Single-Phase AC-DC-AC Multilevel Five-Leg Converter With High-Frequency Link

Abstract

This paper investigates a single-phase ac-dc-ac multilevel converter formed by a three-leg module (converter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$a$</tex-math></inline-formula> ) series connected to an H-bridge (converter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$H$</tex-math></inline-formula> ) at the load side of the system to achieve higher and multilevel output voltage. The proposed solution has two dc-links that are connected through a high-frequency link (HFL) formed by an inverter H-bridge, a small toroidal transformer, and a unidirectional rectifier H-bridge. The use of the HFL permits optimizing the SV-PWM strategy by employing vector redundancies that minimize the switching frequency of the devices. Besides, the control system is simplified compared to the conventional solution with floating capacitors. The proposed system is appropriate for uninterrupted power supply and unified power quality conditioner applications. System equations, dc-link voltage specifications, power distribution analysis, and a space vector PWM are discussed. The proposed control strategy ensures suitable load voltage with constant amplitude and fundamental frequency, guaranteeing a high grid power factor and maintaining controlled dc-link voltages. Compared with the conventional solutions, the proposed topology has a lower dc-link voltage rating, lower harmonic content, and lower switching losses in the converters <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$a$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$H$</tex-math></inline-formula> . Experimental results are presented to validate the developed studies.