1
Introduction
A Multilevel Inverter (MLI) is a specialized field of power electronics field concerned with the conversion of multiple DC sources into a staircase-type AC waveform. There are several MLI configurations available, each employing a different number of semiconductor devices for achieving various MLI levels. Even though there are conventional two-level inverters to convert DC to AC power, MLIs are employed for their ability to generate AC output with less harmonic distortion. The MLIs are also vested with the ability to draw low distortion input current from the input DC sources [1].
An MLI modulation scheme refers to the change in the state of the power semiconductor switches from one operating mode to another operating mode. The modulation scheme can be a High-Switching Frequency (HSF) or a Low-Switching Frequency (LSF) modulation scheme [2]. An HSF modulation scheme refers to the operation of semiconductor switches above 1 kHz while an LSF modulation scheme refers to an operation of Insulated-Gate Bipolar Transistor (IGBT) switches below 1 kHz. The duty cycle of switching pulses to semiconductor switches play a crucial role. There are variations in the performance of the modulation schemes depending on it.
There are various kinds of modulation schemes with their employment depending upon the applications. Enormous research work has been carried out on HSF modulation schemes. Some of the HSF modulation schemes are Sine Pulse Width Modulation (SPWM), Selective Harmonics Elimination (SHE), Space Vector Control (SVC) etc. There are also LSF counterparts for the same modulation schemes [3].
In an HSF modulation scheme, a reference signal and a high-frequency carrier signal are compared. This leads to the generation of switching pulses. The same applies to an LSF modulation scheme with variation in the frequency of the carrier signal.
The rise of multilevel conversion is seen as a welcome revolution in the era of industrialization. The industrial era runs on high and medium voltage but high power applications. Hence, it is necessary to employ multiple semiconductor switches rather than a single semiconductor switch. The drawbacks of conventional two-level inverters are HSF modulation scheme, poor output voltage waveform, higher dv/dt stress, higher electromagnetic interference, need of LC filter etc., [4].
Employing an increased number of semiconductor switches is well suitable for high power conversion. The semiconductor switches are thus allowed to share the voltage and current through the converter. This helps in increasing the life of the converter. The multilevel power conversion has found itself demanding with the advent of renewable energy sources such as solar energy, wind energy [5–7]. MLIs with the convenience of adding isolated and individual sources is well suitable for integrating different types of renewable energy sources.
The MLIs have found widespread applications. Some of the important applications are:
Recently MLIs have found use in the control of variable speed drives and railway transportation electrification. The performance of induction motor drive is enhanced using intelligent rotor resistance estimator [8–10].
The research carried out in [11, 12] have demonstrated that MLIs are employed in renewable energy systems, especially with PV systems for the conversion of the DC input from solar panel to AC output.
In [13, 14], MLIs were applied in the field of power systems and power electronics in STATCOM and induction heater system.
1.1 Classical Multilevel Inverter (C-MLI) Topologies
This section presents the three conventional MLI topologies along with a brief explanation of each topology.
1.1.1 Diode-Clamped MLI
The first MLI topology was invented by Nabae et al. in the year 1981, as shown in Figure 1.1. It was named as a neutral-point-clamped inverter and later also familiarized itself as Diode-Clamped MLI (DC-MLI). The DC-MLI is a combination of two DC sources, two diodes and four switches for a three-level AC output. The modes of operation are shown in Table 1.1. The levels can be extended further with an increase in the number of sources, diodes and switches. The d...
