Introduction

Inverters are very useful for various industrial applications. In the last few years, the voltage-driving method has been adopted. To reduce the semiconductor transient voltage and current rating, a series and parallel connection method is needed. Moreover, the limited standard three-phase converter is also adopted up to the maximum allowable voltage of the load. Also, both the primary and the Pulse width modulation (PWM) switching frequency can be useful. The reduced switching frequency shows the low disappearance and the higher efficiency. In order to synthesize the spectrum signals of the harmonics caused by the capacity, the multi-level inverter has received more attention in recent times. Moreover, a multilevel inverter has a key role in providing improved operating voltage beyond the voltage limits of conventional semiconductors.

The following clarification defines the significance of the multilevel converter. The explanation of a multilevel inverter is: “The multilevel inverter can demonstrate the switching skill very effectively with different voltage and current levels of its input or output nodes”. It is a practical reply to raise the power capacity with a comparably low elements load and decreasing the output harmonics.

The Concept of Multilevel Inverters

Two level inverters are shown in Figure 1. The input voltage of a direct current (DC) is converted to the required alternating current (AC) voltage and frequency [1]. When an inverter operates with Vdc, a two-level inverter can create two different output voltage for a load, Vdc / 2 or (−Vdc) / 2. To generate an AC voltage, both voltages are generally allowable in the PWM. Compared to multilevel inverters, this two-level method creates harmonic distortion, Electromagnetic interference (EMI), and huge dv/dt [2]. The idea of a multilevel inverter is not based on the two-voltage level AC. Alternatively, some voltage levels are connected to get a better waveform and less dv/dt and harmonic distortion. If the voltage level of the inverter is higher than the waveform, it is much better. However, the designs are simple, but increasing voltage levels and more components are needed. The dividing voltage and extending its control scheme become more complex in a three-phase setup [3]. Series-connected multilevel inverters are connected to the capacitor-composing energy tank of the inverter. That provides many nodes in the inverter connecting to the different phases. The term level does not correspond to the voltage level that can be provided by the output inverter. Diodes can provide the path of reactive source from the load to the power supply at the current and voltage of Resistance Inductor load (RL) load having opposite polarity.

Figure 1. Two-level inverter.

A multilevel inverter has positive points compared with the two-level inverter using the PWM method. Also, a multilevel inverter produces very small distortions and dv/dt in the output side. In addition, it produces a common-mode voltage and operates at a lower switching frequency [2] than a two-stage inverter. Besides, there are several features of a multilevel inverter that can be summarized as the following: 1) the multilevel inverter does not produce low distortion output voltage, but it also decreases the dv/dt stresses. Consequently, problems with electromagnetic compatibility are cleared. 2) The multi-level inverter produces a lower voltage in a common mode; as a result, the motor attached load of the multilevel inverter may be decreased. 3) A multilevel inverter draws a small input current distortion. 4) To obtain more output voltage levels using fewer switching devices, many recently structured multilevel inverters have been presented in this publication [4–16].