Fundamental Frequency Switching Strategies of a Seven-Level Hybrid Cascaded H-Bridge Multilevel Inverter
Abstract
This paper presents a cascaded H-bridge multilevel inverter that can be implemented using only a single dc power source and capacitors. Standard cascaded multilevel inverters require n dc sources for 2n + 1 levels. Without requiring transformers, the scheme proposed here allows the use of a single dc power source (e.g., a battery or a fuel cell stack) with the remaining n − 1 dc sources being capacitors, which is referred to as hybrid cascaded H-bridge multilevel inverter (HCMLI) in this paper. It is shown that the inverter can simultaneouslymaintain the dc voltage level of the capacitors and choose a fundamental frequency switching pattern to produce a nearly sinusoidal output. HCMLI using only a single dc source for each phase is promising for high-power motor drive applications as it significantly decreases the number of required dc power supplies, provides high-quality output power due to its high number of output levels, and results in high conversion efficiency and low thermal stress as it uses a fundamental frequency switching scheme. This paper mainly discusses control of seven-level HCMLI with fundamental frequency switching control and how its modulation index range can be extended using triplen harmonic compensation. Index Terms—Fundamental frequency modulation control, hybrid cascaded H-bridge multilevel inverter (HCMLI), triplen harmonic compensation.
I. INTRODUCTION
THE MULTILEVEL inverter is a promising power electronics topology for high-power applications because of its low electromagnetic interference (EMI) and high efficiency with low-switching-frequency control method [1]–[6]. Traditionally, each phase of a cascaded multilevel inverter requires n dc sources for 2n + 1 levels. For many applications, obtaining so many separate dc sources may preclude the use of such an inverter. To reduce the number of dc sources required when the cascaded H-bridge multilevel inverter is applied to a motor drive, a scheme is proposed in this paper that allows the use of a single dc source (such as battery or fuel cell) as the first dc source with the remaining n − 1 dc sources being capacitors in the cascaded H-bridges multilevel inverter, which is referred to as the hybrid cascaded H-bridge multilevel inverter (HCMLI) [7]–[9]. Previous work has shown that pulsewidth modulation (PWM) control can be used on HCMLI [10]. Compared to the traditional cascaded H-bridge multilevel inverter, the proposed HCMLI has a low number of dc sources and retains the lowswitching- frequency advantage. The authors have been working on the multilevel inverter harmonic elimination control technologies based on harmonic elimination mathematics theory, and present several findings, such as complete switching angle solution technology and active harmonic elimination technology. The seven-level multilevel inverter fundamental frequency harmonic elimination method and triplen harmonic injection for modulation index extension method have been published in previous papers. All the technologies published in the previous papers can be applied to normal multilevel inverters to satisfy different application requirements. However, the published technologies cannot be directly applied to HCMLI as the capacitors are not dc sources. The control goal of the HCMLI needs to maintain the balance of the dc voltage level of the capacitors while producing a nearly sinusoidal three-phase output voltage using a low-switchingfrequency harmonic elimination method. This paper focuses on how to apply the seven-level fundamental frequency harmonic elimination method to HCMLI and extend its modulation index range, and presents new findings on HCLMI control other than normal cascaded H-bridge multilevel inverters.
II. WORKING PRINCIPLE OF HCLMI
To operate a cascaded multilevel inverter using a single dc source, capacitors are used as the dc sources for all but the first source. To explain, consider a cascaded multilevel inverter with two H-bridges as shown in Fig. 1. The dc source for the first H-bridge (H1 ) is a battery or fuel cell with an output voltage of Vdc, while the dc source for the second H-bridge (H2) is a capacitor whose voltage is to be held at Vc . The output voltage of the first H-bridge is denoted by v1 and the output of the second H-bridge is denoted by v2 so that the output voltage


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