Three-Phase Four-Wire Nine-Leg AC–DC–AC Converter Based on High-Frequency Link

This work proposes a three-phase four-wire ac–dc–ac converter composed of three single-phase three-leg cells with shared legs. Also, the three dc-links are connected employing a dc–dc converter with a high-frequency transformer (HFT). Each dc-link is connected to a high-frequency H-bridge and each H-bridge is connected to a winding of the transformer. The HFT is responsible to balance the dc-links, providing the necessary power flow. The proposed converter allows dealing with a wide range of unbalances, compensates for grid voltage sags, and provides a high power factor on the grid side with low harmonic content. In addition, the proposed structure can operate with minimum dc-link voltage values in applications where the grid and load frequencies are the same, such as unified power quality conditioners. Converter model is provided, as well as pulse-width modulation, control strategy, design of passive devices, and power analysis. Experimental results are presented for validation purposes. Compared with conventional solutions, the proposed converter offers advantages, such as not requiring line-frequency transformers, being able to operate under severe unbalanced conditions, or requiring a simpler structure.

I. Introduction

In the last decades, the increasing utilization of semiconductor and power electronics devices generated a great amount of nonlinear loads connected to the distribution systems. These loads inject harmonic components in the grid currents that also generate harmonic voltage components at the grid mains [1], [2]. In addition, voltage sags can be caused by faults in the distribution system, starting large motors, turning on/off high-power loads [3]. In four-wire systems, unbalanced conditions may also be found, such as asymmetrical faults and poorly distributed nonlinear loads, leading to unbalanced grid voltages and currents, and neutral currents with excessively high amplitudes, requiring oversized neutral conductors and causing deteriorating of equipment, such as transformers [4]. In this context, unified power quality conditioners (UPQCs) have gained relevance, due to their ability to mitigate disturbances of both grid currents and load voltages [5].