A Highly Efficient Cross-Connected H-Bridge-Style Multilevel Inverter with Lower Power Components ^†

Abstract

Compared to the classical inverters, the multilevel inverter finds remarkable advantages that can be suitably implemented in green energy power generation. Here, an asymmetric multilevel inverter with fewer components is proposed for renewable energy applications. The proposed inverter is a cross between two H-bridge-style devices. To maximize the output voltage, three different algorithms to fix the amplitude of the DC sources are proposed, and the best among them is chosen for implementation. The recommended inverter can generate 19 levels of output voltage using three DC sources with reduced power components. The nearest-level modulation is used as the control course for the inverter. Here, MATLAB software is used to simulate the proposed inverter, and the performance of the inverter is observed. The proposed inverter is constructed in real time, and the performance of the inverter is studied by testing with fixed and variable reactive loads. A comparative study is made between the simulation model and real-time work results interms of efficiency and harmonics in the load waveforms.

1. Introduction

An electrical inverter transforms DC power into AC power from sources like batteries, fuel cells, or solar cells. Current source inverters (CSI) and voltage source inverters (VSI) are two different types of inverters. A multilevel inverter is a device that uses power electronic devices as switches and produces higher voltage levels compared to traditional inverters, depending on the voltage sources used. These multilevel inverters are used in renewable energy systems with high efficiency, electric vehicles, industrial motor drives to improve the motor’s performance, power factor corrections to reduce reactive power consumption, etc. These inverters have high voltage stress, which causes reliability and durability issues, complex control algorithms, harmonic distortions at low switching frequencies, and a high challenge in balancing capacitor voltage. There are different types of MLI, such as diode clamped MLI, Flying Capacitor MLI, Cascaded MLI, Diode-Clamped MLI, and Capacitor-Clamped MLI. These conventional MLIs use capacitors, diodes, and voltage sources, providing a simple topology. A multilevel inverter is proposed on the basis of a series connection [1,2]. The invented MLI gives output voltages of 11 levels, 15 levels, and 19 levels. These inverters are coupled with renewable energy systems as well as fuel cell applications [3]. The designed MLI produces an output voltage of 17 levels with two asymmetrical DC sources. The module had a simple inherent charging for capacitors without any additional circuit. The negative voltage capability is involved [4]. The proposed D-type MLI produces output voltage at 7 levels. The Nearest Level Modulation (NLM) technique is used to control the inverter. The input voltage can be boosted by synthesizing capacitor voltage [5]. Integrating the solar PV system with the inverter is the main goal of the power quality improvement, and the performance of an asymmetric multilevel inverter with different PWM approaches for harmonic suppression is also examined [6]. Three induction motors are driven by a cascaded multilevel inverter using a hybrid control approach. Lower-voltage DC sources are used to improve the voltage rating and solar photovoltaic system power quality. Switching Frequency Optimum (SFO) and Nearest Level Control (NLC) control approaches are used [7]. The control strategy of the neutral point clamped inverter is limited since only one basic vector may be selected as the optimal output during a control period. An extended control set that accelerates the search for the appropriate vector and enhances control performance. The proposed inverter generates three output voltage levels [8]. This proposed topology collects maximum power from the sources and transmits it to the grid in pure AC form with minimal switching and power loss. In this proposed solution, only two switches are switched on for a single state, greatly reducing switching losses and thus increasing efficiency [9]. The proposed inverter produces 19 levels of output voltage without the H-bridge circuit, which is used for power quality improvement in the grid [10]. The proposed inverter produces 7 levels of output voltage in the symmetric case and 15 levels of output voltage in the asymmetric case with 9 switches; however, the total harmonic distortion is high, so the efficiency is reduced [11]. Considering the above papers, a multilevel inverter is proposed that produces 19 levels of output voltage with 3 DC sources and 12 switches. The inverter is simulated using the MATLAB software and tested for real-time applications. This paper comprises Section 2, which discusses the proposed inverter structure; Section 3 presents the comparative analysis of various trends in multilevel inverters; Section 4 consists of the simulation finding; and Section 5 is the conclusion.

2. Proposed 19-Level Inverter

The proposed asymmetric multilevel inverter structure is shown in Figure 1. The inverter consists of 3asymmetric sources and 12 power electronic switches. The proposed inverter produces 19 levels of output voltage, and the inverter is controlled usingt he Nearest Level Modulation (NLM) technique. With the unequal DC magnitude, i.e., V1 = V_DC,V2 = 3V_DC, and V3 = 5V_DC, the configured inverter produces 9 +ve voltage levels, 9 −ve voltage levels, and zero voltage levels. The switching combinations of the proposed inverter are shown in Figure 2, Figure 3, and the switching pattern is provided in

Table 1. Switching pattern of proposed MLI.

VDC	S1	S2	S3	S4	S5	S6	S7	S8	S9	S10	S11	S12
0 V	0	0	1	1	0	0	1	1	1	1	0	0
1 V	1	0	0	1	0	0	1	1	1	1	0	0
9 V	1	0	0	1	1	1	0	0	1	1	0	0
0 V	0	0	1	1	0	0	1	1	1	1	0	0
−1 V	1	0	0	1	0	0	1	1	0	0	1	1
−9 V	1	0	0	1	1	1	0	0	0	0	1	1

.

3. Comparative Analysis

In this section, several kinds of multilevel inverters are compared on the basis of output voltage produced, the number of switches used, voltage sources used, IGBT devices used, total standing voltage (TSV), and the THD of various configurations, as given in

Table 2. Comparison between various configured MLIs.

Ref No.	NL	NSW	NDS	XTHD	N(D+C)
3	11	8	5	4.56	2D
5	17	13	2	3.12	18D + 2C
6	11	12	5	3.35	12D
7	7	9	1	12.17	2C
11	13	11	1	4.9	3D + 3C
Proposed	19	12	3	4.36	-

. The MLI circuit proposed in [3,6] produces eleven levels of output voltage with different switch counts. In [11], the proposed topologies produce thirteen levels of output voltage, whereas the THD is reduced. For the 17 levels of output voltage, 13 switches and 10 switches are used in [5]. The configured inverter produces a total harmonic distortion of 12.17% with seven levels of output voltage [7]. Only one DC voltage source is needed in order to operate at levels 3 and 5 in [7,11].

4. Simulation and Experimentation

The MATLAB/SIMULINK software is used to model the designed multilevel inverter. The asymmetrical DC voltage magnitudes are V1 = 20 VDC, V2 = 60 VDC, and V3 = 100 VDC. The required voltage is attained with the operation of selected switches. The load parameters are R = 90 Ω and L = 30 mH. The nineteen-level inverter is controlled using the Nearest Level Modulation (NLM) technique, which reduces THD. The nearest edge control is selected at the nearest voltage level and fed to logic gates to produce the appropriate output, which is fed to IGBT switches. The load waveforms and harmonic presences are depicted in Figure 4, Figure 5 and Figure 6.

The prototype of the proposed inverter is shown in Figure 7. The inverter receives pulses from the edge control mechanism. The program is flashed to the controller using the FPGA controller. The IGBT devices are used as switches. The four step-down transformers are used to step down the grid voltage and are fed to the driver circuits. The driver circuits are used to drive the 12 IGBT switches. The prototype is validated with variable and fixed resistive impedance loads and parameters, which are provided in

Table 3. Performance parameters—MLI.

Load	Vo	Io	Po	%eff	THD
R	180	2	360	95.16	4.30
RL	180	2	360	94.34	4.26

, and the load waveform is shown in Figure 8.

5. Conclusions

This paper presents the asymmetric multilevel inverter with a reduced number of DC sources and switch counts. The proposed inverter produces 19 levels of output voltage and is controlled using the Nearest Level Modulation technique. The inverter voltage sources are set as V1 = V_DC, V2 = 3V_DC, and V3 = 5 V_DC. The inverter is simulated using MATLAB software that produces a lower THD = 4.26% which is minimum compared to standard IEEE formats. This is also tested in real time for fixed R and RL loads. Because of the variable DC supply, the configured inverter is more suitable for renewable energy systems.