be rewritten as:
The value of α_{1}, α_{2} and α_{3} was selected to satisfy the above condition.
SIMULATION RESULTS
The performance of cascaded 7level inverter and modified 7level inverter is analysed by creating a model of the above in MATLAB/Simulink platform. Figure 4 and 5 shows the MATLAB schematic of cascaded 7level and modified cascaded 7level inverter.

Fig. 4:  MATLAB/Simulink model of cascaded 7level inverter 

Fig. 5:  MATLAB/Simulink model of modified cascaded 7level inverter 
Table 1:  Switching table of modified 7level inverter 

Figure 6 and 8 shows the output voltage waveforms of cascaded 7level and modified cascaded 7level inverter and the line voltage is found to be 150 V.

Fig. 6:  Output voltage of cascaded 7level inverter 

Fig. 7:  FFT analysis of cascaded 7level inverter 

Fig. 8:  Output voltage of modified cascaded 7level inverter 

Fig. 9:  FFT analysis of modified cascaded 7level inverter 

Fig. 10(ab): 
(a) Output voltage and (b) FFT analysis of modified cascaded 9level inverter 
Figure 7 and 9 shows the FFT analysis of cascaded 7level and modified cascaded 7level inverter. Figure 10 shows the output voltage waveform and FFT analysis of modified cascaded 9level inverter.
From the FFT analysis, it is observed that THD value of modified cascaded 7 and 9level inverter is found to be less compared to the conventional cascaded 7level inverter and the comparison chart is shown in Table 2.
EXPERIMENTAL RESULTS
Modified cascaded 7level inverter is fabricated as a prototype model. It consists of 7 switches. Multi conversion unit consists of 3 power stages. One MOSFET and 1 diode is used as a main switch in each power stage. Each power stage is supplied by a DC source.

Fig. 11:  Hardware output voltage waveform of modified cascaded 7level inverter 
Gating signals are generated by PIC 16F877 microcontroller. Based on the signals coming from, the microcontroller the switches are turned ON or OFF. The hardware result of the modified cascaded 7level inverter is shown in Fig. 11.
CONCLUSION
The performance of cascaded 7level and modified cascaded 7level inverter has been evaluated elaborately in MATLAB/Simulink platform. From the simulation and experimental results, modified cascaded multilevel inverter topology is found to be better compared to cascaded multilevel inverter topology in the aspects of Total Harmonic Distortion and the requirement of power semiconductor switches. As steps added to the waveform increases, harmonic distortion of the output decreases as the level increases.