Review of multilevel inverter for high-power applications

Inverters are DC-to-AC converters that use semiconductor switching devices to transfer electric energy parameter (i.e. voltage, current and/or frequency) at a desired value. Two-level inverters were introduced first. But the significant increase in the applications that requires DC-AC conversion has sparked the studies in inverter technologies (Balal et al., 2022; Orfanoudakis et al., 2010; Rahman and Saleh, 2011). However, two-level inverter soon became less-efficient for most of applications due to high total harmonic distortion (THD) in the output waveforms, switching losses and its limitations for medium voltage application due to dv/dt stress over semiconductor switches (Najafi and Yatim, 2011). To overcome these issues, higher switching frequency is used with a suitable filter to achieve output sinusoidal waveform. However, this will create a problem of switching losses (Balal et al., 2022). Multilevel inverters (MLIs) are used to archive better performance using variety of techniques to overcome the limitations of two-level inverter. MLIs are upgraded versions of two-level inverters that offer more output levels in current and voltage waveforms while lowering the dv/dt and di/dt ratios. Depending on the supply type, such as current source inverters (CSIs) or voltage source inverters (VSIs), the output waveforms are produced as staircases of current or voltage. To obtain low THD, by increasing the number of voltage levels, a suitable topology is illustrated in Figure 1. Approximately, 28.88% of THD for three-level flying-capacitor MLI would be decreased to 18.56% with the use of five-level MLI of the same topology (Rana et al., 2019). Also, it is shown that the two-level inverter with 1 kHz switching frequency generates 115% voltage THD, whereas MLIs develop only 22% and 32% voltage THD (Nordvall, 2011). With this significant difference between the two-level and multilevel techniques, MLI has been opted as the preferable one for many applications like renewable energy (solar/wind power inverters) up to megawatt (MW) power levels and motor-drive applications (Abd Halim et al., 2016). The advantages of MLI are listed as:

1. Superior efficiency due to lower switching frequencies.

2. MLI enhances power quality and dynamic stability for the connected utilities.

3. Lower switching stress over the semiconductors.

4. Because to its modular and straightforward design, they can be built up to virtually infinite levels.

5. MLI can be considered as an ideal interface between the utilities and renewable energy sources (Balal et al., 2022; Najafi and Yatim, 2011; Abd Halim et al., 2016).

A comparison between the conventional two-level inverter and MLI is presented in Table 1 (Suresh, 2016).

Table 2 illustrates the resulted THD for conventional two-level and MLIs (Nordvall, 2011).

This paper aims to present an overview of different MLI schemes to identify the pros and cons of each topology. To identify the efficacy of the MLI, a model for MLI with Induction Motor drive (IM) in MATLAB Simulink is developed. At different speed and torque variations, the voltage and current THD responses are obtained and analyzed to demonstrate the inverter performance at different loads.

There are two types of MLIs based on the number of voltage sources utilized to feed the MLIs as follows (Wu and Narimani, 2017a; Akagi, 2019):

1. Single DC source.

2. Multiple DC source.

A single voltage source is divided into a number of capacitors to acquire different voltage levels, whereas the second type requires different sources of voltage, whether from multiple batteries or through separate convertor rectifiers linked to renewable energy sources or many others. The classification of MLI is presented in Figure 1.

The MLI topology that uses multiple voltage supplies is cascaded H-bridge (Saleh and Rahman, 2011). This topology uses multiple units of identical H-bridge power cells connected in a series chain to produce high ac voltages. Each power cell is connected to separate and equal DC voltage supply. Advantages can be obtained from the modular structure contributes to low production costs. Also, redundant switching patterns can be achieved, which provides flexibility in switching design. Moreover, this topology can be extended to infinite number of levels to meet the required standard for the output waveform. However, one of the defining shortcomings of this technology is the significant need for separate voltage sources to power the cells (Wu and Narimani, 2017a; Shehu et al., 2016). A five-level CHB inverter is shown in Figure 2 with its switching pattern in Table 3. It consists of two H-bridge cells per phase.

As per above map, single DC supply MLIs is divided as follows.

Single voltage source is utilized in neutral point clamped MLIs and is split into several capacitors linked in series according to the appropriate number of voltage levels. We shall divide the source into $x−1$ capacitors that are linked to the source in series to get $x$ levels. Figure 3 presents three-level NPC-MLI where the neutral point between the two feeding capacitors (i.e. point z) can be accessed through two clamping diodes. It is most suitable for high-power MV drives with advantages such as the reduced dv/dt and THD for the resultant waveform and, also, the applied voltage is equal on all switching devices in the commutation mode, whereas the drawbacks consider additional clamping diodes for high order of levels, asymmetrical power losses in the switching devices and the necessity of control loop for neutral point voltage deviation (Rana et al., 2019; Nordvall, 2011; Abd Halim et al., 2016; Wu, 2006; Young et al., 2012). Switching pattern is illustrated in Table 4.

This kind of frequency inverter was developed to address one of the drawbacks of the NPC, namely the uneven distribution of losses in the switches, which results in an insufficient distribution of heat inside them. Switches 5 and 6 in Figure 4, for example, control and balance the loads on the switches. However, the entire converter system becomes more expensive and complicated when active clamping switches are used. Table 5 indicates the switching pattern for the three voltage levels.

Bidirectional switches are added between the inverter output terminals and the dc bus neutral point to create the NPP inverter topology, which is evolved from the two-level inverter. As a result, the dv/dt and THD of the three-level voltage waveforms produced by this inverter are minimized. But there are certain restrictions with the NPP inverter where each switch position on the inverter requires a series connection of two or more switches depending on the potential rating. This questions the reliability and complexity of the system. Figure 5 shows three-level NPP inverter, and Table 6 indicates its switching states (Orfanoudakis et al., 2010; Wu and Narimani, 2017b; Guennegues et al., 2009).

Figure 6 displays a 5L-FLC arrangement. This inverter is a two-level inverter with cascaded dc capacitors. Each inverter leg contains three flying capacitors with voltage ratings of $3E$⁠, $2E$ and $E$ (⁠$E=Vd/4$⁠). Inverter legs have four switch pairs. Four gate signals are needed for an inverter leg's eight switches. The main features considered are the modular structure of the switches as well as reduced THD and dv/dt. However, this topology requires numerous dc capacitors equipped with precharging circuits and, also, inverter operating conditions affect flying capacitor voltages. To minimize dc voltage deviation concerns, flying capacitor voltages must be precisely regulated, increasing control system complexity. Because of the aforesaid disadvantages, flying-capacitor inverter's practical usage in MV drives is restricted. However, this inverter topology may be utilized to derive sophisticated converter topologies. Table 7 explains the switching pattern for 5L-FLC inverter (Rana et al., 2019; Nordvall, 2011; Akagi, 2019; Shehu et al., 2016; Wu and Narimani, 2017b; El-Hosainy et al., 2017).

Table 8 presents the comparison of consolidated features of the different MLI topologies discussed in this paper (Suresh, 2016; Hoon et al., 2017; Lai and Peng, 1996; Meynard and Foch, 1992; Rodrí et al., 2007; Debnath et al., 2022; Dyanamina and Kumar Kakodia, 2021).

A model system integrated using detailed circuit simulations in MATLAB®–Simulink program. In this system, a constant voltage source connected to MLI was used to feed Asynchronous motor with squirrel cage rotor type as shown in Figure 7.

Speed reference was applied on the motor with step changes. Also, the values of the reference torque were stepped up from no load to the maximum torque, and this allows to examine the MLI under various conditions and to verify the response of the motor currents. Figure 8, illustrates the speed and torque reference values for this model.

Figure 9 shows the source voltage response along with the motor three phase currents, speed and torque. It is observed that when the speed is increased, the motor draws larger instantaneous currents, which is reflected in the MLI.

With the innovation and growth of industrial sectors on a global and local scale, the need for high-energy converters continues to rise. Increase in significance and demand of MLIs for both low- and high-power applications are observed. This article describes and presents five types of MLI topologies (Rodrí et al., 2007). It is suggested that the MLIs like cascade-five-level and NPC-five-level have shown low current harmonics of around 0.43% and 1.87%, respectively, than two-level inverter showing 5.82%. In order to reduce the number of power switches in multilayer inverters, it is possible to reduce or rearrange the DC input voltages, according to the assessment. Other than that numerous academicians have presented specific topological explanations and resolutions in consideration of the desired use.