Comparative Analysis of PWM and Nearest Level Modulation for a Five-Level Hybrid Multilevel Inverter
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Comparative Analysis of PWM and Nearest Level Modulation for a Five-Level Hybrid Multilevel Inverter
Abstract
The growing demand for high-quality power in medium- and high-power applications necessitates the development of efficient multilevel inverter (MLI) topologies and their corresponding control methods. This paper presents a detailed study of a five-level hybrid multilevel inverter topology designed to reduce semiconductor component count. The performance of this inverter is investigated under two distinct modulation strategies: Nearest Level Modulation (NLM), a fundamental frequency switching technique, and Sinusoidal Pulse Width Modulation (SPWM), a high-frequency switching technique. Using a comprehensive model developed in MATLAB/Simulink, this analysis focuses on the impact of varying the modulation index on the inverter's output characteristics. Key results demonstrate that for NLM, the number of output voltage levels changes discretely based on predefined thresholds, whereas for SPWM, the number of levels and the load current magnitude vary more continuously with the modulation index. The principal finding highlights the fundamental trade-off between the low-frequency, stepped-waveform approach of NLM and the high-frequency, harmonically superior control offered by SPWM, providing valuable insights for selecting an appropriate control strategy based on application requirements.
Keywords
Hybrid Multilevel Inverter, Five-Level Inverter, Pulse Width Modulation (PWM), Nearest Level Modulation (NLM), Modulation Index
I. Introduction
Multilevel inverters (MLIs) have become increasingly important in medium- and high-power applications due to their significant advantages over conventional two-level inverters. By synthesizing an output voltage waveform from multiple DC voltage levels, MLIs produce a stepped output that offers superior voltage quality, reduced total harmonic distortion (THD), and lower electromagnetic interference (EMI). These characteristics make them highly suitable for applications such as motor drives, renewable energy integration, and flexible AC transmission systems.
This paper focuses on a specific five-level hybrid inverter topology that strategically reduces the required number of semiconductor devices compared to classical configurations, thereby aiming to lower system cost and complexity. The core of this investigation is a comparative study of two fundamental modulation techniques applied to this topology: Nearest Level Modulation (NLM) and Sinusoidal Pulse Width Modulation (SPWM). NLM operates at the fundamental frequency, generating a stepped waveform by comparing a reference signal to constant DC levels, while SPWM utilizes high-frequency carriers to shape the output voltage envelope.
The primary contribution of this work is the detailed simulation-based analysis and comparison of these two techniques within the MATLAB/Simulink environment. The analysis places particular emphasis on how variations in the modulation index directly influence the number of generated output voltage levels and the magnitude of the resulting load current. This provides a clear understanding of the control dynamics and operational characteristics of the inverter under each modulation scheme.
The paper is organized as follows: Section II details the system configuration and operating principle of the five-level inverter. Section III describes the theoretical foundations of the NLM and SPWM modulation strategies. Section IV outlines the implementation of the simulation model in MATLAB/Simulink. Section V presents and discusses the simulation results, followed by a conclusion and suggestions for future research in Section VI. The architecture of the proposed inverter will now be detailed.
II. System Configuration and Operating Principle
The development of novel MLI topologies is strategically important for advancing power electronic systems. A primary goal in this field is to achieve a higher number of output voltage levels with a reduced count of power electronic switches and associated gate drive circuits. This reduction directly impacts overall system cost, complexity, physical footprint, and efficiency. The topology examined in this study is designed with this objective in mind.
The investigated five-level hybrid inverter is constructed from a single H-bridge inverter combined with a single bidirectional switch. This power stage is fed by two series-connected, independent DC voltage sources, with each source providing a voltage of VDC/2. This configuration allows for the generation of five distinct voltage levels at the output terminals.
Figure 1. Circuit diagram of the proposed five-level hybrid multilevel inverter.
The five output voltage levels (+VDC, +VDC/2, 0, -VDC/2, -VDC) are synthesized by activating specific combinations of the H-bridge switches (S1, S2, S3, S4) and the bidirectional switch (SA/S5). The operational states are detailed in Table 1.
Table 1: Switching States for Generating Five Voltage Levels
Desired Output Voltage | Required Voltage | Active Switches | Operational Description |
+VDC | V1 + V2 | S1, S4 | The full DC link voltage (V1 + V2) is applied across the load with positive polarity via S1 and S4. |
+VDC/2 | V2 | SA (S5), S2 | Only the second voltage source (V2) is connected to the load with positive polarity via SA and S2. |
0 | 0 | S2, S4 | The load terminals are short-circuited through the lower switches of the H-bridge (S2, S4). |
0 | 0 | S1, S3 | The load terminals are short-circuited through the upper switches of the H-bridge (S1, S3). |
-VDC/2 | -V2 | S3, SA (S5) | The second voltage source (V2) is connected to the load with negative polarity via S3 and SA. |
-VDC | -(V1 + V2) | S2, S3 | The full DC link voltage (V1 + V2) is applied across the load with negative polarity via S2 and S3. |
A primary advantage of this topology is its ability to generate five voltage levels using only five active switches (four in the H-bridge and one bidirectional switch). This represents a significant reduction in component count compared to classical topologies like the cascaded H-bridge, which would require more switches and isolated DC sources to achieve the same number of levels. The control strategies required to generate the precise gating signals for these switches will now be examined.
III. Modulation Strategies
The performance of a multilevel inverter is critically dependent on its modulation strategy. The chosen method directly dictates the harmonic spectrum of the output voltage, the switching losses incurred in the semiconductor devices, and the overall complexity of the control system. This study investigates two distinct methods to control the five-level hybrid inverter: the fundamental frequency Nearest Level Modulation and the high-frequency Sinusoidal Pulse Width Modulation.
A. Nearest Level Modulation (NLM)
Nearest Level Modulation is a fundamental frequency switching method characterized by its simplicity and low switching losses. The core principle involves comparing a sinusoidal reference waveform against a set of predefined, constant DC voltage thresholds. The inverter's output voltage is then switched to the DC level that is closest to the instantaneous value of the reference sine wave, producing a staircase waveform.
In the implementation for the five-level inverter, the sinusoidal reference is compared with four distinct threshold levels: +0.75, +0.25, -0.25, and -0.75. As the reference sine wave rises and crosses the +0.25 threshold, the output transitions to the first positive level (+VDC/2). When it subsequently crosses the +0.75 threshold, the output transitions to the second positive level (+VDC). The process is mirrored for the negative half-cycle, resulting in a stepped output that approximates the reference sinusoid.
B. Sinusoidal Pulse Width Modulation (SPWM)
Sinusoidal Pulse Width Modulation is a high-frequency switching technique widely used in power electronics. The principle involves comparing a low-frequency sinusoidal reference (modulating) signal with one or more high-frequency triangular (carrier) signals to generate the gating pulses for the power switches.
For a multilevel inverter, this technique is typically realized using level-shifted or phase-shifted carrier signals to generate a series of high-frequency pulses whose widths are modulated such that their average value over a switching period follows the sinusoidal reference. This creates a switched output waveform with a low-frequency component that closely tracks the desired sine wave, effectively pushing the dominant harmonics to higher frequencies where they can be more easily filtered.
The practical implementation of these modulation strategies within a comprehensive simulation framework will now be presented.
IV. MATLAB/Simulink Implementation
Simulation plays an indispensable role in power electronics research and development. Environments like MATLAB/Simulink provide a powerful platform for validating circuit topologies and control algorithms, allowing for rapid iteration and analysis while reducing the development time, cost, and risks associated with hardware prototyping.
The simulation model of the five-level hybrid inverter was constructed using standard blocks from the Simulink library. The main power circuit consists of two DC voltage sources, the H-bridge built with ideal switches, the bidirectional switch, and a resistive-inductive (RL) load to represent a typical industrial load.
The control system was designed to accommodate both modulation strategies. For NLM, the control logic was implemented using relational operators to continuously compare the reference sine wave against the four fixed threshold levels (+0.75, +0.25, -0.25, -0.75). The output of these comparators was then used to generate the final gate signals. For SPWM, the controller includes a block to generate the sinusoidal reference signal, which is then compared against high-frequency carrier signals to produce the required PWM pulses. A selector switch was implemented in the model to allow for easy switching between the NLM and SPWM control modes for comparative analysis.
The simulation was configured with typical parameters representative of a low-voltage laboratory setup. The key parameters are summarized in Table 2.
Table 2: Key Simulation Parameters
Parameter | Value |
DC Link Voltage (VDC) | 100 V |
Source Voltages (V1, V2) | 50 V each |
Fundamental Frequency | 50 Hz |
SPWM Carrier Frequency | 2 kHz |
Load Type | R-L Load |
Resistance (R) | 10 Ω |
Inductance (L) | 20 mH |
The subsequent section will analyze and discuss the waveforms and performance data obtained from this simulation model under various operating conditions.
V. Results and Discussion
This section presents the simulation results, offering a direct comparison of the inverter's performance when controlled by the NLM and SPWM strategies. The analysis focuses primarily on the output voltage and current waveforms and investigates the system's response to variations in the modulation index (ma), which is a key control parameter for both techniques.
A. Performance under Nearest Level Modulation (NLM)
Under NLM control, the inverter produces a characteristic stepped output voltage waveform, with each step corresponding to one of the five available DC levels. The generation of these levels is directly tied to the amplitude of the reference sinusoid, controlled by the modulation index.
The simulation reveals a discrete relationship between the modulation index and the number of output levels:
• When ma < 0.25, the reference sine wave does not cross the first threshold. As a result, no output voltage is produced.
• When ma > 0.25 (e.g., ma = 0.3), the reference wave crosses the ±0.25 thresholds, resulting in a three-level output waveform (0, +VDC/2, -VDC/2).
• A full five-level output (+VDC, +VDC/2, 0, -VDC/2, -VDC) is achieved only when ma > 0.75, allowing the reference to cross all four voltage thresholds.
This demonstrates the quantized nature of NLM, where output voltage amplitude cannot be controlled continuously but only in discrete steps determined by the fixed threshold placement.
B. Performance under Sinusoidal Pulse Width Modulation (SPWM)
In contrast to NLM, the SPWM technique generates an output voltage composed of high-frequency pulses. The width of these pulses is modulated to create a low-frequency sinusoidal envelope that approximates the reference signal.
Varying the modulation index under SPWM control has a more continuous effect on the output:
• The number of effective voltage levels in the output envelope is dependent on ma. For example, simulation shows that reducing ma from 0.9 to 0.5 causes the number of discernible voltage levels to decrease from five to three.
• The magnitude of the load current is directly proportional to the modulation index. As ma increases from 0.2 to 0.9, the amplitude of the sinusoidal output current increases accordingly. This linear relationship between modulation index and output voltage (and thus current) is a hallmark of SPWM, offering fine-grained control over power delivery, a significant advantage over NLM's stepped control.
Figure 5. Simulated output voltage and current waveforms for the SPWM technique at ma = 0.9.
C. Comparative Analysis
The simulation results reveal fundamental differences between the two modulation strategies. NLM produces a clean, staircase waveform with switching events occurring only at the fundamental frequency. SPWM, on the other hand, produces a high-frequency pulsed waveform that offers more precise control over the output voltage envelope.
The practical trade-offs are significant. The low switching frequency of NLM results in substantially lower switching losses, potentially leading to higher inverter efficiency. Conversely, SPWM offers superior control over the output voltage and pushes harmonics to higher frequencies, but at the cost of increased switching losses. The impact of the modulation index on the inverter's output for both techniques is summarized in Table 3.
Table 3: Impact of Modulation Index (ma) on Inverter Output
Modulation Index (ma) | NLM: Observed Output | SPWM: Observed Output |
0.2 | No output voltage generated. | Low-amplitude sinusoidal current; effective output appears as three levels. |
0.3 | Three-level output (+VDC/2, 0, -VDC/2). | Increased sinusoidal current; effective output is three levels. |
0.5 | Three-level output. | Higher sinusoidal current; effective output is three levels. |
0.9 | Full five-level output. | Full five-level envelope; maximum sinusoidal current. |
In summary, the choice between NLM and SPWM depends heavily on the specific application's priorities, whether they be maximizing efficiency (favoring NLM) or achieving high-fidelity waveform control and better harmonic performance (favoring SPWM).
VI. Conclusion and Future Scope
A simulation-based comparative analysis of NLM and SPWM control for a five-level hybrid inverter has been presented. This research successfully demonstrated the operation of a reduced-component-count five-level hybrid inverter topology through detailed modeling and simulation in MATLAB/Simulink.
The key conclusions drawn from this work are:
• The five-level hybrid topology was successfully implemented and validated in the simulation environment, confirming its capability to generate the desired five voltage levels.
• Both NLM and SPWM were shown to be effective methods for controlling the inverter, though they exhibit fundamentally different operational characteristics.
• The modulation index was identified as a critical control parameter for both strategies, directly dictating the number of active voltage levels and the amplitude of the output current.
• A clear performance trade-off was identified: NLM offers the advantage of low-frequency switching and potentially higher efficiency, while SPWM provides superior control over the output voltage envelope and harmonic profile at the expense of higher switching losses.
To build upon this foundational work, several avenues for future research are recommended:
1. Quantitative Harmonic Analysis: A detailed comparative study of the Total Harmonic Distortion (THD) of the output voltage and current for both modulation techniques across a range of modulation indices.
2. Efficiency and Loss Analysis: A comprehensive analysis of the conduction and switching losses for NLM and SPWM to quantitatively assess the overall inverter efficiency under different operating conditions.
3. Advanced Modulation Strategies: The implementation and evaluation of more advanced control methods, such as Space Vector Modulation (SVM), to potentially improve DC bus utilization and further optimize harmonic performance.
4. Experimental Validation: The construction of a hardware prototype of the five-level inverter to experimentally validate the simulation results and assess the real-world performance of the topology and control algorithms.
VII. YouTube Video
VIII. Purchase link of the Model
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